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1.IntroductionFar-infrared astronomy, defined broadly as encompassing science at wavelengths of 30 to , is an invaluable tool in understanding all aspects of our cosmic origins. Tracing its roots to the late 1950s, with the advent of infrared detectors sensitive enough for astronomical applications, far-infrared astronomy has developed from a niche science, pursued by only a few teams of investigators, to a concerted worldwide effort pursued by hundreds of astronomers, targeting areas ranging from the origins of our Solar System to the ultimate origin of the Universe. By their nature, far-infrared observations study processes that are mostly invisible at other wavelengths, such as young stars still embedded in their natal dust clouds or the obscured, rapid assembly episodes of supermassive black holes. Moreover, the 30 to wavelength range includes a rich and diverse assembly of diagnostic features. The most prominent of these are as follows:
These profusion and diversity of diagnostics allow for advances across a wide range of disciplines. We briefly describe four examples in the following paragraphs: Planetary systems and the search for life: Far-infrared continuum observations in multiple bands over 50 to measure the size distributions, distances, and orbits of both trans-Neptunian objects1–4 and zodiacal dust,5 which give powerful constraints on the early formation stages of our Solar System and of others. Molecular and water features determine the composition of these small bodies, provide the first view of how water pervaded the early Solar System via deuterated species ratios, and constrain how water first arrived on Earth.6–8 Far-infrared observations are also important for characterizing the atmospheric structure and composition of the gas giant planets and their satellites, especially Titan. Far-infrared continuum observations also give a direct view of the dynamics and evolution of protoplanetary disks, thus constraining the early formation stages of other solar systems.9–12 Deuterated species can be used to measure disk masses and ice features, and water lines give a census of water content and thus the earliest seeds for life,13 whereas the water lines and other molecular features act as biomarkers, providing the primary tool in the search for life beyond Earth.14,15 The early lives of stars: The cold, obscured early stages of star formation make them especially amenable to study at far-infrared wavelengths. Far-infrared continuum observations are sensitive to the cold dust in star-forming regions, from the filamentary structures in molecular clouds16 to the envelopes and disks that surround individual premain-sequence stars.17 They, thus, trace the luminosities of young stellar objects and can constrain the masses of circumstellar structures. Conversely, line observations such as [O I], CO, and probe the gas phase, including accretion flows, outflows, jets, and associated shocks.18–24 For protostars, as their spectral energy distributions (SEDs) peak in the far-infrared, photometry in this regime is required to refine estimates of their luminosities and evolutionary states25–27 and can break the degeneracy between inclination angle and evolutionary state (at midinfrared and shorter wavelengths, a more evolved protostar seen through its edge-on disk has an SED similar to a deeply embedded protostar viewed from an intermediate angle28). With Herschel, it has become possible to measure temperatures deep within starless cores,29 and young protostars have been discovered that were only visible at far-infrared and longer wavelengths.30 These protostars have ages of , only 5% of the estimated protostellar lifetime. In the T Tauri phase, where the circumstellar envelope disperses, far-infrared observations probe the circumstellar disk.31 At later phases, the far-infrared traces extrasolar analogs of the Kuiper belt in stars, such as Fomalhaut.32 Future far-infrared observations hold the promise of understanding the photometric variability of protostars. Herschel has shown that the far-infrared emission from embedded protostars in Orion could vary by as much as 20% over a timescale of weeks,33 but such studies are limited by the -year lifetime of Herschel. Future observatories will allow for sensitive mapping of entire star-forming regions several times over the durations of their missions. This will enable a resolution to the long-running question of whether protostellar mass accretion happens gradually over a few hundred thousand years or more stochastically as a series of short, episodic bursts.34 The physics and assembly history of galaxies: The shape of the mid/far-infrared dust continuum is a sensitive diagnostic of the dust grain size distribution in the interstellar medium (ISM) of our Milky Way, and nearby galaxies, which in turn diagnoses energy balance in the ISM.35–38 Emission and absorption features measure star formation, metallicity gradients, gas-phase abundances and ionization conditions, and gas masses, all independently of extinction, providing a valuable perspective on how our Milky Way, and other nearby galaxies, formed and evolved.39–43 Continuum and line surveys at far-infrared wavelengths measure both obscured star formation rates and black hole accretion rates over the whole epoch of galaxy assembly, up to , and are essential to understand why the comoving rates of both star formation and supermassive black hole accretion peaked at redshifts of , when the Universe was only 2 or 3 billion years old and have declined strongly since then.44,45 Of particular relevance in this context are the infrared-luminous galaxies in which star formation occurs embedded in molecular clouds, hindering the escape of optical and ultraviolet radiation; however, the radiation heats dust, which reradiates infrared light, enabling star-forming galaxies to be identified and their star formation rates to be inferred. These infrared-luminous galaxies may dominate the comoving star formation rate density at and are most optimally studied via infrared observations.46–50 Furthermore, far-infrared telescopes can study key processes in understanding stellar and black hole mass assemblies, whether or not they depend directly on each other and how they depend on environment, redshift, and stellar mass.51–53 The origins of the Universe: Millimeter-wavelength investigations of primordial B- and E-modes in the cosmic microwave background (CMB) provide the most powerful observational constraints on the very early Universe, at least until the advent of space-based gravitational-wave observatories.54,55 However, polarized dusty foregrounds are a pernicious barrier to such observations, as they limit the ability to measure B-modes produced by primordial gravitational waves, and thus to probe epochs up to s after the Big Bang. Observations at far-infrared wavelengths are the only way to isolate and remove these foregrounds. CMB instruments that also include far-infrared channels thus allow for internally consistent component separation and foreground subtraction. The maturation of far-infrared astronomy as a discipline has been relatively recent, in large part catalyzed by the advent of truly sensitive infrared detectors in the early 1990s. Moreover, the trajectory of this development over the past two decades has been steep, going from one dedicated satellite and a small number of other observatories by the mid-1980s to at least eight launched infrared-capable satellites, three airborne facilities, and several balloon/sub-orbital and dedicated ground-based observatories by 2018. New detector technologies are under development, and advances in areas such as mechanical coolers enable those detectors to be deployed within an expanding range of observing platforms. Even greater returns are possible in the future, as far-infrared instrumentation capabilities remain far from the fundamental sensitivity limits of a given aperture. This recent, rapid development of the far-infrared is reminiscent of the advances in optical and near-infrared astronomy from the 1940s to the 1990s. Optical astronomy has benefited greatly from developments in sensor, computer, and related technologies that had been driven in large part by commercial and other applications, and which by now are fairly mature. Far-infrared astronomers have only recently started to benefit from comparable advances in capability. The succession of rapid technological breakthroughs, coupled with a wider range of observing platforms, means that far-infrared astronomy holds the potential for paradigm-shifting advances in several areas of astrophysics over the next decade. We here review the technologies and observing platforms for far-infrared astronomy and discuss potential technological developments for those platforms, including in detectors and readout systems; optics; telescope and detector cooling; platform infrastructure on the ground, sub-orbital, and in space; software development; and community cohesion. We aim to identify the technologies needed to address the most important science goals accessible in the far-infrared. We do not review the history of infrared astronomy, as informative historical reviews can be found elsewhere.56–65 We focus on the 30- to wavelength range, though we do consider science down to , and into the millimeter range, as well. We primarily address the U.S. mid/far-infrared astronomy community; there also exist roadmaps for both European66 and Canadian67 far-infrared astronomy, and for THz technology covering a range of applications.68–70 2.Observatories: Atmosphere-Based2.1.Ground-BasedFar-infrared astronomy from the ground faces the fundamental limitation of absorption in Earth’s atmosphere, primarily by water vapor. The atmosphere is mostly opaque in the midinfrared through far-infrared, with only a few narrow wavelength ranges with modest transmission. This behavior is shown in Fig. 1, which compares atmospheric transmission for ground-based observing, observing from Stratospheric Observatory for Infrared Astronomy (SOFIA) (Sec. 2.2), with two higher altitudes that are accessible by balloon-based platforms. The difficulties of observing from the ground at infrared wavelengths are evident. Moreover, the transmissivity and widths of these windows are heavily weather-dependent. Nevertheless, there do exist spectral windows at 34, 350, 450, 650, and with good, albeit weather-dependent transmission at dry, high-altitude sites, with a general improvement toward longer wavelengths. At wavelengths longward of about 1 mm, there are large bands with good transmission. These windows have enabled an extensive program of ground-based far-infrared astronomy, using both single-dish and interferometer facilities. 2.1.1.Single-dish facilitiesSingle-dish telescopes dedicated to far-infrared through millimeter astronomy have been operated for over 30 years. Examples include the 15-m James Clerk Maxwell Telescope (JCMT), the 12-m Caltech Submillimeter Observatory (CSO, closed September 2015), the 30-m telescope operated by the Institut de Radioastronomie Millimétrique (IRAM), the 12-m Atacama Pathfinder Experiment (APEX), the 50-m Large Millimeter Telescope (LMT) in Mexico, the 10-m Submillimeter Telescope (formerly the Heinrich Hertz SMT) in Arizona, and the 10-m South Pole Telescope. These facilities have made major scientific discoveries in almost every field of astronomy, from planet formation to high-redshift galaxies. They have also provided stable development platforms, resulting in key advances in detector technology, and pioneering techniques that subsequently found applications in balloon-borne and space missions. There is an active program of ground-based single-dish far-infrared astronomy, with current and near-future single-dish telescopes undertaking a range of observation types, from wide-field mapping to multiobject wideband spectroscopy. This, in turn, drives a complementary program of technology development. In general, many applications for single-dish facilities motivate development of detector technologies capable of very large pixel counts (Sec. 5.1). Similarly, large pixel counts are envisioned for planned space-based far-infrared observatories, including the Origins Space Telescope (OST; see Sec. 3.3). As far-infrared detector arrays have few commercial applications, they must be built and deployed by the science community itself. Thus, ground-based instruments represent a vital first step toward achieving NASA’s long-term far-infrared goals. We here briefly describe two new ground-based facilities: CCAT-prime (CCAT-p), and the LMT: CCAT-p: It is a 6-m telescope at 5600-m altitude, near the summit of Cerro Chajnantor in Chile.72 CCAT-p is being built by Cornell University and a German consortium that includes the universities of Cologne and Bonn, and in joint venture with the Canadian Atacama Telescope Corporation. In addition, CCAT-p collaborates with CONICYT and several Chilean universities. The project is funded by a private donor and by the collaborating institutions and is expected to achieve first light in 2021. The design of CCAT-p is an optimized crossed-Dragone73 system that delivers an 8-deg field of view (FoV) with a nearly flat image plane. At , the FoV with adequate Strehl ratio reduces to about 4 deg. The wavelength coverage of the anticipated instruments will span wavelengths of to 1.3 mm. With the large FoV and a telescope surface root mean square of below , CCAT-p is an exceptional observatory for survey observations. As the zenith transmission is in the first quartile at the CCAT-p site,74 a observing capability will be added in a second-generation upgrade. The primary science drivers for CCAT-p are (1) tracing the epoch or reionization via [CII] intensity mapping; (2) studying the evolution of galaxies at high redshifts; (3) investigating dark energy, gravity, and neutrino masses via measurements of the Sunyaev–Zel’dovich effect; and (4) studying the dynamics of the ISM in the Milky Way and nearby galaxies via high spectral resolution mapping of fine structure and molecular lines. CCAT-p will host two facility instruments: the CCAT Heterodyne Array Instrument (CHAI), and the direct detection instrument Prime-Cam (P-Cam). CHAI is being built by the University of Cologne and will have two focal plane arrays that simultaneously cover the 370- and bands. The arrays will initially have elements, with a planned expansion to 128 elements each. The direct detection instrument P-Cam, which will be built at Cornell University, will encompass seven individual optic tubes. Each tube has a FoV of about 1.3 deg. For first light, three tubes will be available: (1) a four-color, polarization-sensitive camera with 9000 pixels that simultaneously covers the 1400-, 1100-, 850-, and bands; (2) a 6000-pixel Fabry–Pérot spectrometer; and (3) a 18,000-pixel camera for the band. LMT: The LMT is a 50-m diameter telescope sited at 4600 m on Sierra Negra in Mexico. The LMT has a FoV of 4′ and is optimized for maximum sensitivity and small beam size at far-infrared and millimeter wavelengths. It too will benefit from large-format new instrumentation in the coming years. A notable example is TolTEC, a wide-field imager operating at 1.1, 1.4, and 2.1 mm, and with an anticipated mapping speed at 1.1 mm of (Table 1). At 1.1 mm, the TolTEC beam size is anticipated to be , which is smaller than the 6″ beam size of the Spitzer extragalactic survey maps. As a result, the LMT confusion limit at 1.1 mm is predicted to be , thus making LMT capable of detecting sources with star formation rates below at . This makes TolTEC an excellent “discovery machine” for high-redshift obscured galaxy populations. As a more general example of the power of new instruments mounted on single-aperture ground-based telescopes, a -object steered-beam multiobject spectrometer mounted on the LMT would exceed the abilities of any current ground-based facility, including Atacama large millimeter/submillimeter array (ALMA), for survey spectroscopy of galaxies, and would require an array of . Table 1Selected examples of sensitivities achieved by far-infrared to millimeter-wave detector arrays, along with some required for future missions.
Note: Requirements for the SPICA/SAFARI instrument are taken from Ref. 75. Requirements for the SPIRIT interferometer (whose aperture is the effective aperture diameter for an interferometer with two 1-m diameter telescopes) are taken from Ref. 76. 2.1.2.InterferometryInterferometry at far-infrared wavelengths is now routinely possible from the ground and has provided order of magnitude improvements in spatial resolution and sensitivity over single-dish facilities. Three major ground-based far-infrared/millimeter interferometers are currently operational. The NOEMA array (the successor to the IRAM Plateau de Bure interferometer) consists of nine 15-m dishes at 2550-m elevation in the French Alps. The submillimeter array (SMA) consists of eight 6-m dishes on the summit of Mauna Kea in Hawaii (4200-m elevation). Both NOEMA and the SMA are equipped with heterodyne receivers. NOEMA has up to 16-GHz instantaneous bandwidth, whereas the SMA has up to 32 GHz of instantaneous bandwidth (or 16 GHz with dual polarization) with 140-KHz uniform spectral resolution. Finally, the ALMA is sited on the Chajnantor Plateau in Chile at an elevation of 5000 m. It operates from 310 to in eight bands covering the primary atmospheric windows. ALMA uses heterodyne receivers based on superconductor–insulator–superconductor (SIS) mixers in all bands, with 16-GHz maximum instantaneous bandwidth split across two polarizations and four basebands. ALMA consists of two arrays: the main array of fifty 12-m dishes (of which typically 43 are in use at any one time), and the Morita array (also known as the Atacama Compact Array), which consists of up to twelve 7-m dishes and up to four 12-m dishes equipped as single-dish telescopes. At the ALMA site (which is the best of the three ground-based interferometer sites), the precipitable water vapor (PWV) is below 0.5 mm for 25% of the time during the five best observing months (May to September). This corresponds to a transmission of about 50% in the best part of the 900-GHz window (ALMA band 10). In more typical weather (), the transmission at 900 GHz is 25%. There are plans to enhance the abilities of ALMA over the next decade by (1) increasing the bandwidth, (2) achieving finer angular resolutions, (3) improving wide-area mapping speeds, and (4) improving the data archive. The primary improvement in bandwidth is expected to come from an upgrade to the ALMA correlator, which will effectively double the instantaneous bandwidth and increase the number of spectral points by a factor of 8. This will improve ALMA’s continuum sensitivity by a factor and will make ALMA more efficient at line surveys. Further bandwidth improvements include the addition of a receiver covering 35 to 50 GHz (ALMA band 1, expected in 2022) and 67 to 90 GHz (ALMA band 2). To improve angular resolution, studies are underway to explore the optimal number and placement of antennas for baseline lengths of up to tens of kilometers. Other possible improvements include increasing the number of antennas in the main array to 64, the incorporation of focal-plane arrays to enable wider field imaging, and improvements in the incorporation of ALMA into the global very long baseline interferometry network. 2.2.Stratospheric Observatory for Infrared AstronomyThe SOFIA77 is an effective 2.5-m diameter telescope mounted within a Boeing 747SP aircraft that flies at altitudes of 13,700 m to get above over 99.9% of the Earth’s atmospheric water vapor. The successor to the Learjet observatory and NASA’s Kuiper Airborne Observatory (KAO), SOFIA saw first light in May 2010, began prime operations in May 2014, and offers per year for community science observations.78 SOFIA is the only existing public observatory with access to far-infrared wavelengths inaccessible from the ground, though CMB polarization studies at millimeter wavelengths have also been proposed from platforms at similar altitudes to SOFIA.79 SOFIA’s instrument suite can be changed after each flight and is evolvable with new or upgraded instruments as capabilities improve. SOFIA is also a versatile platform, allowing for (1) continuous observations of single targets for up to 5 h, (2) repeated observations over multiple flights in a year, and (3) in principle, observations in the visible though millimeter-wavelength range. Example flight paths for SOFIA are shown in Fig. 2. Each flight path optimizes observing conditions (e.g., elevation, percentage of water vapor, and maximal on-target integration time). SOFIA can be positioned to where the science needs, enabling all-sky access. Annually, SOFIA flies from Christchurch, New Zealand, to enable Southern Hemisphere observations. SOFIA’s instruments include the 5- to camera and grism spectrometer FORCAST,80,81 the high-resolution (up to ) 4.5- to spectrometer EXES,82 the 51- to integral field spectrometer FIFI-LS,83 the 50- to camera and polarimeter HAWC,84 and the heterodyne spectrometer GREAT.85,86 The first-generation HIPO87 and FLITECAM88 instruments retired in early 2018. The sensitivities of these instruments as a function of wavelength are presented in Fig. 3. Upgrades to instruments over the past few years have led to new science capabilities, such as adding a polarimetry channel to HAWC (HAWC+89), and including larger arrays and simultaneous channels on GREAT (upGREAT90 & 4GREAT, commissioned in 2017), making it into an efficient mapping instrument. Figure 4 shows early polarimetry measurements from HAWC+. Given the versatility and long-term nature of SOFIA, there is a continuous need for more capable instruments throughout SOFIA’s wavelength range. However, the unique niche of SOFIA, given its warm telescope and atmosphere, the imminent era of the James Webb Space Telescope (JWST), and ever more capable ground-based platforms, is high-resolution spectroscopy. This is presently realized with two instruments (GREAT and EXES). An instrument under development, the high-resolution midinfrared spectrometer (HIRMES), scheduled for commissioning in 2019, will enhance SOFIA’s high-resolution spectroscopy capabilities. HIRMES covers 25 to , with three spectroscopic modes (, , and ), and an imaging spectroscopy mode (). As SOFIA can renew itself with new instruments, it provides both new scientific opportunities and maturation of technology to enable future far-infrared space missions. SOFIA offers a 20-kVA cryocooler with two compressors capable of servicing two cold heads. The heads can be configured to operate two cryostats or in parallel within one cryostat to increase heat-pumping capacity, with second-stage cooling capacity of at 4.2 K and first-stage cooling capacity of at 70 K. Instruments aboard SOFIA can weigh up to 600 kg, excluding the instrument electronics in the counterweight rack and PI Rack(s), and can draw power up to 6.5 kW. Their volume is limited by the aircraft’s door access and must fit within the telescope assembly constraints. Capabilities that would be invaluable in a next-generation SOFIA instrument include, but are not limited to, the following:
Other possible improvements to the SOFIA instrument suite include: (1) upgrading existing instruments (e.g., replacing the FIFI-LS germanium photoconductors to achieve finer spatial sampling through higher multiplexing factors), and (2) instruments that observe in current gaps in SOFIA wavelength coverage (e.g., 1 to , 90 to , and 210 to ). More general improvements include the ability to swap instruments faster than a 2-day timescale, or the ability to mount multiple instruments. Mounting multiple instruments improves observing efficiency if both instruments can be used on the same source, covering different wavelengths or capabilities. This would also allow for flexibility to respond to targets of opportunity, time domain, or transient phenomena, and increase flexibility as a development platform to raise technology readiness levels (TRLs91,92) of key technologies. 2.3.Scientific BallooningBalloon-based observatories allow for observations at altitudes of up to (130,000 ft). At these altitudes, of the atmosphere remains above the instrument, with negligible water vapor. Scientific balloons, thus, give access, relatively cheaply, to infrared discovery space that is inaccessible to any ground-based platform, and in some cases even to SOFIA. For example, several key infrared features are inaccessible even at aircraft altitudes (Fig. 1), including low-energy water lines and the [N II] line. Scientific ballooning is, thus, a valuable resource for infrared astronomy. Both standard balloons, with flight times of , and long duration balloons (LDBs) with typical flight times of 7 to 15 days (though flights have lasted as long as 55 days) have been used. Balloon projects include the Balloon-borne Large Aperture Submillimeter Telescopes (BLAST93–95), PILOT,96 the Stratospheric Terahertz Observatory97 (Fig. 5), and Far-Infrared Interferometric Telescope Experiment (FITE)99 and Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII),100 both described in Sec. 4.1. Approved future missions include GUSTO, scheduled for launch in 2021. With the development of ultra-LDBs (ULDBs), with potential flight times of over 100 days, new possibilities for far-infrared observations have become available. A further advantage of ballooning, in a conceptually similar manner to SOFIA, is that the payloads are typically recovered and available to refly on -year timescales, meaning that balloons are a vital platform for technology development and TRL raising. For example, far-infrared direct-detector technology shares many common elements (detection approaches, materials, and readouts) with CMB experiments, which are conducted on the ground,101–103 from balloons,104–106 and in space. These platforms have been useful for developing bolometer and readout technology applicable to the far-infrared. All balloon projects face challenges, as the payload must include the instrument and all of the ancillary equipment needed to obtain scientific data. For ULDBs, however, there are two additional challenges: Payload mass: Whereas zero-pressure balloons (including LDBs) can lift up to about 2700 kg, ULDBs have a mass limit of about 1800 kg. Designing a payload to this mass limit is nontrivial, as science payloads can have masses in excess of 2500 kg. For example, the total mass of the GUSTO gondola is estimated to be 2700 kg. Cooling: All far-infrared instruments must operate at cryogenic temperatures. Liquid cryogens have been used for instruments on both standard and LDBs, with additional refrigerators (e.g., , adiabatic demagnetization) to bring detector arrays down to the required operating temperatures, which can be as low as 100 mK. These cooling solutions typically operate on timescales commensurate with LDB flights. For the ULDB flights, however, it is not currently possible to achieve the necessary cryogenic hold times. Use of mechanical coolers to provide first-stage cooling would solve this problem, but current technology does not satisfy the needs of balloon missions. Low-cost cryocoolers for use on the ground are available, but have power requirements that are hard to meet on balloons, which currently offer total power of up to about 2.5 kW. Low-power cryocoolers exist for space use, but their cost (typically ) does not fit within typical balloon budgets. Cryocoolers are discussed in detail in Sec. 5.5. In addition to addressing the challenges described above, there exist several avenues of development that would enhance many balloon experiments. Three examples are as follows:
2.4.Short Duration Rocket FlightsSounding rockets inhabit a niche between high-altitude balloons and fully orbital platforms, providing 5 to 10 min of observation time above the Earth’s atmosphere, at altitudes of 50 to . They have been used for a wide range of astrophysical studies, with a heritage in infrared astronomy stretching back to the 1960s.107–110 Though an attractive way to access space for short periods, the mechanical constraints of sounding rockets are limiting in terms of size and capability of instruments. However, sounding rockets observing in the infrared are flown regularly,111 and rockets are a viable platform for both technology maturation and certain observations in the far-infrared. In particular, measurements of the absolute brightness of the far-infrared sky, intensity mapping, and development of ultra-low-noise far-infrared detector arrays are attractive applications of this platform. Regular access to sounding rockets is now a reality, with the advent of larger, more capable Black Brant XI vehicles to be launched from southern Australia via the planned Australian NASA deployment in 2019–2020. Similarly, there are plans for recovered flights from Kwajalein Atoll using the recently tested NFORCE water recovery system. Long-duration sounding rockets capable of providing limited access to orbital trajectories and observation times have been studied,112 and NASA is continuing to investigate this possibility. However, no missions using this platform are currently planned, and as a result the associated technology development is moving slowly. 3.Observatories: Space-BasedAll atmospheric-based observing platforms, including SOFIA and balloons, suffer from photon noise from atmospheric emission. Even at balloon altitudes, of order 1% emissivity on average through the far-infrared remains from residual water vapor, which can contaminate astrophysical water lines, unless they are shifted by velocities of at least a few tens of . The telescope optics is another source of loading, with an unavoidable 2% to 4% emissivity. Though the total emissivity can be , these ambient-temperature () background sources dominate that of the zodiacal and galactic dusts. Space-based platforms are, thus, for several paths of inquiry, the only way to perform competitive infrared observations. There exists a rich history of space-based mid/far-infrared observatories (Fig. 6), including Infrared Atmospheric Sounder (IRAS),113 Midcourse Space Experiment,114 the Infrared Telescope in Space,115 Infrared Space Observatory (ISO)116 Submillimeter Wave Astronomy Satellite (SWAS)117 Odin,118 Akari,119 Herschel,120 Wide-Field Infrared Survey Explorer,121 and Spitzer.122 Far-infrared detector arrays are also used on space-based CMB missions, with past examples, including Planck,123 Wilkinson microwave anisotropy probe,124 and Cosmic Background Explorer,125,126 as well as concepts, such as primordial inflation explorer,127 LiteBIRD,128 and Cosmic Origins Explorer.129 It is notable, however, that the performance of many past and present facilities is limited by thermal emission from telescope optics (Fig. 7). The comparison between infrared telescopes operating at 270 K and temperatures of a few kelvins is analogous to the comparison between the sky brightness during the day and at night in the optical. Even with Herschel and its telescope, the telescope emission was the dominant noise term for both its Photodetector Array Camera and Spectrometer (PACS133) and Spectral and Photometric Imaging Receiver (SPIRE134). Thus, the ultimate scientific promise of the far-infrared is in orbital missions with actively cooled telescopes and instruments. Cooling the telescope to a few kelvins effectively eliminates its emission through most of the far-infrared band. When combined with appropriate optics and instrumentation, this results in orders-of-magnitude improvement in discovery speed over what is achievable from atmospheric-based platforms (Figs. 8 and 9). A “cold” telescope can bring sensitivities at observed frame of 30 to into parity with those at shorter (JWST) and longer (ALMA) wavelengths. A further limiting factor is source confusion—the fluctuation level in image backgrounds below which individual sources can no longer be detected. Unlike instrument noise, confusion noise cannot be reduced by increasing integration time. Source confusion can arise from both smooth diffuse emission and fluctuations on scales smaller than the beam size of the telescope. Outside of the plane of the Milky Way, the primary contributors to source confusion are structures in Milky Way dust emissions, individually undetected extragalactic sources within the telescope beam, and individually detected sources that are blended with the primary source. Source confusion is, thus, a strong function of the location on the sky of the observations, the telescope aperture, and observed wavelength. Source confusion is a concern for all previous and current single-aperture infrared telescopes, especially space-based facilities whose apertures are modest compared to ground-based facilities. A summary of the confusion limits of some previous infrared telescopes is given in Fig. 10. A related concept is line confusion, which is caused by the blending and overlapping of individual lines in spectral line surveys. Although this is barely an issue in, e.g., H I surveys as the 21-cm H I line is bright and isolated,142 it is potentially a pernicious source of uncertainty at far-infrared wavelengths, where there are a large number of bright spectral features. This is true in galactic studies143 and in extragalactic surveys. Carefully chosen spatial and spectral resolutions are required to minimize line confusion effects.144 Several approaches have been adopted to extract information on sources below the standard confusion limit. They include detection methods applied to single-band maps,145 the use of prior positional information from higher spatial resolution images to deconvolve single far-infrared sources,146,147 and combination of priors on positions with priors from SED modeling.148,149 Finally, the spatial–spectral surveys from upcoming facilities such as SAFARI on Space Infrared Telescope for Cosmology and Astrophysics (SPICA) or the OST Survey Spectrometer on the OST should push significantly below the classical confusion limit by including spectral information to break degeneracies in the third spatial dimension.150 There are two further challenges that confront space-based far-infrared observatories, which are unfamiliar to suborbital platforms: Dynamic range: Moving to “cold” telescopes, sensitivity is limited only by the far-infrared sky background. We enter a regime where the dominant emission arises from the sources under study, and the sky has genuinely high contrast. This imposes a new requirement on the detector system—to observe the full range of source brightness—that is simple from suborbital platforms but challenging for cooled space-based platforms, as the saturation powers of currently proposed high-resolution detector arrays are within orders of magnitude of their noise equivalent power (NEP is, briefly, the input signal power that results in a signal-to-noise ratio of unity in a 1-Hz bandwidth—the minimum detectable power per square root of bandwidth. Thus, a lower NEP is better. In-depth discussions of the concept of NEP can be found in Refs. 151152.–153.). This would limit observations to relatively faint sources. Dynamic range limitations were even apparent for previous-generation instruments such as the multiband imaging photometer onboard spitzer and PACS onboard Herschel, with saturation limits at of 57 and 220 Jy, respectively. Thus, we must either design detector arrays with higher dynamic range or populate the focal plane with detector arrays, each suited to part of the range of intensities. Interference: The susceptibility of cooled detector arrays to interference from ionizing radiation in space was noted in the development of microcalorimeter arrays for x-ray telescopes.154–156 Moreover, this susceptibility was clearly demonstrated by the bolometers on Planck. An unexpectedly high rate and magnitude of ionizing radiation events were a major nuisance for this mission, requiring corrections to be applied to nearly all of the data. Had this interference been a factor of worse, it would have caused significant loss of science return from Planck. Techniques are being developed and demonstrated to mitigate this interference for x-ray microcalorimeters by the addition of a few-micron-thick layer of gold on the back of the detector frame. It is likely that a similar approach can mitigate interference in high-resolution far-infrared detector arrays as well. Moreover, work on reducing interference in far-infrared detector arrays is being undertaken in the SPACEKIDS program (Sec. 5.1.2). NASA, the ESA, and the Japan Aerospace Exploration Agency (JAXA), in collaboration with astronomers and technologists around the world, are studying various options for cryogenic space observatories for the far-infrared. There are also opportunities to broaden the far-infrared astrophysics domain to new observing platforms. We give an overview of these space-based observing platforms in the following sections. We do not address the JWST, as comprehensive overviews of this facility are given elsewhere.157 We also do not review non-U.S./E.U. projects, such as Millimetron/Spektr-M.158,159 3.1.Space Infrared Telescope for Cosmology and AstrophysicsFirst proposed by JAXA scientists in 1998, the SPICA160–164 garnered worldwide interest due to its sensitivity in the mid- and far-infrared, enabled by the combination of the actively cooled telescope and the sensitive far-infrared detector arrays. Both ESA and JAXA have invested in a concurrent study, and an ESA–JAXA collaboration structure has gelled. ESA will provide the 2.5-m telescope, science instrument assembly, satellite integration and testing, and the spacecraft bus. JAXA will provide the passive and active cooling systems (supporting a telescope cooled to below 8 K), cryogenic payload integration, and launch vehicle. JAXA has indicated commitment to their portion of the collaboration, and the ESA selected SPICA as one of the three candidates for the Cosmic Visions M5 mission. The ESA phase-A study is underway now, and the downselect among the three missions will occur in 2021. Launch is envisioned for 2031. An example concept design of SPICA is shown in Fig. 11. SPICA will have three instruments. JAXA’s SPICA MIRI will offer imaging and spectroscopy from 12 to . It is designed to complement JWST-MIRI with wide-field mapping (broadband and spectroscopic), spectroscopy with an immersion grating, and an extension to with antimony-doped silicon detector arrays. A polarimeter from a French-led consortium will provide dual-polarization imaging in 2 to 3 bands using high-impedance semiconductor bolometers similar to those developed for Herschel-PACS, but modified for the lower background and to provide differential polarization. A sensitive far-infrared spectrometer, SAFARI, is being provided by an SRON-led consortium.165,166 It will provide full-band instantaneous coverage over 35 to , with a longer wavelength extension under study, using four grating modules. A Fourier transform module, which can be engaged in front of the grating modules, will offer a boost to the resolving power, up to . A U.S. team is working in collaboration with the European team and aims to contribute detector arrays and spectrometer modules to SAFARI167 through NASA’s Mission of Opportunity. 3.2.Probe-Class MissionsRecognizing the need for astronomical observatories beyond the scope of Explorer-class missions but with a cadence more rapid than flagship observatories, such as the Hubble Space Telescope (HST), JWST, and the Wide-Field Infrared Survey Telescope, NASA announced a call for Astrophysics Probe concept studies in 2017. Ten probe concepts were selected in Spring 2017 for 18-month studies. Probe study reports will be submitted to NASA and to the Astro 2020 Decadal Survey to advocate for the creation of a probe observatory line, with budgets of $400 million to $1 billion. Among the probe concepts under development is the far-infrared Galaxy Evolution Probe (GEP), led by the University of Colorado Boulder and the Jet Propulsion Laboratory. The GEP concept is a two-meter-class, mid/far-infrared observatory with both wide-area imaging and follow-up spectroscopy capabilities. The primary aim of the GEP is to understand the roles of star formation and black hole accretion in regulating the growth of stellar and black hole mass. In the first year of the GEP mission, it will detect galaxies, including galaxies at , beyond the peak in redshift of cosmic star formation, by surveying several hundred square degrees of the sky. A unique and defining aspect of the GEP is that it will detect galaxies by bands of rest-frame midinfrared emission from polycyclic aromatic hydrocarbons (PAHs), which are indicators of star formation, while also using the PAH emission bands and silicate absorption bands to measure photometric redshifts. The GEP will achieve these goals with an imager using photometric bands spanning to at least , giving a spectral resolution of (Fig. 12). Traditionally, an imager operating at these wavelengths on a 2-m telescope would be significantly confusion-limited, especially at the longer wavelengths (see e.g., the discussion in the introduction to Sec. 3). However, the combination of many infrared photometric bands, and advanced multi-wavelength source extraction techniques, will allow the GEP to push significantly below typical confusion limits. The GEP team is currently simulating the effects of confusion on their surveys, with results expected in early 2019. The imaging surveys from the GEP will, thus, enable new insights into the roles of redshift, environment, luminosity, and stellar mass in driving obscured star formation and black hole accretion over most of the cosmic history of galaxy assembly. In the second year of the GEP survey, a grating spectrometer will observe a sample of galaxies from the first-year survey to identify embedded active galactic nucleus (AGN). The current concept for the spectrometer includes four or five diffraction gratings with , and spectral coverage from to at least . The spectral coverage is chosen to enable detection of the high-excitation [NeV] line, which is an AGN indicator, over , and the [OI] line, which is predominantly a star formation indicator, over . Recent advances in the far-infrared detector array technology have made an observatory such as the GEP feasible. It is now possible to fabricate large arrays of sensitive kinetic inductance detectors (KIDs; see Sec. 5.1.2) that have a high-frequency multiplex factor. The GEP concept will likely employ Silicon blocked impurity band arrays (similar to those used on JWST-MIRI) for wavelengths from 10 to and KIDs at wavelengths longer than . Coupled with a cold () telescope such that the GEP’s sensitivity would be photon-limited by astrophysical backgrounds (Fig. 7), the GEP will detect the progenitors of Milky Way-type galaxies at . Far-infrared KID sensitivities have reached the NEPs required for the GEP imaging to be background-limited (169,170), although they would need to be lowered further, by a factor of at least 3, for the spectrometer to be background-limited. The GEP would serve as a pathfinder for the OST (Sec. 3.3), which would have a greater reach in redshift by virtue of its larger telescope. SOFIA and balloons will also serve as technology demonstrators for the GEP and OST. The technology drivers for the GEP center on detector array size and readout technology. Whereas KID arrays with are within reach, investment must be made for the development of low-power-consumption readout technology (Sec. 5.1.4). Large KID (or other direct-detection technology) arrays with low-power readouts on SOFIA and balloons would raise their respective TRLs, enabling the GEP and OST. 3.3.Origins Space TelescopeAs part of the preparations for the 2020 Decadal Survey, NASA is supporting four studies of flagship astrophysics missions. One of these studies is for a far-infrared observatory. A Science and Technology Definition Team (STDT) is pursuing this study with support from NASA Goddard Space Flight Center (GSFC). The STDT has settled on a single-dish telescope, and coined the name “Origins Space Telescope.” The OST will trace the history of our origins, starting with the earliest epochs of dust and heavy-element production through to the search for extrasolar biomarkers in the local universe. It will answer textbook-altering questions, such as “How did the universe evolve in response to its changing ingredients?” and “How common are planets that support life?” Two concepts for the OST are being investigated, based on an Earth-Sun L2 orbit, and a telescope and instrument module actively cooled with 4 K-class cryocoolers. Concept 1 (Fig. 13) has an open architecture, similar to that of JWST. It has a deployable segmented 9-m telescope with five instruments covering the mid-infrared through the submillimeter. Concept 2 is smaller and simpler and resembles the Spitzer Space Telescope architecturally. It has a 5.9-m diameter telescope (with the same light collecting area as JWST) with no deployable components. Concept 2 has four instruments, which span the same wavelength range and have comparable spectroscopic and imaging capabilities as the instruments in concept 1. Because OST would commence in the middle of the next decade, improvements in far-infrared detector arrays are anticipated, both in per-pixel sensitivity and array format, relative to what is currently mature for spaceflight (Sec. 5.1). Laboratory demonstrations, combined with initial OST instrument studies which consider the system-level readout requirements, suggest that total pixel counts of 100,000 to 200,000 will be possible, with each pixel operating at the photon background limit. This is a huge step forward over the 3200 pixels total on Herschel PACS and SPIRE, and the anticipated for SPICA. The OST is studying the impact of confusion on both wide- and deep-survey concepts. Their approach is as follows. First, a model of the far-infrared sky is used to generate a three-dimensional (3-D) hyperspectral data cube. Each slice of the cube is then convolved with the telescope beam, and the resulting cube is used to conduct a search for galaxies with no information given on the input catalogs. Confusion noise is then estimated by comparing the input galaxy catalog to the recovered galaxy catalog. The results from this work are not yet available, but this approach is a significant step forward in robustness compared to prior methods.144 3.4.CubeSatsCubeSats are small satellites built in multiples of 1U (, ). Because they are launched within containers, they are safe secondary payloads, reducing the cost of launch for the payload developer. In addition, a large ecosystem of CubeSat vendors and suppliers is available, which further reduces costs. CubeSats, thus, provide quick, affordable access to space, making them attractive technology pathfinders and risk mitigation missions toward larger observatories. Moreover, according to a 2016 National Academies report,171 CubeSats have demonstrated their ability to perform high-value science, especially via missions to make a specific measurement, and/or that complement a larger project. To date, well over 700 CubeSats have been launched, most of them 3Us. Within general astrophysics, CubeSats can produce competitive science, although the specific area needs to be chosen carefully.172,173 For example, long-duration pointed monitoring is a unique niche. So far, the Astrophysics division within NASA’s Science Mission Directorate has funded four CubeSat missions: in -rays (BurstCube174), in x-rays (HaloSat175), and in the ultraviolet (SPARCS;176 CUTE177). For the far-infrared, the CubeSat technology requirements are daunting. Most far-infrared detectors require cooling to reduce the thermal background to acceptable levels, to 4 K or even 0.1 K, although CubeSats equipped with Schottky-based instruments that do not require active cooling may be sufficiently sensitive for certain astronomical and Solar System applications (see also e.g., Ref. 178). CubeSat platforms are, thus, constrained by the lack of low-power, high-efficiency cryocoolers. Some applications are possible at 40 K, and small Stirling coolers can provide 1 W of heat lift at this temperature (see also Sec. 5.5). However, this would require the majority of the volume and power budget of even a large CubeSat (which typically have total power budgets of a few tens of watts), leaving little for further cooling stages, electronics, detector systems, and telescope optics. CubeSats are also limited by the large beam size associated with small optics. A diffraction-limited 10-cm aperture operating at would have a beam size of about 3.5′. There are concepts for larger, deployable apertures,179 up to , but none has been launched. For these reasons, it is not currently feasible to perform competitive far-infrared science with CubeSats. However, CubeSats can serve to train the next generation of space astronomers, as platforms for technology demonstrations that may be useful to far-infrared astronomy, and as complements to larger observing systems. For example, the CubeSat Infrared Atmospheric Sounder (CIRAS) is an Earth Observation 6U mission with a 4.78 to imaging spectrograph payload. The detector array will be cooled to 120 K, using a Lockheed Martin Coaxial MPT Cryocooler, which provides a 1-W heat lift (Fig. 14). At longer wavelengths, the Aerospace Corporation’s CUMULOS181 has demonstrated 8 to Earth imaging with an uncooled bolometer from a CubeSat. CubeSats can also serve as support facilities. In the submillimeter range, CalSat uses a CubeSat as a calibration source for CMB polarization observatories.182 3.5.International Space StationThe International Space Station (ISS) is a stable platform for both science and technology development. Access to the ISS is currently provided to the U.S. astronomical community through Mission of Opportunity calls, which occur approximately every two years and have cost caps. Several payload sites are available for hosting U.S. instruments, with typically of volume, at least 0.5 and up to 6 kW of power, wired and wireless Ethernet connectivity, and at least 20 kbps serial data bus downlink capability.183 In principle, the ISS is an attractive platform for astrophysics, as it offers a long-term platform at a mean altitude of 400 km, with the possibility for regular instrument servicing. Infrared observatories have been proposed for space station deployment at least as far back as 1990.184 There are, however, formidable challenges in using the ISS for infrared astronomy. The ISS environment is, for infrared science, significantly unstable, with 16 sunrises in every 24-h period, “glints” from equipment near the FoV, and vibrations and electromagnetic fields from equipment in the ISS. Furthermore, the external instrument platforms are not actively controlled and are subject to various thermal instabilities over an orbit, which would require active astrometric monitoring. Even with these challenges, there are two paths forward for productive infrared astronomy from the ISS:
Efforts, thus, exist to enable infrared observing from the ISS. For example, the Terahertz Atmospheric/Astrophysics Radiation Detection in Space is a proposed infrared experiment that will observe both in the upper atmosphere of Earth and in the ISM of the Milky Way. 4.New Instruments and MethodsContinuing advances in telescope and detector technology will enable future-generation observatories to have much greater capabilities than their predecessors. Technological advancement also raises the possibility of new observing techniques in the far-infrared, with the potential for transformational science. We discuss two such techniques in this section: interferometry and time-domain astronomy. 4.1.InterferometryMost studies of future far-infrared observatories focus on single-aperture telescopes. There is, however, enormous potential for interferometry in the far-infrared (Fig. 15). Far-infrared interferometry is now routine from the ground (as demonstrated by ALMA, NOEMA, and the SMA) but has been barely explored from space- and balloon-based platforms. However, the combination of access to the infrared without atmospheric absorption and angular resolutions that far exceed those of any single-aperture facility and enables entirely new areas of investigation.185–187 In our Solar System, far-infrared interferometry can directly measure the emission from icy bodies in the Kuiper belt and Oort cloud. Around other stars, far-infrared interferometry can probe planetary disks to map the spatial distribution of water, water ice, gas, and dust, and search for structure caused by planets. At the other end of the scale, far-infrared interferometry can measure the rest-frame near/midinfrared emission from high-redshift galaxies without the information-compromising effects of spatial confusion. This was recognized within NASA’s 2010 long-term roadmap for Astrophysics, Enduring Quests/Daring Visions,188 which stated that, within the next few decades, scientific goals will begin to outstrip the capabilities of single-aperture telescopes. For example, imaging of exo-Earths, determining the distribution of molecular gas in protoplanetary disks, and directly observing the event horizon of a black hole, all require single-aperture telescopes with diameters of hundreds of meters, over an order of magnitude larger than is currently possible. Conversely, interferometry can provide the angular resolution needed for these goals with much less difficulty. Far-infrared interferometry is also an invaluable technology development platform. Because certain technologies for interferometry, such as ranging accuracy, are more straightforward for longer wavelengths, far-infrared interferometry can help enable interferometers operating in other parts of the electromagnetic spectrum (interferometer technology has, however, been developed for projects outside the infrared; examples include the Keck Interferometer, CHARA, LISA Pathfinder, the Terrestrial Planet Finder, and several decades of work on radio interferometry). This was also recognized within Enduring Quests/Daring Visions: “the technical requirements for interferometry in the far-infrared are not as demanding as for shorter wavelength bands, so far-infrared interferometry may again be a logical starting point that provides a useful training ground while delivering crucial science.” Far-infrared interferometry, thus, has broad appeal, beyond the far-infrared community, as it holds the potential to catalyze development of space-based interferometry across multiple wavelength ranges. The 2000 Decadal Survey189 recommended development of a far-infrared interferometer (FIRI), and the endorsed concept [the submillimeter probe of the evolution of cosmic structure (SPECS)] was subsequently studied as a “vision mission.”190 Recognizing that SPECS was extremely ambitious, a smaller, structurally connected interferometer was studied as a potential origins probe—the Space Infrared Interferometric Telescope (SPIRIT,191 Fig. 16). At around the same time, several interferometric missions were studied in Europe, including FIRI192 and the heterodyne interferometer Exploratory Submm Space Radio-Interferometric Telescope.193 Another proposed mission, TALC,194,195 is a hybrid between a single-aperture telescope and an interferometer and, thus, demonstrates technologies for a structurally connected interferometer. There are also concepts using nanosats.196 Recently, the European community carried out the Far-Infrared Space Interferometer Critical Assessment (FP7-FISICA), resulting in a design concept for the Far-Infrared Interferometric Telescope. Finally, the “double Fourier” technique that would enable simultaneous high spatial and spectral observations over a wide FoV is maturing through laboratory experimentation, simulation, and algorithm development.197–201 Two balloon payloads have been developed to provide scientific and technical demonstration of interferometry. They are the FITE99 and the BETTII,100 first launched in June 2017. The first BETTII launch resulted in a successful engineering flight, demonstrating nearly all of the key systems needed for future science flights. Unfortunately, an anomaly at the end of the flight resulted in complete loss of the payload. A rebuilt BETTII should fly before 2020. Together, BETTII and FITE will serve as an important development step toward future space-based interferometers, while also providing unique scientific return. Their successors, taking advantage of many of the same technologies as other balloon experiments (e.g., new cryocoolers and lightweight optics), will provide expanded scientific capability while continuing the path toward space-based interferometers. FIRIs have many of the same technical requirements as their single-aperture cousins. In fact, an interferometer could be used in “single aperture” mode, with instruments similar to those on a single-aperture telescope. However, in interferometric mode, the development requirements for space-based far-infrared interferometry are as follows:
Finally, we comment on the temporal performance requirements. The temporal performance requirements of different parts of an interferometer depend on several factors, including the FoV, sky and telescope backgrounds, rate of baseline change, and desired spectral resolution. We do not discuss these issues in detail here, as they are beyond the scope of a review paper. We do, however, give an illustrative example; a 1′ FoV, with a baseline of 10 m, spectral resolution of , and 16 points per fringe results in a readout speed requirement of 35 Hz. However, increasing the spectral resolution to (at the same scan speed) raises the readout speed requirement to 270 Hz. These correspond to detector time constants of 17 and 3 ms. A baseline requirement for a relatively modest interferometer (e.g., SHARP-IR202) is, thus, a detector time constant of a few milliseconds. The exact value is, however, tied tightly to the overall mission architecture and operation scheme. 4.2.Time-Domain and Rapid-Response AstronomyTime-domain astronomy is an established field at x-ray through optical wavelengths, with notable observations including Swift’s studies of transient high-energy events and the Kepler mission using optical photometry to detect exoplanets. Time-domain astronomy in the far-infrared holds the potential for similarly important studies of phenomena on timescales of days to years, namely, (1) searching for infrared signatures of (dust-obscured) -ray bursts, (2) monitoring the temporal evolution of waves in debris disks to study the earliest stages of planet formation, and (3) monitoring supernovae light curves to study the first formation stages of interstellar dust. To date, however, such capabilities in the far-infrared have been limited. For example, Spitzer was used to measure secondary transits of exoplanets,203 but only when the ephemeris of the target was known. The limitations of far-infrared telescopes for time-domain astronomy are twofold. First, to achieve high photometric precision in the time domain, comparable to that provided by Kepler, the spacecraft must be extremely stable, to requirements beyond those typically needed for cameras and spectrographs. This is not a fundamental technological challenge, but the stability requirements must be taken into consideration from the earliest design phase of the observatory. Second, if the intent is to discover transient events in the far-infrared (rather than monitor known ones), then the FoV of the telescope must be wide, as most transient events cannot be predicted and, thus, must be found via observations of a large number of targets. 5.Technology PrioritiesThe anticipated improvements in existing far-infrared observatories, as well as the realization of next-generation space-based far-infrared telescopes, all require sustained, active development in key technology areas. We, here, review the following areas: direct-detector arrays (Sec. 5.1), medium-resolution spectroscopy (Sec. 5.2), heterodyne spectroscopy (Sec. 5.3), Fabry–Pérot interferometry (Sec. 5.4), cooling systems (Sec. 5.5), and mirrors (Sec. 5.6). We briefly discuss a selection of other topics in Sec. 5.7. 5.1.Direct-Detector ArraysA key technical challenge for essentially any future far-infrared space observatory (whether single aperture or interferometer) is the development of combined direct-detector + multiplexer readout systems. These systems are not typically developed by the same industrial teams that build near-infrared device and midinfrared device. Instead, they are usually developed by dedicated groups at universities or national laboratories. These systems have two core drivers:
There are also the challenges of interference and dynamic range (Sec. 3). The world leaders in far-infrared detector technology include SRON in the Netherlands, Cambridge and Cardiff in the U.K., and NASA in the USA, with at least three approaches under development. In order of technical readiness they are as follows:
All are potentially viable for future far-infrared missions. We consider each one in turn, along with a short discussion of multiplexing. 5.1.1.Transition edge sensorsA TES (Fig. 17) consists of a superconducting film operated near its superconducting transition temperature. This means that the functional form of the temperature dependence of resistance, , is very sharp. The sharpness of the function allows for substantially better sensitivity than semiconducting thermistors (though there are other factors to consider, such as readout schemes; see Sec. 5.1.4). Arrays of TES bolometers have been used in CMB experiments206,207–210 and in calorimeters in the -ray,211 x-ray,212,213 ultraviolet, and optical. They are also anticipated for future x-ray missions, such as Athena.214,215 In the infrared, TES bolometers are widely used. A notable ground-based example is SCUBA2 on the JCMT216 (Table 1). Other sub-orbital examples include HAWC+ and the upcoming HIRMES instrument, both on SOFIA. TES bolometers are also planned for use in the SAFARI instrument for SPICA.217–220 In terms of sensitivity, groups at SRON and Jet Propulsion Laboratory (JPL) have demonstrated TES sensitivities of .219,221,222 The advantages of TES arrays over KIDs and QCD arrays are technological maturity and versatility in readout schemes (see Sec. 5.1.4). However, TES detector arrays do face challenges. The signal in TES bolometers is a current through a (sub-) resistive film at sub-kelvin temperatures, so conventional amplifiers are not readily impedance matched to conventional low-noise amplifiers (LNAs) with high-input impedance. Instead, superconducting quantum interference devices (SQUIDs) are used as first-stage amplifiers and SQUID-based circuits have been fashioned into a switching time-domain multiplexers (the TDMs, from NIST and UBC223), which has led to array formats of up to . Although this time-domain multiplexing system is mature and field-tested in demanding scientific settings, it is not an approach that can readily scale above , due primarily to wire count considerations. Other issues with TES arrays include (1) challenging array fabrication, (2) relatively complex SQUID-based readout systems, and (3) no on-chip multiplexing (yet). 5.1.2.Kinetic inductance detectorsThe simplest approach to high-multiplex-factor frequency-domain multiplexing (FDM; see also Sec. 5.1.4) thus far is the KID224,225 (Fig. 18). In a KID, photons incident on a superconducting film break Cooper pairs, which results in an increase in the inductance of the material. When embedded in a resonant circuit, the inductance shift creates a measureable frequency shift, which is encoded as a phase shift of the probe tone. KIDs originated as far-infrared detector arrays, with on-telescope examples, including MAKO226 and MUSIC227 at the CSO, A-MKID228 at APEX, NIKA/NIKA2229–231 at IRAM, the extremely compact -Spec,232,233 SuperSpec,234 and the submillimeter wave imaging spectrograph DESHIMA.235 KIDs were later adapted for the optical/near-infrared,236 where they provide advances in time resolution and energy sensitivity. Examples include ARCONS,237 DARKNESS and MEC,238,239 the KRAKENS IFU,240 and PICTURE-C.241 KIDs are also usable for millimeter-wave/CMB studies,242–246 although there are challenges in finding materials with suitably low ’s when operating below 100 GHz. KIDs are now being built in large arrays for several ground-based and sub-orbital infrared observatories, including the BLAST-Pol2 balloon experiment. There exist three primary challenges in using KIDs in space-based infrared observatories: Sensitivity: Sub-orbital far-infrared observatories have relatively high backgrounds and thus have sensitivities that are 2 to 3 orders of magnitude above those needed for background-limited observations from space. For space-based KID instruments, better sensitivities are needed. The state of the art is from SPACEKIDs, for which NEPs of have been demonstrated in aluminum devices coupled via an antenna.169,247,248 This program has also demonstrated 83% yield in a 961-pixel array cooled to 120 mK. A further important outcome of the SPACEKIDs program was the demonstration that the effects of cosmic ray impacts can be effectively minimized.169,249 In the U.S., the Caltech/JPL group and the SuperSpec collaboration have demonstrated sensitivities below in a small-volume titanium nitride devices at 100 mK, also with radiation coupled via an antenna. Structural considerations: KIDs must have both small active volume (to increase response to optical power) and a method of absorbing photons directly without using superconducting transmission lines. Options under development include:
Antenna coupling at high frequencies: Although straightforward for the submillimeter band, the antenna coupling becomes nontrivial for frequencies above the superconducting cutoff of the antenna material (e.g., for Nb and 1.2 THz for NbTiN). To mitigate this, one possible strategy is to integrate the antenna directly into the KID, using only aluminum for the parts of the detector that interact with the THz signal. This approach has been demonstrated at 1.55 THz, using a thick aluminum ground plane and a thin aluminum central line to limit ground plane losses to 10%.169,170 This approach does not rely on superconducting stripline technology and could be extended to frequencies up to . A final area of research for KIDs, primarily for CMB experiments, is the KID-sensed bolometer, in which the thermal response of the KID is used to sense the temperature of a bolometer island. These devices will be limited by the fundamental phonon transport sensitivity of the bolometer and so are likely to have sensitivity limits comparable to TES bolometers, but may offer advantages, including simplified readout, on-array multiplexing, lower sensitivity to magnetic fields, and larger dynamic range. 5.1.3.Quantum capacitance detectorsThe QCD250–254 is based on the single Cooper-pair box (SCB), a superconducting device initially developed as a qubit for quantum computing applications. The SCB consists of a small island of superconducting material connected to a ground electrode via a small () tunnel junction. The island is biased with respect to ground through a gate capacitor, and because it is sufficiently small to exhibit quantum behavior, its capacitance becomes a strong function of the presence or absence of a single free electron. By embedding this system capacitively in a resonator (similar to that used for a KID), a single electron entering or exiting the island (via tunneling through the junction) produces a detectable frequency shift. To make use of this single-electron sensitivity, the QCD is formed by replacing the ground electrode with a superconducting photon absorber. As with the KIDs, photons with energy larger than the superconducting gap breaks Cooper pairs, thereby establishing a density of free electrons in the absorber that then tunnel onto (and rapidly back out of) the island through the tunnel junction. The rate of tunneling into the island, and thus the average electron occupation in the island, is determined by the free-electron density in the absorber, set by the photon flux. Because each photo-produced electron tunnels back and forth many times before it recombines, and because these tunneling events can be detected individually, the system has the potential to be limited by the photon statistics with no additional noise. This has indeed been demonstrated. QCDs have been developed to the point where a 25-pixel array yields a few devices which are photon-noise-limited for radiation under a load of , corresponding to a NEP of . The system seems to have good efficiency as well, with inferred detection of 86% of the expected photon flux for the test setup. As an additional demonstration, a fast-readout mode has been developed which can identify individual photon arrival events based on the subsequent increase in tunneling activity for a timescale on order of the electron recombination time (Fig. 19). With its demonstrated sensitivity and natural FDM, the QCD is promising for future far-infrared space systems. Optical NEPs of below at have been demonstrated, with the potential for photon counting at far-infrared wavelengths.255 However, QCDs are some way behind both TES and KID arrays in terms of technological maturity. To be viable for infrared instruments, challenges in (1) yield and array-level uniformity, (2) dark currents, and (3) dynamic range must all be overcome. The small tunnel junctions are challenging, but it is hoped that advances in lithography and processing will result in improvements. 5.1.4.System considerations for direct-detector readoutsThere exist three commonly used multiplexing (muxing) schemes256 for readout of arrays: FDM, TDM, and Code Division Muxing (CDM). In this section, we briefly review their applicability and advantages. FDM is a promising path for reading out the large arrays anticipated in future infrared observatories. In FDM, a single readout circuit services up to , each coupled through a microresonator tuned to a distinct frequency. Each pixel is then probed individually with a radio frequency (RF) or microwave tone at its particular frequency. The warm electronics must create the suite of tones, which is transmitted to the array for each circuit, then digitize, Fourier transform, and channel the output data stream to measure the phase and amplitude shifts of each tone independently. The number of resonators (and thus pixels) that can be arrayed onto a single readout circuit depends on the quality factor (Q) of the resonators and the bandwidth available in the circuit. For micro-resonators patterned in superconducting films, resonator Qs exceeding are possible but more typical values are around , which permits per octave of readout bandwidth to be operated with sufficiently low cross talk. In these systems, all of the challenging electronics are on the warm side, and the detector array is accessed via low-loss RF/microwave lines (one from the warm side down through the cryostat stages and another for the return signal). Moreover, FDM readout schemes can be applied to both TES and KID arrays, whereas other multiplexing schemes are TES-only. An example of recent progress is the development of a FDM scheme that can read out 132 TES pixels simultaneously, using a single SQUID, without loss of sensitivity.220 This is very close to the 160 detectors per SQUID targeted for SPICA/SAFARI. There are, however, the following limitations to FDM schemes:
A further challenge, which applies to readout schemes for any far-infrared resonant detector array (including TES, KID, and QCD systems), is the power required to read out detector arrays, due in part to the signal-processing requirements. The power requirements are such that they may pose a significant obstacle to reading out detector arrays on any balloon- or space-based platform. For the OST, power dissipation in the warm electronics will be a particular challenge. An example is the medium-resolution survey spectrometer (MRSS), which targets 200,000 pixels among all six spectrometer bands. The concept assumes resonator frequencies between 75 MHz and 1 GHz, and 1500 pixels can be arrayed in this bandwidth (a relatively comfortable multiplexing density assuming 400 per readout octave). This requires 130 readout circuits, each with two coaxial lines all the way to the cold stage, and a cold amplifier on the output. The conducted loads through the coaxial lines, as well as reasonable assumptions about the LNA dissipation (1 mW at 4 K plus 3 mW at 20 K for each circuit), do not stress the observatory thermal design. However, the electronics for each circuit requires a 2 giga-sample-per-second analog-to-digital converter working at -bits depth, followed by FFTs of this digital signal stream in real time—1024 point FFTs every . Systems such as these implemented in field programmable gate array used in the laboratory dissipate for each readout circuit, which is not compatible with having 130 such systems on a space mission. For these reasons, development of muxing schemes is a high priority for large-format arrays, irrespective of the detector technology used. A promising path for such development is to employ a dedicated application-specific integrated circuit (ASIC), designed to combine the digitization, FFT, and tone extraction in a single chip. Power dissipation estimates obtained for the MRSS study based on custom spectrometer chips developed for flight systems, and extrapolating to small-gate CMOS technology, suggest that such a custom chip could have a power dissipation of per circuit, including all aspects. At this level, the total scales to . This power dissipation is well within the range of that of other subsystems on future missions—for example, such missions will require several kilowatts to operate the cryocoolers—and thus does not pose a unique problem. Finally, we make four observations:
5.2.Medium-Resolution SpectroscopyA variety of spectrometer architectures can be used to disperse light at far-infrared wavelengths. Architectures that have been successfully used on air-borne and space instruments include grating dispersion such as FIFI-LS on SOFIA260 and PACS on Herschel,133 Fourier transform spectrometers such as the Herschel/SPIRE-FTS,134 and Fabry–Pérot etalons such as FIFI on the KAO telescope.261 These technologies are well understood and can achieve spectral resolutions of . However, future spectrometers will need to couple large FoVs to many thousands of imaging detectors, a task for which all three of these technologies have drawbacks. Grating spectrometers are mechanically simple devices that can achieve but are challenging to couple to wide FoVs as the spectrum is dispersed along one spatial direction on the detector array. FTS systems require moving parts and suffer from noise penalties associated with the need for spectral scanning. They are also not well suited for studying faint objects because of systematics associated with long-term stability of the interferometer and detectors.262 Fabry–Pérot systems are also mechanically demanding, requiring tight parallelism tolerances of mirror surfaces, and typically have restricted free spectral range due to the difficulty of manufacturing sufficiently precise actuation mechanisms.263 A new technology that can couple the large FoVs anticipated in next-generation far-infrared telescopes to kilo-pixel or larger detector arrays would be transformative for far-infrared spectroscopy. A promising approach to this problem is the far-infrared filter bank technology.264,265 This technology has been developed as a compact solution to the spectral dispersion problem and has potential for use in space. These devices require the radiation to be dispersed to propagate down a transmission line or waveguide. The radiation encounters a series of tuned resonant filters, each of which consists of a section of transmission line of length , where is the resonant wavelength of channel . These half-wave resonators are evanescently coupled to the feedline with designable coupling strengths described by the quality factors and for the feedline and detector, respectively. The filter bank is formed by arranging a series of channels monotonically increasing in frequency, with a spacing between channels equal to an odd multiple of . The ultimate spectral resolution is given as where accounts for any additional sources of dissipation in the circuit and is the net quality factor. This arrangement has several advantages in low- and medium-resolution spectroscopy from space, including (1) compactness (fitting on a single chip with area of tens of square centimeters), (2) integrated on-chip dispersion and detection, (3) high end-to-end efficiency equal to or exceeding existing technologies, and (4) a mechanically stable architecture. A further advantage of this architecture is the low intrinsic background in each spectrometer, which only couples to wavelengths near its resonance. This means that very low backgrounds can be achieved, requiring detector NEPs below . Filter banks do, however, have drawbacks.264 For example, although filter banks are used in instruments operating from millimeter to radio wavelengths, they are currently difficult to manufacture for use at wavelengths shortward of about .Two ground-based instruments are being developed that make use of filter banks. A prototype transmission-line system has been fabricated for use in SuperSpec266,267 for the LMT. SuperSpec will have near 250 GHz and will allow photon-background-limited performance. A similar system is WSPEC, a 90-GHz filter bank spectrometer that uses machined waveguide to propagate the radiation.268 This prototype instrument has five channels covering the 130- to 250-GHz band. Though neither instrument is optimized for space applications, this technology can be adapted to space, and efforts are underway to deploy it on suborbital rockets. 5.3.High-Resolution SpectroscopySeveral areas of investigation in mid/far-infrared astronomy call for spectral resolution of , higher than can be achieved with direct-detection approaches. At this very high spectral resolution, heterodyne spectroscopy is routinely used,269,270 with achievable spectral resolution of up to . In heterodyne spectroscopy, the signal from the “sky” source is mixed with a spectrally pure, large-amplitude, locally generated signal, called the “local oscillator (LO),” in a nonlinear device. The nonlinearity generates the sum and difference of the sky and LO frequencies. The latter, the “intermediate frequency (IF),” is typically in the 1- to 10-GHz range and can be amplified by LNAs and subsequently sent to a spectrometer, which now is generally implemented as a digital signal processor. A heterodyne receiver is a coherent system, preserving the phase and amplitude of the input signal. Although the phase information is not used for spectroscopy, it is available and can be used in, e.g., interferometry. The general requirements for LOs are as follows: narrow linewidth, high stability, low noise, tunability over the required frequency range, and sufficient output power to couple effectively to the mixer. For far-infrared applications, LO technologies are usually one of the two following types: multiplier chain and quantum cascade laser (QCL). Multiplier chains offer relatively broad tuning, high spectral purity, and known output frequency. The main limitation is reaching higher frequencies (). QCLs are attractive at higher frequencies, as their operating frequency range extends to 5 THz and above, opening up the entire far-infrared range for high-resolution spectroscopy. For mixers, most astronomical applications use one or more of the following three technologies: Schottky diodes, SIS mixers, and hot electron bolometer (HEB) mixers.271 Schottky diodes function at temperatures of , can operate at frequencies as high as (), and provide large IF bandwidths of , but offer sensitivities that can be an order of magnitude or more poorer than either SIS or HEB mixers. They also require relatively high LO power, in the order of 1 mW. SIS and HEB mixers, in contrast, have operating temperatures of and require LO powers of only . SIS mixers are most commonly used at frequencies up to about 1 THz, whereas HEB mixers are used over the 1 to 6 THz range. At present, the SIS mixers offer IF bandwidths and sensitivities both a factor of 2 to 3 better than the HEB mixers. All three mixer types have been used on space-flown hardware: SIS and HEB mixers in the Herschel HIFI instrument,272,273 and Schottky diodes on instruments in SWAS and Odin. Heterodyne spectroscopy can currently achieve spectral resolutions of , and in principle the achievable spectral resolution is limited only by the purity of the signal from the LO. Moreover, heterodyne spectroscopy preserves the phase of the sky signal as well as its frequency, lending itself naturally to interferometric applications. Heterodyne arrays are used on SOFIA, as well as many ground-based platforms. They are also planned for use in several upcoming observatories, including GUSTO. A further example is FIRSPEX, a concept study for a small-aperture telescope with heterodyne instruments to perform several large-area surveys targeting bright far-infrared fine-structure lines, using a scanning strategy similar to that used by Planck.274 There are, however, challenges for the heterodyne approach. We highlight five here:
On a final note, for the higher frequency () arrays, high-power (5 to 10 mW) QCL LOs are a priority for development, along with power division schemes (e.g., Fourier phase gratings) to utilize QCLs effectively.277–279 At , frequency-multiplied sources remain the system of choice and have been successfully used in missions including SWAS, Herschel-HIFI, STO2, and in GREAT and upGREAT on SOFIA. However, to support large-format heterodyne arrays, and to allow operation with reduced total power consumption for space missions, further development of this technology is necessary. Further valuable developments include SIS and HEB mixers that can operate at temperatures of and integrated focal planes of mixers and low-noise IF amplifiers. 5.4.Fabry–Pérot InterferometryFabry–Pérot Interferometers (FPIs) have been used for astronomical spectroscopy for decades, with examples, such as FIFI,280 KWIC,281 ISO-SWS/LWS,282,283 and SPIFI.284 FPIs similar to the one used in ISO have also been developed for balloon-borne telescopes.285 FPIs consist of two parallel, highly reflective (typically with reflectivities of ), very flat mirror surfaces. These two mirrors create a resonator cavity. Any radiation whose wavelength is an integral multiple of twice the mirror separation meets the condition for constructive interference and passes the FPI with high transmission. As the radiation bounces many times between the mirrors before passing, FPIs can be fabricated very compactly, even for high spectral resolution, making them attractive for many applications. In addition, FPIs allow for large FoVs, making them an excellent choice as devices for spectroscopic survey instruments. Observations with FPI are most suitable for extended objects and surveys of large fields, where moderate-to-high spectral resolution () is required. For example:
FPIs do, however, face challenges. We highlight four examples here:
5.5.Small and Low-Power CoolersFor any spaceborne observatory operating at mid/far-infrared wavelengths, achieving high sensitivity requires that the telescope, instrument, and detectors be cooled, with the level of cooling dependent on the detector technology, the observation wavelength, and the goals of the observations. Cooling technology is thus fundamentally enabling for all aspects of mid/far-infrared astronomy. The cooling required for the telescope depends on the wavelengths being observed (Fig. 7). For some situations, cooling the telescope to 30 to 40 K is sufficient. At these temperatures, it is feasible to use radiative (passive) cooling solutions if the telescope is space-based and if the spacecraft orbit and attitude allow for a continuous view of deep space.287 Radiative coolers typically resemble a set of thermal/solar shields in front of a black radiator to deep space (Fig. 6). This is a mature technology, having been used on Spitzer, Planck, and JWST (for an earlier proposed example, see Ref. 288). For many applications, however, cooling the telescope to a few tens of kelvins is suboptimal. Instead, cooling to an order of 4 K is required, e.g., zodiacal background limited observations (see also Sec. 3). Moreover, detector arrays require cooling at least to this level. For example, SIS and HEB mixers need cooling up to 4 K, whereas TES, KID, and QCD arrays need cooling to 0.1 K or below. Achieving cooling at these temperatures requires a cooling chain—a staged series of cooling technologies selected to maximize the cooling per-mass and per-input power. To achieve temperatures below , or where a continuous view of deep space is not available, cryocoolers are necessary. In this context, the Advanced Cryocooler Technology Development Program (ACTDP289), initiated in 2001, has made excellent progress in developing cryogen-free multiyear cooling for low-noise detector arrays at temperatures of 6 K and below (Fig. 20). The state of the art for these coolers include those onboard Planck, JWST, and Hitomi.290 Similar coolers that could achieve 4 K are at TRL 4-5, having been demonstrated as a system in a laboratory environment291 or as a variant of a cooler that has a high TRL (JWST/MIRI). Mechanical cryocoolers for higher temperatures have already demonstrated impressive on-orbit reliability (Table 2). The moving components of a 4 K cooler are similar (expanders) or the same (compressors) as those that have flown. Further development of these coolers to maximize cooling per input power for small cooling loads ( at 4 K) and lower mass is however needed. There is also a need to minimize the vibration from the cooler system. The miniature reverse-Brayton cryocoolers under development by Creare are examples of reliable coolers with negligible exported vibration. These coolers are at TRL 6 for 80 K and TRL 4 for 10 K operation. Table 2Long-life space cryocooler operating experiences as of May 2016.
Note: Almost all cryocoolers have continued to operate normally until turned off at the end of the instrument life. Mid/far-infrared and CMB astrophysics observatories are highlighted in bold. The data in this table are courtesy of Ron Ross, Jr. For cooling to below 0.1 K, adiabatic demagnetization refrigerators (ADRs) are currently the only proven technology, although work has been funded by ESA to develop a continuously recirculating dilution refrigerator [continuous adiabatic demagnetization refrigerator (CADR)]. A single-shot DR was flown on Planck producing of cooling at 100 mK for about 1.5 years, whereas a three-stage ADR was used on Hitomi producing of cooling at 50 mK with an indefinite lifetime. In contrast, a TRL 4 CADR has demonstrated of cooling at 50 mK with no life-limiting parts292 (Fig. 21). This technology is being advanced toward TRL 6 by 2020 via funding from the NASA SAT/TPCOS program.293 Demonstration of a 10-K upper stage for this machine, as is planned, would enable coupling to a higher temperature cryocooler, such as that of Creare, that has near-zero vibration. The flight control electronics for this ADR are based on the flight-proven Hitomi ADR control and has already achieved TRL 6. ADR coolers are the current reference design for the Athena x-ray observatory. For the OST, all three of the above technologies are required to maintain the telescope at near 4 K and the detector arrays at near 50 mK. Continuous development of 0.1 and 4 k coolers with cooling powers of tens of milliwatt, high reliability, and lifetimes of 10+ years is of great importance for future far-infrared observatories. Moreover, the development of smaller, lighter, vibration-resistant, power-efficient cryocoolers enables expansion of infrared astronomy to new observing platforms. An extremely challenging goal would be the development of a 0.1 K cooler with power, space, and vibration envelopes that enable its use inside a 6U CubeSat, while leaving adequate resources for detector arrays, optics, and downlink systems (see also Sec. 3.4). More generally, the ubiquity of cooling in infrared astronomy means that the development of low-mass, low-power, and low-cost coolers will reduce mission costs and development time across all observational domains. 5.6.High Surface Accuracy Lightweight MirrorsAs far-infrared observing platforms mature and develop, there emerge new opportunities to use large aperture mirrors for which the only limitations are (1) mirror mass and (2) approaches to active control and correction of the mirror surface. This raises the possibility of a high-altitude, long-duration far-infrared-observing platform with a mirror factors of 2 to 5 larger than on facilities such as SOFIA or Herschel. The key enabling technology for such an observing platform is the manufacturing of lightweight, high surface accuracy mirrors, and their integration into observing platforms. This is especially relevant for ULDBs, which are well suited for this activity. Lightweight mirrors with apertures of 3 m to several tens of meters are ideal for observations from balloon-borne platforms. Carbon-fiber mirrors are an attractive option; they have low mass and can offer high sensitivity in the far-infrared, at low cost of manufacture. Apertures of 2.5 m are used on projects, such as BLAST-TNG.95 Apertures of up to are undergoing ground-based tests, including the phase 2 NIAC study for the large balloon reflector.294–296 A conceptually related topic is the physical size and mass of optical components. The physical scale of high-resolution spectrometers in the far-infrared is determined by the optical path difference required for the resolution. For resolutions of , this implies scales of several meters for a grating spectrometer. This scale can be reduced by folding, but mass remains a potentially limiting problem. Moreover, larger physical sizes are needed for optical components to accommodate future large format arrays, posing challenges for uniformity, thermal control, and antireflection coatings. The development of low-mass optical elements suitable for diffraction-limited operation at would open the range of technical solutions available for the highest performance instruments. 5.7.Other NeedsThere exist several further areas for which technology development would be beneficial. We briefly summarize them below: Lower-loss THz optics: Lenses, polarizers, filters, and duplexers. Digital backends: Low-power (of order a few watts or less) digital backends with channels covering up to several tens of gigahertz of bandwidth. Wide-field imaging Fourier transform spectrometers: Expanding on the capabilities of, e.g., SPIRE on Herschel, balloon- or space-based imaging Fourier transform spectrometer with FoVs of tens of square arcminutes.297 Examples include the concept H2EX.298 Deployable optics: Development of deployable optic schemes across a range of aperture sizes would be enabling for a range of platforms. Examples range from 20- to 50-cm systems for CubeSats to 5- to 10-m systems for JWST. Data downlinking and archiving: The advent of infrared observatories with large-format detector arrays presents challenges in downlinking and archiving. Infrared observatories have, to date, not unduly stressed downlinking systems, but this could change in the future with multiple instruments each with to on a single observatory. Moreover, the increasing number and diversity of PI and facility-class infrared observatories poses challenges to data archiving, in particular for enabling investigators to efficiently use data from multiple observatories in a single study. One way to mitigate this challenge is by increasing the use of onboard data processing and compression, as is already done for missions operating at shorter wavelengths. Commonality and community in instrument software: Many tasks are similar across a single platform, and even between platforms (e.g., pointing algorithms, focus, and data download). Continuous adherence to software development best practices, code sharing via repositories via GitHub, and fully open-sourcing software, will continue to drive down associated operating costs, speed up development, and facilitate ease of access. 6.Conclusions: The Instrument Development Landscape for Infrared AstronomyThe picture that coalesces from this review is that far-infrared astronomy is still an emerging field, even after over 40 years of development. Optical and near-infrared astronomy has a mature and well-understood landscape in terms of technology development for different platforms. In contrast, far-infrared astronomy has more of the “wild west” about it; there are several observing platforms that range widely in maturity, all with overlapping but complementary domains of excellence. Moreover, considering the state of technology, all areas have development paths where huge leaps forward in infrared observing capability can be obtained. In some cases, entirely new platforms can be made possible. To conclude this review, we bring together and synthesize this information in order to lay out how the capabilities of each platform can be advanced. To do so, we use the following definitions:
These definitions correspond closely to the definitions of enabling (a pull technology) and enhancing (a push technology) as used in the 2015 NASA Technology Roadmap. As different technology fields vary in relevance for different platforms, technologies can be enabling for some platforms and enhancing for others. In Fig. 22, we assess the status of selected technology areas as enabling or enhancing, as a function of observing platform. This table is solely the view of the authors and not obtained via a community consultation. With this caveat in mind, based on Fig. 22, we present a non-exhaustive list of important technology development areas for far-infrared astronomy: Large-format detectors: Existing and near-future infrared observatories include facilities with large FoVs, or those designed to perform extremely high-resolution spectroscopy. These facilities motivate the development of large-format arrays that can fill telescope FoVs, allowing for efficient mapping and high spatial resolutions. A reference goal is to increase the number of pixels in arrays to for direct detectors and for heterodyne detectors. This is a small number compared to arrays for optical and near-infrared astronomy, for which millions of pixels can be fielded in a single chip, but is still 1 to 2 orders of magnitude larger than any array currently used in the far-infrared. Detector readout electronics: Increases in detector array sizes are inevitably accompanied by increases in complexity and power required for the readout electronics and power dissipation of the cold amplifiers for these arrays. At present, the power requirements for detector array readout systems are a key limitation for their use in any space-based or suborbital platform, restricting them to use in ground-based facilities. For these reasons, development of multiplexing schemes is a high priority for large-format arrays, irrespective of the technology used. The main driver for power dissipation is the bandwidth of the multiplexers. Low-power cryogenic amplifiers, in particular parametric amplifiers, can mitigate this problem at 4 K. ASICs, which combine digitization, FFT, and tone extraction in a single chip, can greatly reduce the power required for the warm readout system. A reference goal for the use of arrays on space-based observatories such as the OST is a total power dissipation in the readout system of below 2 kW. This requires a denser spacing of individual channels in frequency-domain multiplexers. For balloon-based facilities, sub-kilowatt power dissipation is desirable. Direct-detector sensitivity and dynamic range: The performance of 4 K-cooled space-based and high-altitude suborbital telescopes will be limited by astrophysical backgrounds such as zodiacal light, galactic cirrus, and the microwave background, rather than telescope optics or the atmosphere. Increasing pixel sensitivity to take advantage of this performance is of paramount importance to realize the potential of future infrared observatories. A reference goal is large-format detector arrays with per-pixel NEP of . This sensitivity is enabling for all imaging and medium-resolution spectroscopy applications. It meets the requirement of spectroscopy for the OST and exceeds the medium-resolution spectroscopy requirement for SPICA by a factor of 5. However, for high spectral resolutions (, e.g., the proposed HRS on the OST), even greater sensitivities are required, of , and ideally photon-counting. Turning to dynamic range, the dynamic range of detector arrays for high-background applications, such as ground-based observatories, is sufficient. However, the situation is problematic for the low background of cold space-based observatories. This is particularly true of observatories with apertures, since the saturation powers of currently proposed high-resolution detector arrays are within orders of magnitude of their NEPs. It would be advantageous to increase the dynamic range of detector arrays to 5 or more orders of magnitude of their NEPs, as this would mitigate the need to populate the focal plane with multiple detector arrays, each with different NEPs. LOs for heterodyne spectroscopy: The extremely high spectral resolutions achievable by heterodyne spectroscopy at mid/far-infrared wavelengths are of great value, both for scientific investigations in their own right and for complementarity with the moderate spectral resolutions of facilities, such as JWST. This motivates continued development of high-quality LO sources to increase the sensitivity and bandwidth of heterodyne receivers. An important development area is high spectral purity, narrow-line, phase-locked, high-power (5 to 10 mW) QCL LOs, as the QCL LOs operate effectively for the higher frequency () arrays. A complementary development area is power division schemes (e.g., Fourier phase gratings) to utilize QCLs effectively. High bandwidth heterodyne mixers: The current bandwidth of heterodyne receivers means that only very small spectral ranges can be observed at any one time, meaning that some classes of observation, such as multiple line scans of single objects, are often prohibitively inefficient. There is, thus, a need to increase the IF bandwidth of 1- to 5-THz heterodyne mixers. A reference goal is a minimum of 8-GHz bandwidth required at frequencies of . This will allow for simultaneous observation of multiple lines, improving both efficiency and calibration accuracy. A related development priority is low-noise 1- to 5-THz mixers that can operate at temperatures of . At present, the most promising paths toward such mixers align with the HEB and SIS technologies. Interferometry: Ground-based observations have conclusively demonstrated the extraordinary power of interferometry in the centimeter to submillimeter, with facilities such as the VLA and ALMA providing orders of magnitude increases in spatial resolution and sensitivity over any existing single-dish telescope. As Fig. 22 illustrates, the technology needs for space-based far-infrared interferometry are relatively modest and center on direct-detector developments. For interferometry, high-speed readout is more important than a large pixel count or extremely low NEP. For example, SPIRIT requires arrays of detectors with a NEP of and a detector time constant of .76 Detailed simulations, coupled with rigorous laboratory experimentation and algorithm development, are the greatest priorities for interferometry. Cryocoolers: As cooling to 4- and 0.1-K temperatures is required for all far-infrared observations, improvements in the efficiency, power requirements, size, and vibration of cryocoolers are valuable for all far-infrared space- and suborbital-based platforms. For coolers, there is a need for further development of both CADRs and DRs that enable cooling of up to tens of at , to enable cooling of larger arrays. For 4 K coolers, further development to maximize cooling power per input power for small cooling loads ( at 4 K) and lower mass is desirable, along with minimizing the exported vibration from the cooler system. For coolers, development of a cooling solution with power, space, and vibration envelopes that enable its use inside a 6U CubeSat, while leaving adequate resources for detector arrays, optics, and downlink systems, would enable far-infrared observations from CubeSat platforms, as well as enhancing larger observatories. Deployable and/or lightweight telescope mirrors: The advent of long-duration, high-altitude observing platforms, and the expanded capabilities of future launch vehicles, enable the consideration of mirrors for far-infrared observatories with diameters 2 to 5 times larger than on facilities such as SOFIA and Herschel. The most important limitations on mirror size are then the (1) mass and (2) approaches to active control of the mirror surface. The development of large-aperture, lightweight, high-surface-accuracy mirrors is thus an important consideration, including those in a deployable configuration. A related area is the development of optical components that accommodate large-format arrays or very high-resolution spectroscopy. Technology maturation platforms: Suborbital far-infrared platforms, including ground-based facilities, SOFIA, and balloon-borne observatories, continue to make profound advances in all areas of astrophysics. However, they also serve as a tiered set of platforms for technology maturation and raising TRLs. The continuous use of all these platforms for technology development is essential to realize the long-term ambitions of the far-infrared community for large, actively cooled, space-based infrared telescopes. A potentially valuable addition to this technology maturation tier is the ISS, which offers a long-term, stable orbital platform with abundant power. Software and data archiving: In the post-Herschel era, SOFIA and other sub-orbital platforms will play a critical role in mining the information-rich far-infrared spectral range, and in keeping the community moving forward. For example, the instruments flying on SOFIA and currently under development did not exist when Herschel instrumentation was defined. During this time, and henceforth, there is an urgent need to ensure community best practices in software design, code sharing, and open sourcing via community-wide mechanisms. It is also important to maintain and enhance data-archiving schemes that effectively bridge multiple complex platforms in a transparent way and which enable access to the broadest possible spectrum of the community. AcknowledgmentsWe thank George Nelson and Kenol Jules for their help on the capabilities of the ISS, and Jochem Baselmans for insights into KIDs. We also thank all speakers who took part in the FIR SIG Webinar series. This report is developed in part from the presentations and discussions at the Far-Infrared Next Generation Instrumentation Community Workshop, held in Pasadena, California, in March 2017. It is written as part of the activities of the Far-Infrared Science Interest Group. This work is supported by CNES. A portion of the research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. ReferencesD. E. Backman, A. Dasgupta and R. E. Stencel,
“Model of a Kuiper belt small grain population and resulting far-infrared emission,”
Astrophys. J., 450 L35
(1995). https://doi.org/10.1086/309660 ASJOAB 0004-637X Google Scholar
P. Santos-Sanz et al.,
““TNOs are Cool”: a survey of the trans-Neptunian region. IV. Size/albedo characterization of 15 scattered disk and detached objects observed with Herschel-PACS,”
Astron. Astrophys., 541 A92
(2012). https://doi.org/10.1051/0004-6361/201118541 AAEJAF 0004-6361 Google Scholar
J. Lebreton et al.,
“An icy Kuiper belt around the young solar-type star HD 181327,”
Astron. Astrophys., 539 A17
(2012). https://doi.org/10.1051/0004-6361/201117714 AAEJAF 0004-6361 Google Scholar
C. Eiroa et al.,
“DUst around NEarby stars. The survey observational results,”
Astron. Astrophys., 555 A11
(2013). https://doi.org/10.1051/0004-6361/201321050 AAEJAF 0004-6361 Google Scholar
D. Nesvorný et al.,
“Cometary origin of the zodiacal cloud and carbonaceous micrometeorites. implications for hot debris disks,”
Astrophys. J., 713 816
–836
(2010). https://doi.org/10.1088/0004-637X/713/2/816 ASJOAB 0004-637X Google Scholar
A. Morbidelli et al.,
“Source regions and time scales for the delivery of water to Earth,”
Meteorit. Planet. Sci., 35 1309
–1320
(2000). https://doi.org/10.1111/maps.2000.35.issue-6 Google Scholar
M. J. Mumma and S. B. Charnley,
“The chemical composition of comets—emerging taxonomies and natal heritage,”
Annu. Rev. Astron. Astrophys., 49 471
–524
(2011). https://doi.org/10.1146/annurev-astro-081309-130811 ARAAAJ 0066-4146 Google Scholar
P. Hartogh et al.,
“Ocean-like water in the Jupiter-family comet 103P/Hartley 2,”
Nature, 478 218
–220
(2011). https://doi.org/10.1038/nature10519 Google Scholar
W. S. Holland et al.,
“Submillimetre images of dusty debris around nearby stars,”
Nature, 392 788
–791
(1998). https://doi.org/10.1038/33874 Google Scholar
S. M. Andrews and J. P. Williams,
“Circumstellar dust disks in Taurus–Auriga: the submillimeter perspective,”
Astrophys. J., 631 1134
–1160
(2005). https://doi.org/10.1086/apj.2005.631.issue-2 ASJOAB 0004-637X Google Scholar
G. Bryden et al.,
“Frequency of debris disks around solar-type stars: first results from a Spitzer MIPS survey,”
Astrophys. J., 636 1098
–1113
(2006). https://doi.org/10.1086/apj.2006.636.issue-2 ASJOAB 0004-637X Google Scholar
M. C. Wyatt,
“Evolution of debris disks,”
Annu. Rev. Astron. Astrophys., 46 339
–383
(2008). https://doi.org/10.1146/annurev.astro.45.051806.110525 ARAAAJ 0066-4146 Google Scholar
M. R. Hogerheijde et al.,
“Detection of the water reservoir in a forming planetary system,”
Science, 334 338
–340
(2011). https://doi.org/10.1126/science.1208931 SCIEAS 0036-8075 Google Scholar
L. Kaltenegger, W. A. Traub and K. W. Jucks,
“Spectral evolution of an Earth-like planet,”
Astrophys. J., 658 598
–616
(2007). https://doi.org/10.1086/509283 ASJOAB 0004-637X Google Scholar
P. Hedelt et al.,
“Spectral features of Earth-like planets and their detectability at different orbital distances around F, G, and K-type stars,”
Astron. Astrophys., 553 A9
(2013). https://doi.org/10.1051/0004-6361/201117723 AAEJAF 0004-6361 Google Scholar
P. André et al.,
“From filamentary clouds to prestellar cores to the stellar IMF: initial highlights from the Herschel Gould Belt Survey,”
Astron. Astrophys., 518 L102
(2010). https://doi.org/10.1051/0004-6361/201014666 AAEJAF 0004-6361 Google Scholar
ALMA Partnershipet al.,
“The 2014 ALMA long baseline campaign: first results from high angular resolution observations toward the HL Tau Region,”
Astrophys. J., 808 L3
(2015). https://doi.org/10.1088/2041-8205/808/1/L3 ASJOAB 0004-637X Google Scholar
F. Motte, P. Andre and R. Neri,
“The initial conditions of star formation in the rho Ophiuchi main cloud: wide-field millimeter continuum mapping,”
Astron. Astrophys., 336 150
–172
(1998). AAEJAF 0004-6361 Google Scholar
II N. J. Evans et al.,
“The Spitzer c2d legacy results: star-formation rates and efficiencies; evolution and lifetimes,”
Astrophys. J. Suppl. Ser., 181 321
–350
(2009). https://doi.org/10.1088/0067-0049/181/2/321 APJSA2 0067-0049 Google Scholar
F. Schuller et al.,
“ATLASGAL—the APEX telescope large area survey of the galaxy at ,”
Astron. Astrophys., 504 415
–427
(2009). https://doi.org/10.1051/0004-6361/200811568 AAEJAF 0004-6361 Google Scholar
L. E. Kristensen et al.,
“Water in star-forming regions with Herschel (WISH). II. Evolution of 557 GHz emission in low-mass protostars,”
Astron. Astrophys., 542 A8
(2012). https://doi.org/10.1051/0004-6361/201118146 AAEJAF 0004-6361 Google Scholar
P. Manoj et al.,
“Herschel/PACS spectroscopic survey of protostars in orion: the origin of far-infrared CO emission,”
Astrophys. J., 763 83
(2013). https://doi.org/10.1088/0004-637X/763/2/83 ASJOAB 0004-637X Google Scholar
E. F. van Dishoeck et al.,
“Water in star-forming regions with the Herschel Space Observatory (WISH). I. Overview of key program and first results,”
Publ. Astron. Soc. Pac., 123 138
–170
(2011). https://doi.org/10.1086/658676 PASPAU 0004-6280 Google Scholar
D. M. Watson et al.,
“Evolution of mass outflow in protostars,”
Astrophys. J., 828 52
(2016). https://doi.org/10.3847/0004-637X/828/1/52 ASJOAB 0004-637X Google Scholar
M. M. Dunham et al.,
“Identifying the low-luminosity population of embedded protostars in the c2d observations of clouds and cores,”
Astrophys. J. Suppl. Ser., 179 249
–282
(2008). https://doi.org/10.1086/520860 Google Scholar
E. Furlan et al.,
“The Herschel orion protostar survey: spectral energy distributions and fits using a Grid of protostellar models,”
Astrophys. J. Suppl. Ser., 224 5
(2016). https://doi.org/10.3847/0067-0049/224/1/5 APJSA2 0067-0049 Google Scholar
W. J. Fischer et al.,
“The Herschel orion protostar survey: luminosity and envelope evolution,”
Astrophys. J., 840 69
(2017). https://doi.org/10.3847/1538-4357/aa6d69 ASJOAB 0004-637X Google Scholar
B. A. Whitney et al.,
“Two-dimensional radiative transfer in protostellar envelopes. II. An evolutionary sequence,”
Astrophys. J., 598 1079
–1099
(2003). https://doi.org/10.1086/apj.2003.598.issue-2 ASJOAB 0004-637X Google Scholar
R. Launhardt et al.,
“The Earliest Phases of Star Formation (EPoS): a Herschel key project. The thermal structure of low-mass molecular cloud cores,”
Astron. Astrophys., 551 A98
(2013). https://doi.org/10.1051/0004-6361/201220477 AAEJAF 0004-6361 Google Scholar
A. M. Stutz et al.,
“A Herschel and APEX census of the reddest sources in Orion: searching for the youngest protostars,”
Astrophys. J., 767 36
(2013). https://doi.org/10.1088/0004-637X/767/1/36 ASJOAB 0004-637X Google Scholar
C. D. Howard et al.,
“Herschel/PACS survey of protoplanetary disks in Taurus/Auriga – observations of [O I] and [C II], and far-infrared continuum,”
Astrophys. J., 776 21
(2013). https://doi.org/10.1088/0004-637X/776/1/21 ASJOAB 0004-637X Google Scholar
B. Acke et al.,
“Herschel images of Fomalhaut. An extrasolar Kuiper belt at the height of its dynamical activity,”
Astron. Astrophys., 540 A125
(2012). https://doi.org/10.1051/0004-6361/201118581 AAEJAF 0004-6361 Google Scholar
N. Billot et al.,
“Herschel far-infrared photometric monitoring of protostars in the Orion Nebula Cluster,”
Astrophys. J., 753 L35
(2012). https://doi.org/10.1088/2041-8205/753/2/L35 ASJOAB 0004-637X Google Scholar
S. J. Kenyon et al.,
“An IRAS survey of the Taurus–Auriga molecular cloud,”
Astron. J., 99 869
–887
(1990). https://doi.org/10.1086/115380 Google Scholar
D. Calzetti et al.,
“The dust content and opacity of actively star-forming galaxies,”
Astrophys. J., 533 682
–695
(2000). https://doi.org/10.1086/apj.2000.533.issue-2 ASJOAB 0004-637X Google Scholar
A. Li and B. T. Draine,
“Infrared emission from interstellar dust. II. The diffuse interstellar medium,”
Astrophys. J., 554 778
–802
(2001). https://doi.org/10.1086/apj.2001.554.issue-2 ASJOAB 0004-637X Google Scholar
D. A. Dale et al.,
“The infrared spectral energy distribution of normal star-forming galaxies,”
Astrophys. J., 549 215
–227
(2001). https://doi.org/10.1086/apj.2001.549.issue-1 ASJOAB 0004-637X Google Scholar
S. Molinari et al.,
“Hi-GAL: the Herschel infrared galactic plane survey,”
Publ. Astron. Soc. Pac., 122 314
–325
(2010). https://doi.org/10.1086/648999 PASPAU 0004-6280 Google Scholar
M. K. Crawford et al.,
“Far-infrared spectroscopy of galaxies—the 158 micron C(+) line and the energy balance of molecular clouds,”
Astrophys. J., 291 755
–771
(1985). https://doi.org/10.1086/163113 ASJOAB 0004-637X Google Scholar
P. Panuzzo et al.,
“Probing the molecular interstellar medium of M82 with Herschel-SPIRE spectroscopy,”
Astron. Astrophys., 518 L37
(2010). https://doi.org/10.1051/0004-6361/201014558 AAEJAF 0004-6361 Google Scholar
J. Fischer et al.,
“Herschel-PACS spectroscopic diagnostics of local ULIRGs: conditions and kinematics in Markarian 231,”
Astron. Astrophys., 518 L41
(2010). https://doi.org/10.1051/0004-6361/201014676 AAEJAF 0004-6361 Google Scholar
T. Daz-Santos et al.,
“Explaining the [C II] deficit in luminous infrared galaxies—first results from a Herschel/PACS study of the GOALS sample,”
Astrophys. J., 774 68
(2013). https://doi.org/10.1088/0004-637X/774/1/68 ASJOAB 0004-637X Google Scholar
D. Farrah et al.,
“Far-infrared fine-structure line diagnostics of ultraluminous infrared galaxies,”
Astrophys. J., 776 38
(2013). https://doi.org/10.1088/0004-637X/776/1/38 ASJOAB 0004-637X Google Scholar
G. Lagache, J.-L. Puget and H. Dole,
“Dusty infrared galaxies: sources of the cosmic infrared background,”
Annu. Rev. Astron. Astrophys., 43 727
–768
(2005). https://doi.org/10.1146/annurev.astro.43.072103.150606 ARAAAJ 0066-4146 Google Scholar
P. Madau and M. Dickinson,
“Cosmic star-formation history,”
Annu. Rev. Astron. Astrophys., 52 415
–486
(2014). https://doi.org/10.1146/annurev-astro-081811-125615 ARAAAJ 0066-4146 Google Scholar
C. J. Lonsdale, D. Farrah, H. E. Smith,
“Ultraluminous Infrared Galaxies,”
Astrophysics Update 2, 285 2006). Google Scholar
G. Rodighiero et al.,
“The lesser role of starbursts in star formation at z = 2,”
Astrophys. J., 739 L40
(2011). https://doi.org/10.1088/2041-8205/739/2/L40 ASJOAB 0004-637X Google Scholar
D. Lutz et al.,
“PACS evolutionary probe (PEP)—a Herschel key program,”
Astron. Astrophys., 532 A90
(2011). https://doi.org/10.1051/0004-6361/201117107 AAEJAF 0004-6361 Google Scholar
S. J. Oliver et al.,
“The Herschel multi-tiered extragalactic survey: HerMES,”
MNRAS, 424 1614
–1635
(2012). https://doi.org/10.1111/mnr.2012.424.issue-3 Google Scholar
C. M. Casey, D. Narayanan and A. Cooray,
“Dusty star-forming galaxies at high redshift,”
Phys. Rep., 541 45
–161
(2014). https://doi.org/10.1016/j.physrep.2014.02.009 PRPLCM 0370-1573 Google Scholar
R. Genzel et al.,
“A study of the gas-star formation relation over cosmic time,”
MNRAS, 407 2091
–2108
(2010). https://doi.org/10.1111/mnr.2010.407.issue-4 Google Scholar
A. C. Fabian,
“Observational evidence of active galactic nuclei feedback,”
Annu. Rev. Astron. Astrophys., 50 455
–489
(2012). https://doi.org/10.1146/annurev-astro-081811-125521 ARAAAJ 0066-4146 Google Scholar
D. Farrah et al.,
“Direct evidence for termination of obscured star formation by radiatively driven outflows in reddened QSOs,”
Astrophys. J., 745 178
(2012). https://doi.org/10.1088/0004-637X/745/2/178 ASJOAB 0004-637X Google Scholar
L. Page et al.,
“Three-year Wilkinson microwave anisotropy probe (WMAP) observations: polarization analysis,”
Astrophys. J. Suppl. Ser., 170 335
–376
(2007). https://doi.org/10.1086/509230 APJSA2 0067-0049 Google Scholar
Planck Collaboration,
“Planck 2015 results. XV. Gravitational lensing,”
Astron. Astrophys., 594 A15
(2016). https://doi.org/10.1051/0004-6361/201525941 AAEJAF 0004-6361 Google Scholar
B. T. Soifer and J. L. Pipher,
“Instrumentation for infrared astronomy,”
Annu. Rev. Astron. Astrophys., 16 335
–369
(1978). https://doi.org/10.1146/annurev.aa.16.090178.002003 ARAAAJ 0066-4146 Google Scholar
F. J. Low, G. H. Rieke and R. D. Gehrz,
“The beginning of modern infrared astronomy,”
Annu. Rev. Astron. Astrophys., 45 43
–75
(2007). https://doi.org/10.1146/annurev.astro.44.051905.092505 ARAAAJ 0066-4146 Google Scholar
G. H. Rieke,
“Infrared detector arrays for astronomy,”
Annu. Rev. Astron. Astrophys., 45 77
–115
(2007). https://doi.org/10.1146/annurev.astro.44.051905.092436 ARAAAJ 0066-4146 Google Scholar
P. H. Siegel,
“THz instruments for space,”
IEEE Trans. Antennas Propag., 55 2957
–2965
(2007). https://doi.org/10.1109/TAP.2007.908557 Google Scholar
S. Price,
“History of space-based infrared astronomy and the air force infrared celestial backgrounds program,”
(2008). Google Scholar
S. D. Price,
“Infrared sky surveys,”
Space Sci. Rev., 142 233
–321
(2009). https://doi.org/10.1007/s11214-008-9475-4 SPSRA4 0038-6308 Google Scholar
G. H. Rieke,
“History of infrared telescopes and astronomy,”
Exp. Astron., 25 125
–141
(2009). https://doi.org/10.1007/s10686-009-9148-7 Google Scholar
J. Lequeux,
“Early infrared astronomy,”
J. Astron. Hist. Heritage, 12
(2), 125
–140
(2009). Google Scholar
M. Rowan-Robinson, Night Vision: Exploring the Infrared Universe, Cambridge University Press, Cambridge
(2013). Google Scholar
D. L. Clements, Infrared Astronomy–Seeing the Heat: from William Herschel to the Herschel Space Observatory, CRC Press, Florida
(2014). Google Scholar
D. Rigopoulou et al.,
“The European far-infrared space roadmap,”
(2017). Google Scholar
T. Webb et al.,
“A roadmap for canadian submillimetre astronomy,”
(2013). Google Scholar
C. Walker, Terahertz Astronomy, CRC Press, Florida
(2015). Google Scholar
Y. Lee, Principles of Terahertz Science and Technology, Springer US, New York
(2009). Google Scholar
S. S. Dhillon et al.,
“The 2017 terahertz science and technology roadmap,”
J. Phys. D: Appl. Phys., 50
(4), 043001
(2017). https://doi.org/10.1088/1361-6463/50/4/043001 Google Scholar
S. D. Lord,
“A new software tool for computing Earth’s atmospheric transmission of near- and far-infrared radiation,”
(1992). Google Scholar
B. Koopman et al.,
“The CCAT-prime extreme field-of-view submillimeter telescope on Cerro Chajnantor,”
437.01
(2017). Google Scholar
M. D. Niemack,
“Designs for a large-aperture telescope to map the CMB 10x faster,”
Appl. Opt., 55 1688
(2016). https://doi.org/10.1364/AO.55.001688 APOPAI 0003-6935 Google Scholar
S. J. E. Radford and J. B. Peterson,
“Submillimeter atmospheric transparency at Maunakea, at the south pole, and at Chajnantor,”
Publ. Astron. Soc. Pac., 128 075001
(2016). https://doi.org/10.1088/1538-3873/128/965/075001 PASPAU 0004-6280 Google Scholar
B. Jackson et al.,
“The spica-safari detector system: Tes detector arrays with frequency-division multiplexed squid readout,”
IEEE Trans. Terahertz Sci. Technol., 2
(1), 12
–21
(2012). https://doi.org/10.1109/TTHZ.2011.2177705 Google Scholar
D. J. Benford et al.,
“Cryogenic far-infrared detectors for the Space Infrared Interferometric Telescope (SPIRIT),”
Proc. SPIE, 6687 66870E
(2007). https://doi.org/10.1117/12.734751 PSISDG 0277-786X Google Scholar
P. Temi et al.,
“The SOFIA observatory at the start of routine science operations: mission capabilities and performance,”
Astrophys. J. Suppl. Ser., 212 24
(2014). https://doi.org/10.1088/0067-0049/212/2/24 APJSA2 0067-0049 Google Scholar
E. T. Young et al.,
“Early science with SOFIA, the stratospheric observatory for infrared astronomy,”
Astrophys. J., 749 L17
(2012). https://doi.org/10.1088/2041-8205/749/2/L17 ASJOAB 0004-637X Google Scholar
S. M. Feeney et al.,
“Cosmic microwave background science at commercial airline altitudes,”
MNRAS, 469 L6
–L10
(2017). https://doi.org/10.1093/mnrasl/slx040 Google Scholar
J. D. Adams et al.,
“FORCAST: a first light facility instrument for SOFIA,”
Proc. SPIE, 7735 77351U
(2010). https://doi.org/10.1117/12.857049 PSISDG 0277-786X Google Scholar
T. L. Herter et al.,
“First science observations with SOFIA/FORCAST: the FORCAST mid-infrared camera,”
Astrophys. J., 749 L18
(2012). https://doi.org/10.1088/2041-8205/749/2/L18 ASJOAB 0004-637X Google Scholar
M. J. Richter et al.,
“High-resolution mid-infrared spectroscopy from SOFIA using EXES,”
Proc. SPIE, 4857 37
–46
(2003). https://doi.org/10.1117/12.458628 PSISDG 0277-786X Google Scholar
S. Colditz et al.,
“The SOFIA far-infrared spectrometer FIFI-LS: spearheading a post Herschel era,”
Proc. SPIE, 8446 844617
(2012). https://doi.org/10.1117/12.924510 PSISDG 0277-786X Google Scholar
D. A. Harper et al.,
“Development of the HAWC far-infrared camera for SOFIA,”
Proc. SPIE, 5492 1064
–1073
(2004). https://doi.org/10.1117/12.552151 PSISDG 0277-786X Google Scholar
S. Heyminck et al.,
“GREAT: the SOFIA high-frequency heterodyne instrument,”
Astron. Astrophys., 542 L1
(2012). https://doi.org/10.1051/0004-6361/201218811 AAEJAF 0004-6361 Google Scholar
B. Klein et al.,
“High-resolution wide-band fast Fourier transform spectrometers,”
Astron. Astrophys., 542 L3
(2012). https://doi.org/10.1051/0004-6361/201218864 AAEJAF 0004-6361 Google Scholar
E. W. Dunham et al.,
“HIPO: a high-speed imaging photometer for occultations,”
Proc. SPIE, 5492 592
–603
(2004). https://doi.org/10.1117/12.552152 PSISDG 0277-786X Google Scholar
I. S. McLean et al.,
“FLITECAM: a 1-5 micron camera and spectrometer for SOFIA,”
Proc. SPIE, 6269 62695B
(2006). https://doi.org/10.1117/12.672173 PSISDG 0277-786X Google Scholar
C. D. Dowell et al.,
“HAWC+: a detector, polarimetry, and narrow-band imaging upgrade to SOFIA’s far-infrared facility camera,”
Am. Astron. Soc. Meeting Abstr., 221 345.14
(2013). Google Scholar
C. Risacher et al.,
“The upGREAT 1.9 THz multi-pixel high resolution spectrometer for the SOFIA Observatory,”
Astron. Astrophys., 595 A34
(2016). https://doi.org/10.1051/0004-6361/201629045 AAEJAF 0004-6361 Google Scholar
J. C. Mankins,
“Technology readiness levels,”
White Paper,
(1995). Google Scholar
J. C. Mankins,
“Technology readiness assessments: a retrospective,”
Acta Astronaut., 65
(9), 1216
–1223
(2009). https://doi.org/10.1016/j.actaastro.2009.03.058 AASTCF 0094-5765 Google Scholar
G. S. Tucker et al.,
“The balloon-borne large aperture sub-millimeter telescope,”
Adv. Space Res., 33 1793
–1796
(2004). https://doi.org/10.1016/j.asr.2003.05.022 Google Scholar
L. M. Fissel et al.,
“The balloon-borne large-aperture submillimeter telescope for polarimetry: BLAST-Pol,”
Proc. SPIE, 7741 77410E
(2010). https://doi.org/10.1117/12.857601 PSISDG 0277-786X Google Scholar
N. Galitzki et al.,
“The next generation BLAST experiment,”
J. Astron. Instrum., 3 1440001
(2014). https://doi.org/10.1142/S2251171714400017 Google Scholar
J.-P. Bernard et al.,
“PILOT: a balloon-borne experiment to measure the polarized FIR emission of dust grains in the interstellar medium,”
Exp. Astron., 42 199
–227
(2016). https://doi.org/10.1007/s10686-016-9506-1 Google Scholar
C. Walker et al.,
“The stratospheric THz observatory (STO),”
Proc. SPIE, 7733 77330N
(2010). https://doi.org/10.1117/12.857765 PSISDG 0277-786X Google Scholar
H. Shibai et al.,
“Far-infrared interferometric experiment (FITE): toward the first flight,”
Pathways Towards Habitable Planets, 430 541 2010). Google Scholar
S. A. Rinehart et al.,
“The balloon experimental twin telescope for infrared interferometry (BETTII): an experiment for high angular resolution in the far-infrared,”
Publ. Astron. Soc. Pac., 126 660
(2014). http://dx.doi.org/10.1086/677402 PASPAU 0004-6280 Google Scholar
“Detection of B-mode polarization at degree angular scales by BICEP2,”
Phys. Rev. Lett., 112 241101
(2014). https://doi.org/10.1103/PhysRevLett.112.241101 Google Scholar
R. Flauger, J. C. Hill and D. N. Spergel,
“Toward an understanding of foreground emission in the BICEP2 region,”
J. Cosmol. Astropart. Phys., 2014 039
–039
(2014). https://doi.org/10.1088/1475-7516/2014/08/039 JCAPBP 1475-7516 Google Scholar
BICEP2/Keck Collaborationet al.,
“Joint analysis of BICEP2/Keck array and Planck data,”
Phys. Rev. Lett., 114 101301
(2015). https://doi.org/10.1103/PhysRevLett.114.101301 Google Scholar
A. Kogut et al.,
“The primordial inflation polarization explorer (PIPER),”
Proc. SPIE, 8452 84521J
(2012). https://doi.org/10.1117/12.925204 PSISDG 0277-786X Google Scholar
N. N. Gandilo et al.,
“The primordial inflation polarization explorer (PIPER),”
Proc. SPIE, 9914 99141J
(2016). https://doi.org/10.1117/12.2231109 PSISDG 0277-786X Google Scholar
“The EBEX balloon borne experiment—optics, receiver, and polarimetry,”
(2017). Google Scholar
K. Shivanandan, J. R. Houck and M. O. Harwit,
“Preliminary observations of the far-infrared night-sky background radiation,”
Phys. Rev. Lett., 21 1460
–1462
(1968). https://doi.org/10.1103/PhysRevLett.21.1460 Google Scholar
J. R. Houck and M. Harwit,
“Far-infrared observations of the night sky,”
Science, 164 1271
–1273
(1969). https://doi.org/10.1126/science.164.3885.1271 SCIEAS 0036-8075 Google Scholar
S. D. Price and R. G. Walker, The AFGL Four Color Infrared Sky Survey: Catalog of Observations at 4.2, 11.0, 19.8, and 27.4 Micrometer, 1976). Google Scholar
G. Seibert,
“The history of sounding rockets and their contribution to European Space Research,”
(2006). Google Scholar
M. Zemcov et al.,
“The Cosmic Infrared Background Experiment (CIBER): a sounding rocket payload to study the near infrared extragalactic background light,”
Astrophys. J. Suppl. Ser., 207 31
(2013). https://doi.org/10.1088/0067-0049/207/2/31 APJSA2 0067-0049 Google Scholar
N. R. Council, Revitalizing NASA’s Suborbital Program: Advancing Science, Driving Innovation, and Developing Workforce, The National Academies Press, Washington, DC
(2010). Google Scholar
G. Neugebauer et al.,
“The Infrared Astronomical Satellite (IRAS) mission,”
Astrophys. J., 278 L1
–L6
(1984). https://doi.org/10.1086/184209 ASJOAB 0004-637X Google Scholar
J. D. Mill et al.,
“Midcourse space experiment: introduction to the spacecraft, instruments, and scientific objectives,”
J. Spacecr. Rockets, 31 900
–907
(1994). https://doi.org/10.2514/3.55673 Google Scholar
H. Murakami et al.,
“The IRTS (Infrared Telescope in Space) mission,”
Publ. Astron. Soc. Jpn., 48 L41
–L46
(1996). https://doi.org/10.1093/pasj/48.5.L41 Google Scholar
M. F. Kessler et al.,
“The Infrared Space Observatory (ISO) mission,”
Astron. Astrophys., 315
(2), L27
–L31
(1996). AAEJAF 0004-6361 Google Scholar
G. J. Melnick et al.,
“The submillimeter wave astronomy satellite: science objectives and instrument description,”
Astrophys. J., 539 L77
–L85
(2000). https://doi.org/10.1086/312856 ASJOAB 0004-637X Google Scholar
H. L. Nordh et al.,
“The Odin orbital observatory,”
Astron. Astrophys., 402 L21
–L25
(2003). https://doi.org/10.1051/0004-6361:20030334 AAEJAF 0004-6361 Google Scholar
H. Murakami et al.,
“The infrared astronomical mission AKARI*,”
Publ. Astron. Soc. Jpn., 59 S369
–S376
(2007). https://doi.org/10.1093/pasj/59.sp2.S369 Google Scholar
G. L. Pilbratt et al.,
“Herschel space observatory: an ESA facility for far-infrared and submillimetre astronomy,”
Astron. Astrophys., 518 L1
(2010). https://doi.org/10.1051/0004-6361/201014759 AAEJAF 0004-6361 Google Scholar
E. L. Wright et al.,
“The Wide-Field Infrared Survey Explorer (WISE): mission description and initial on-orbit performance,”
Astron. J., 140 1868
–1881
(2010). https://doi.org/10.1088/0004-6256/140/6/1868 Google Scholar
M. W. Werner et al.,
“The Spitzer space telescope mission,”
Astrophys. J. Suppl. Ser., 154 1
–9
(2004). https://doi.org/10.1086/apjs.2004.154.issue-1 APJSA2 0067-0049 Google Scholar
“Planck early results. I. The Planck mission,”
Astron. Astrophys., 536 A1
(2011). https://doi.org/10.1051/0004-6361/201116464 AAEJAF 0004-6361 Google Scholar
C. L. Bennett et al.,
“The microwave anisotropy probe mission,”
Astrophys. J., 583 1
–23
(2003). https://doi.org/10.1086/apj.2003.583.issue-1 ASJOAB 0004-637X Google Scholar
N. W. Boggess et al.,
“The COBE mission—its design and performance two years after launch,”
Astrophys. J., 397 420
–429
(1992). https://doi.org/10.1086/171797 ASJOAB 0004-637X Google Scholar
D. J. Fixsen et al.,
“The spectrum of the extragalactic far-infrared background from the COBE FIRAS observations,”
Astrophys. J., 508 123
–128
(1998). https://doi.org/10.1086/apj.1998.508.issue-1 ASJOAB 0004-637X Google Scholar
A. Kogut et al.,
“The primordial inflation explorer (PIXIE),”
Proc. SPIE, 9904 99040W
(2016). https://doi.org/10.1117/12.2231090 PSISDG 0277-786X Google Scholar
T. Matsumura et al.,
“Mission design of LiteBIRD,”
J. Low Temp. Phys., 176 733
–740
(2014). https://doi.org/10.1007/s10909-013-0996-1 Google Scholar
J. Delabrouille et al.,
“Exploring cosmic origins with CORE: survey requirements and mission design,”
J. Cosmol. Astropart. Phys., 2018
(2018). https://doi.org/10.1088/1475-7516/2018/04/014 Google Scholar
D. J. Fixsen,
“The temperature of the cosmic microwave background,”
Astrophys. J., 707 916
–920
(2009). https://doi.org/10.1088/0004-637X/707/2/916 ASJOAB 0004-637X Google Scholar
D. Paradis et al.,
“Far-infrared to millimeter astrophysical dust emission. II. Comparison of the two-level systems (TLS) model with astronomical data,”
Astron. Astrophys., 534 A118
(2011). https://doi.org/10.1051/0004-6361/201116862 AAEJAF 0004-6361 Google Scholar
C. Leinert et al.,
“The 1997 reference of diffuse night sky brightness,”
Astron. Astrophys. Suppl. Ser., 127 1
–99
(1998). https://doi.org/10.1051/aas:1998105 AAESB9 0365-0138 Google Scholar
A. Poglitsch et al.,
“The photodetector array camera and spectrometer (PACS) on the Herschel space observatory,”
Astron. Astrophys., 518 L2
(2010). https://doi.org/10.1051/0004-6361/201014535 AAEJAF 0004-6361 Google Scholar
M. J. Griffin et al.,
“The Herschel-SPIRE instrument and its in-flight performance,”
Astron. Astrophys., 518 L3
(2010). https://doi.org/10.1051/0004-6361/201014519 AAEJAF 0004-6361 Google Scholar
P. Hacking and J. R. Houck,
“A very deep IRAS survey at , ,”
Astrophys. J. Suppl. Ser., 63 311
–333
(1987). https://doi.org/10.1086/191167 APJSA2 0067-0049 Google Scholar
C. Kiss, U. Klaas and D. Lemke,
“Determination of confusion noise for far-infrared measurements,”
Astron. Astrophys., 430 343
–353
(2005). https://doi.org/10.1051/0004-6361:20041422 AAEJAF 0004-6361 Google Scholar
H. Dole et al.,
“Confusion of extragalactic sources in the mid- and far-infrared: spitzer and beyond,”
Astrophys. J. Suppl. Ser., 154 93
–96
(2004). https://doi.org/10.1086/apjs.2004.154.issue-1 APJSA2 0067-0049 Google Scholar
H. T. Nguyen et al.,
“HerMES: the SPIRE confusion limit,”
Astron. Astrophys., 518 L5
(2010). https://doi.org/10.1051/0004-6361/201014680 AAEJAF 0004-6361 Google Scholar
M. Negrello et al.,
“Confusion noise at far-infrared to millimetre wavelengths,”
MNRAS, 352 493
–500
(2004). https://doi.org/10.1111/mnr.2004.352.issue-2 Google Scholar
N. Fernandez-Conde et al.,
“Simulations of the cosmic infrared and submillimeter background for future large surveys. I. Presentation and first application to Herschel/SPIRE and Planck/HFI,”
Astron. Astrophys., 481 885
–895
(2008). https://doi.org/10.1051/0004-6361:20078188 AAEJAF 0004-6361 Google Scholar
T. T. Takeuchi et al.,
“Estimation of the confusion limit for spica,”
in JAXA Spec. Publ.: Proc. SPICA Sci. Conf. from Exoplanets to Distant Galaxies: SPICA’s New Window on the Cool Univ.,
157
(2018). Google Scholar
M. G. Jones et al.,
“When is stacking confusing? The impact of confusion on stacking in deep H I galaxy surveys,”
MNRAS, 455 1574
–1583
(2016). https://doi.org/10.1093/mnras/stv2394 Google Scholar
B. Tercero et al.,
“A line confusion limited millimeter survey of Orion KL. I. Sulfur carbon chains,”
Astron. Astrophys., 517 A96
(2010). https://doi.org/10.1051/0004-6361/200913501 AAEJAF 0004-6361 Google Scholar
A. Kogut, E. Dwek and S. H. Moseley,
“Spectral confusion for cosmological surveys of Redshifted C II emission,”
Astrophys. J., 806 234
(2015). https://doi.org/10.1088/0004-637X/806/2/234 ASJOAB 0004-637X Google Scholar
K. K. Knudsen et al.,
“An ultradeep submillimetre map: beneath the SCUBA confusion limit with lensing and robust source extraction,”
MNRAS, 368 487
–496
(2006). https://doi.org/10.1111/j.1365-2966.2006.10138.x Google Scholar
I. G. Roseboom et al.,
“The Herschel Multi-Tiered Extragalactic Survey: source extraction and cross-identifications in confusion-dominated SPIRE images,”
MNRAS, 409 48
–65
(2010). https://doi.org/10.1111/mnr.2010.409.issue-1 Google Scholar
M. Safarzadeh et al.,
“A novel technique to improve photometry in confused images using graphs and Bayesian priors,”
Astrophys. J., 798 91
(2015). https://doi.org/10.1088/0004-637X/798/2/91 ASJOAB 0004-637X Google Scholar
T. P. MacKenzie, D. Scott and M. Swinbank,
“SEDEBLEND: a new method for deblending spectral energy distributions in confused imaging,”
MNRAS, 463 10
–23
(2016). https://doi.org/10.1093/mnras/stw1890 Google Scholar
P. D. Hurley et al.,
“HELP: XID+, the probabilistic de-blender for Herschel SPIRE maps,”
MNRAS, 464 885
–896
(2017). https://doi.org/10.1093/mnras/stw2375 Google Scholar
G. Raymond et al.,
“The effectiveness of mid IR / far IR blind, wide area, spectral surveys in breaking the confusion limit,”
Publ. Astron. Soc. Jpn., 62 697
–708
(2010). https://doi.org/10.1093/pasj/62.3.697 Google Scholar
J. M. Lamarre,
“Photon noise in photometric instruments at far-infrared and submillimeter wavelengths,”
Appl. Opt., 25 870
–876
(1986). https://doi.org/10.1364/AO.25.000870 APOPAI 0003-6935 Google Scholar
P. L. Richards,
“Bolometers for infrared and millimeter waves,”
J. Appl. Phys., 76
(1), 1
–24
(1994). https://doi.org/10.1063/1.357128 Google Scholar
D. J. Benford, T. R. Hunter and T. G. Phillips,
“Noise equivalent powers of background limited thermal detectors at submillimeter wavelengths,”
Int. J. Infrared Millimeter Waves, 19 931
–938
(1998). https://doi.org/10.1023/A:1022671223858 Google Scholar
C. K. Stahle et al.,
“Design and performance of the ASTRO-E/XRS microcalorimeter array and anticoincidence detector,”
Proc. SPIE, 3765 128
–136
(1999). https://doi.org/10.1117/12.366494 PSISDG 0277-786X Google Scholar
C. K. Stahle et al.,
“Cosmic ray effects in microcalorimeter arrays,”
Nucl. Instrum. Methods Phys. Res. A, 520 472
–474
(2004). https://doi.org/10.1016/j.nima.2003.11.376 Google Scholar
T. Saab et al.,
“GEANT modeling of the low-Earth-orbit cosmic-ray background for the Astro-E2 XRS instrument,”
Proc. SPIE, 5501 320
–327
(2004). https://doi.org/10.1117/12.551945 PSISDG 0277-786X Google Scholar
J. P. Gardner et al.,
“The James Webb space telescope,”
Space Sci. Rev., 123 485
–606
(2006). https://doi.org/10.1007/s11214-006-8315-7 SPSRA4 0038-6308 Google Scholar
A. V. Smirnov et al.,
“Space mission Millimetron for terahertz astronomy,”
Proc. SPIE, 8442 84424C
(2012). https://doi.org/10.1117/12.927184 PSISDG 0277-786X Google Scholar
N. S. Kardashev et al.,
“Review of scientific topics for the Millimetron space observatory,”
Phys. Usp., 57 1199
–1228
(2014). https://doi.org/10.3367/UFNe.0184.201412c.1319 Google Scholar
T. Nakagawa et al.,
“HII/L2 mission: future Japanese infrared astronomical mission,”
Proc. SPIE, 3356 462
–470
(1998). https://doi.org/10.1117/12.324469 PSISDG 0277-786X Google Scholar
T. NakagawaSpica Working Group,
“SPICA: space infrared telescope for cosmology and astrophysics,”
Adv. Space Res., 34 645
–650
(2004). https://doi.org/10.1016/j.asr.2003.04.044 Google Scholar
B. Swinyard et al.,
“The space infrared telescope for cosmology and astrophysics: SPICA A joint mission between JAXA and ESA,”
Exp. Astron., 23 193
–219
(2009). https://doi.org/10.1007/s10686-008-9090-0 Google Scholar
T. Nakagawa et al.,
“The next-generation infrared space mission Spica: project updates,”
Publ. Korean Astron. Soc., 32 331
–335
(2017). https://doi.org/10.5303/PKAS.2017.32.1.331 Google Scholar
B. Sibthorpe et al.,
“The SPICA mission,”
EAS Publ. Ser., 75 411
–417
(2015). https://doi.org/10.1051/eas/1575083 Google Scholar
P. Roelfsema et al.,
“SAFARI new and improved: extending the capabilities of SPICA’s imaging spectrometer,”
Proc. SPIE, 9143 91431K
(2014). https://doi.org/10.1117/12.2056449 PSISDG 0277-786X Google Scholar
C. Pastor et al.,
“SAFARI optical system architecture and design concept,”
Proc. SPIE, 9904 99043U
(2016). https://doi.org/10.1117/12.2232786 PSISDG 0277-786X Google Scholar
C. BradfordSPICA Consortium, and SAFARI Consortium,
“The space infrared telescope for cosmology and astrophysics and pending US contribution,”
Am. Astron. Soc. Meeting Abstr., 229 238.25
(2017). Google Scholar
D. A. Dale and G. Helou,
“The infrared spectral energy distribution of normal star-forming galaxies: calibration at far-infrared and submillimeter wavelengths,”
Astrophys. J., 576 159
–168
(2002). https://doi.org/10.1086/apj.2002.576.issue-1 ASJOAB 0004-637X Google Scholar
J. J. A. Baselmans et al.,
“A kilo-pixel imaging system for future space based far-infrared observatories using microwave kinetic inductance detectors,”
Astron. Astrophys., 601 A89
(2017). https://doi.org/10.1051/0004-6361/201629653 AAEJAF 0004-6361 Google Scholar
J. Bueno et al.,
“Full characterisation of a background limited antenna coupled KID over an octave of bandwidth for THz radiation,”
Appl. Phys. Lett., 110 233503
(2017). https://doi.org/10.1063/1.4985060 Google Scholar
T. H. Zurbuchen,
“Achieving Science with CubeSats: Thinking Inside the Box,”
Proc. SPIE, 9978
(2016). https://doi.org/10.1117/12.2238764 Google Scholar
D. R. Ardila, E. Shkolnik and V. Gorjian,
“Cubesats for astrophysics: the current perspective,”
Am. Astron. Soc. Meeting Abstr., 229 206.05
(2017). Google Scholar
E. L. Shkolnik,
“On the verge of an astronomy CubeSat revolution,”
Nat. Astron., 2 374
–378
(2018). https://doi.org/10.1038/s41550-018-0438-8 Google Scholar
J. S. Perkins et al.,
“BurstCube: a CubeSat for gravitational wave counterparts,”
Am. Astron. Soc. Meeting Abstr., 231 361.14
(2018). Google Scholar
P. Kaaret,
“HaloSat—a CubeSat to study the hot galactic halo,”
Am. Astron. Soc. Meeting Abstr., 229 328.03
(2017). Google Scholar
E. L. Shkolnik et al.,
“Monitoring the high-energy radiation environment of exoplanets around low-mass stars with SPARCS (Star-Planet Activity Research CubeSat),”
Am. Astron. Soc. Meeting Abstr., 231
(2018). Google Scholar
B. T. Fleming et al.,
“Colorado ultraviolet transit experiment: a dedicated CubeSat mission to study exoplanetary mass loss and magnetic fields,”
J. Astron. Telesc. Instrum. Syst., 4 014004
(2018). https://doi.org/10.1117/1.JATIS.4.1.014004 Google Scholar
A. Joseph, E. Barrentine and A. Brown,
“A thermal imaging instrument with uncooled detectors,”
18357
(2018). Google Scholar
E. Agasid, K. Ennico-Smith and A. Rademacher,
“Collapsible space telescope (CST) for nanosatellite imaging and observation,”
(2013). Google Scholar
T. S. Pagano,
“Cubesat infrared atmospheric sounder (CIRAS) NASA invest technology demonstration,”
Proc. SPIE, 10177 101770K
(2017). https://doi.org/10.1117/12.2266282 PSISDG 0277-786X Google Scholar
D. Ardila and D. Pack,
“The cubesat multispectral observation system (cumulos),”
(2016). Google Scholar
B. R. Johnson et al.,
“A CubeSat for calibrating ground-based and sub-orbital millimeter-wave polarimeters (CalSat),”
J. Astron. Instrum., 4 1550007
(2015). https://doi.org/10.1142/S2251171715500075 Google Scholar
L. Primm, K. Jules and L. Bullock, External Payloads Proposer’s Guide to the International Space Station, Goddard Space Flight Center(2016). Google Scholar
R. L. Brown et al.,
“High-resolution imaging spectroscopy at terahertz frequencies,”
IAU Colloq. 123: Observatories in Earth Orbit and Beyond, 166 509
–515 1990). Google Scholar
M. Sauvage et al.,
“Sub-arcsecond far-infrared space observatory: a science imperative,”
Submission to ESA Science Programme M-class Mission Call,
(2013). Google Scholar
D. Leisawitz et al.,
“Advancing toward far-infrared interferometry in space through coordinated international efforts,”
Proc. SPIE, 8860 88600A
(2013). https://doi.org/10.1117/12.2024432 PSISDG 0277-786X Google Scholar
R. Juanola-Parramon,
“A far-infrared spectro-spatial space interferometer: instrument simulator and testbed implementation,”
Springer International Publishing,
(2016). Google Scholar
C. Kouveliotou et al.,
“Enduring quests-daring visions (NASA astrophysics in the next three decades),”
(2014). Google Scholar
N. R. Council, Astronomy and Astrophysics in the New Millennium, The National Academies Press, Washington, DC
(2001). Google Scholar
M. Harwit, D. Leisawitz and S. Rinehart,
“A far-infrared/submillimeter kilometer-baseline interferometer in space,”
New Astron. Rev., 50 228
–234
(2006). https://doi.org/10.1016/j.newar.2005.11.030 Google Scholar
D. Leisawitz et al.,
“The space infrared interferometric telescope (SPIRIT): high-resolution imaging and spectroscopy in the far-infrared,”
Adv. Space Res., 40 689
–703
(2007). https://doi.org/10.1016/j.asr.2007.05.081 Google Scholar
F. P. Helmich and R. J. Ivison,
“FIRI—a far-infrared interferometer,”
Exp. Astron., 23 245
–276
(2009). https://doi.org/10.1007/s10686-008-9100-2 Google Scholar
W. Wild et al.,
“ESPRIT: a study concept for a far-infrared interferometer in space,”
Proc. SPIE, 7013 70132R
(2008). https://doi.org/10.1117/12.789603 PSISDG 0277-786X Google Scholar
G. Durand et al.,
“TALC: a new deployable concept for a 20 m far-infrared space telescope,”
Proc. SPIE, 9143 91431A
(2014). https://doi.org/10.1117/12.2055895 PSISDG 0277-786X Google Scholar
M. Sauvage et al.,
“A development roadmap for critical technologies needed for TALC: a deployable 20 m annular space telescope,”
Proc. SPIE, 9904 99041L
(2016). https://doi.org/10.1117/12.2231867 PSISDG 0277-786X Google Scholar
K. Dohlen et al.,
“Design of a nano-satellite demonstrator of an infrared imaging space interferometer: the HyperCube,”
Proc. SPIE, 9146 914603
(2014). https://doi.org/10.1117/12.2057226 PSISDG 0277-786X Google Scholar
II N. M. Elias et al.,
“The mathematics of double-Fourier interferometers,”
Astrophys. J., 657 1178
–1200
(2007). https://doi.org/10.1086/509282 ASJOAB 0004-637X Google Scholar
D. Leisawitz et al.,
“Developing wide-field spatio-spectral interferometry for far-infrared space applications,”
Proc. SPIE, 8445 84450A
(2012). https://doi.org/10.1117/12.926812 PSISDG 0277-786X Google Scholar
W. F. Grainger et al.,
“Demonstration of spectral and spatial interferometry at thz frequencies,”
Appl. Opt., 51
(12), 2202
–2211
(2012). https://doi.org/10.1364/AO.51.002202 Google Scholar
P. Ade et al.,
“Progress in spectral-spatial interferometry at multi-thz frequencies: potential applications,”
in 2015 8th UK, Eur., China Millimeter Waves and THz Technol. Workshop (UCMMT),
1
–4
(2015). Google Scholar
C. Bracken et al.,
“Quasi-optical analysis of a far-infrared spatio-spectral space interferometer concept,”
Infrared Phys. Technol., 77 171
–178
(2016). https://doi.org/10.1016/j.infrared.2016.05.030 Google Scholar
S. A. Rinehart et al.,
“The space high angular resolution probe for the infrared (SHARP-IR),”
Proc. SPIE, 9904 99042L
(2016). https://doi.org/10.1117/12.2231790 PSISDG 0277-786X Google Scholar
D. Deming et al.,
“Spitzer transit and secondary eclipse photometry of GJ 436b,”
Astrophys. J., 667 L199
–L202
(2007). https://doi.org/10.1086/522496 ASJOAB 0004-637X Google Scholar
M. J. Griffin, J. J. Bock and W. K. Gear,
“Relative performance of filled and feedhorn-coupled focal-plane architectures,”
Appl. Opt., 41 6543
–6554
(2002). https://doi.org/10.1364/AO.41.006543 APOPAI 0003-6935 Google Scholar
T. Suzuki et al.,
“Development of ultra-low-noise TES bolometer arrays,”
J. Low Temp. Phys., 184 52
–59
(2016). https://doi.org/10.1007/s10909-015-1401-z Google Scholar
“Antenna-coupled TES bolometers used in BICEP2, Keck array, and spider,”
Astrophys. J., 812 176
(2015). https://doi.org/10.1088/0004-637X/812/2/176 ASJOAB 0004-637X Google Scholar
S. W. Henderson et al.,
“Readout of two-kilopixel transition-edge sensor arrays for Advanced ACTPol,”
Proc. SPIE, 9914 99141G
(2016). https://doi.org/10.1117/12.2233895 PSISDG 0277-786X Google Scholar
J. Hubmayr et al.,
“Design of 280 GHz feedhorn-coupled TES arrays for the balloon-borne polarimeter SPIDER,”
Proc. SPIE, 9914 99140V
(2016). https://doi.org/10.1117/12.2231896 PSISDG 0277-786X Google Scholar
R. J. Thornton et al.,
“The Atacama cosmology telescope: the polarization-sensitive ACTPol instrument,”
Astrophys. J. Suppl. Ser., 227 21
(2016). https://doi.org/10.3847/1538-4365/227/2/21 APJSA2 0067-0049 Google Scholar
K. L. Denis et al.,
“Fabrication of feedhorn-coupled transition edge sensor arrays for measurement of the cosmic microwave background polarization,”
J. Low Temp. Phys., 184 668
–673
(2016). https://doi.org/10.1007/s10909-015-1366-y Google Scholar
O. Noroozian et al.,
“High-resolution gamma-ray spectroscopy with a microwave-multiplexed transition-edge sensor array,”
Appl. Phys. Lett., 103 202602
(2013). https://doi.org/10.1063/1.4829156 Google Scholar
K. D. Irwin et al.,
“X-ray detection using a superconducting transition-edge sensor microcalorimeter with electrothermal feedback,”
Appl. Phys. Lett., 69 1945
–1947
(1996). https://doi.org/10.1063/1.117630 Google Scholar
D. A. Wollman et al.,
“Superconducting transition-edge-microcalorimeter X-ray spectrometer with 2 eV energy resolution at 1.5 keV,”
Nucl. Instrum. Methods Phys. Res. A, 444 145
–150
(2000). https://doi.org/10.1016/S0168-9002(99)01351-0 Google Scholar
S. J. Smith et al.,
“Transition-edge sensor pixel parameter design of the microcalorimeter array for the x-ray integral field unit on Athena,”
Proc. SPIE, 9905 99052H
(2016). https://doi.org/10.1117/12.2231749 PSISDG 0277-786X Google Scholar
L. Gottardi et al.,
“Development of the superconducting detectors and read-out for the X-IFU instrument on board of the X-ray observatory Athena,”
Nucl. Instrum. Methods Phys. Res. A, 824 622
–625
(2016). https://doi.org/10.1016/j.nima.2015.09.072 Google Scholar
W. S. Holland et al.,
“SCUBA-2: the 10 000 pixel bolometer camera on the James Clerk Maxwell Telescope,”
MNRAS, 430 2513
–2533
(2013). https://doi.org/10.1093/mnras/sts612 Google Scholar
D. J. Goldie et al.,
“Ultra-low-noise transition edge sensors for the SAFARI L-band on SPICA,”
Proc. SPIE, 8452 84520A
(2012). https://doi.org/10.1117/12.925861 PSISDG 0277-786X Google Scholar
D. J. Goldie et al.,
“Performance of horn-coupled transition edge sensors for L- and S-band optical detection on the SAFARI instrument,”
Proc. SPIE, 9914 99140A
(2016). https://doi.org/10.1117/12.2232740 PSISDG 0277-786X Google Scholar
P. Khosropanah et al.,
“Ultra-low noise TES bolometer arrays for SAFARI instrument on SPICA,”
Proc. SPIE, 9914 99140B
(2016). https://doi.org/10.1117/12.2233472 PSISDG 0277-786X Google Scholar
R. A. Hijmering et al.,
“Readout of a 176 pixel FDM system for SAFARI TES arrays,”
Proc. SPIE, 9914 99141C
(2016). https://doi.org/10.1117/12.2231714 PSISDG 0277-786X Google Scholar
A. D. Beyer et al.,
“Development of fast, background-limited transition-edge sensors for the background-limited infrared/sub-mm spectrograph (BLISS) for SPICA,”
Proc. SPIE, 8452 84520G
(2012). https://doi.org/10.1117/12.926326 PSISDG 0277-786X Google Scholar
B. S. Karasik et al.,
“Normal metal hot-electron nanobolometer with Johnson noise thermometry readout,”
IEEE Trans. Terahertz Sci. Technol., 5
(1), 16
–21
(2014). Google Scholar
K. D. Irwin and G. C. Hilton, Transition-Edge Sensors, 63 2005). Google Scholar
P. K. Day et al.,
“A broadband superconducting detector suitable for use in large arrays,”
Nature, 425 817
–821
(2003). https://doi.org/10.1038/nature02037 Google Scholar
J. Zmuidzinas,
“Superconducting microresonators: physics and applications,”
Ann. Rev. Condens. Matter Phys., 3
(1), 169
–214
(2012). https://doi.org/10.1146/annurev-conmatphys-020911-125022 Google Scholar
L. J. Swenson et al.,
“MAKO: a pathfinder instrument for on-sky demonstration of low-cost 350 micron imaging arrays,”
Proc. SPIE, 8452 84520P
(2012). https://doi.org/10.1117/12.926223 PSISDG 0277-786X Google Scholar
P. R. Maloney et al.,
“MUSIC for sub/millimeter astrophysics,”
Proc. SPIE, 7741 77410F
(2010). https://doi.org/10.1117/12.857751 PSISDG 0277-786X Google Scholar
S. Heyminck et al.,
“Development of a MKID Camera for APEX,”
in Twenty-First Int. Symp. Space Terahertz Technol.,
262
(2010). Google Scholar
A. Monfardini et al.,
“NIKA: a millimeter-wave kinetic inductance camera,”
Astron. Astrophys., 521 A29
(2010). https://doi.org/10.1051/0004-6361/201014727 AAEJAF 0004-6361 Google Scholar
A. Monfardini et al.,
“A dual-band millimeter-wave kinetic inductance camera for the IRAM 30 m telescope,”
Astrophys. J. Suppl. Ser., 194 24
(2011). https://doi.org/10.1088/0067-0049/194/2/24 APJSA2 0067-0049 Google Scholar
R. Adam et al.,
“The NIKA2 large-field-of-view millimetre continuum camera for the 30 m IRAM telescope,”
Astron. Astrophys., 609 A115
(2018). https://doi.org/10.1051/0004-6361/201731503 AAEJAF 0004-6361 Google Scholar
G. Cataldo et al.,
“Micro-spec: an ultracompact, high-sensitivity spectrometer for far-infrared and submillimeter astronomy,”
Appl. Opt., 53
(6), 1094
–1102
(2014). https://doi.org/10.1364/AO.53.001094 Google Scholar
E. M. Barrentine et al.,
“Design and performance of a high resolution spec: an integrated sub-millimeter spectrometer,”
Proc. SPIE, 9914 99143O
(2016). https://doi.org/10.1117/12.2234462 Google Scholar
E. Shirokoff et al.,
“Design and performance of superspec: an on-chip, kid-based, mm-wavelength spectrometer,”
J. Low Temp. Phys., 176
(5–6), 657
–662
(2014). https://doi.org/10.1007/s10909-014-1122-8 Google Scholar
A. Endo et al.,
“Development of DESHIMA: a redshift machine based on a superconducting on-chip filterbank,”
Proc. SPIE, 8452 84520X
(2012). https://doi.org/10.1117/12.925637 PSISDG 0277-786X Google Scholar
B. A. Mazin et al.,
“A superconducting focal plane array for ultraviolet, optical, and near-infrared astrophysics,”
Opt. Express, 20 1503
(2012). https://doi.org/10.1364/OE.20.001503 Google Scholar
B. A. Mazin et al.,
“ARCONS: a 2024 pixel optical through near-IR cryogenic imaging spectrophotometer,”
Publ. Astron. Soc. Pac., 125 1348
–1361
(2013). https://doi.org/10.1086/674013 PASPAU 0004-6280 Google Scholar
S. Meeker et al.,
“Design and development status of mkid integral field spectrographs for high contrast imaging,”
in Adapt. Opt. Extrem. Large Telesc. 4–Conf. Proc.,
(2015). Google Scholar
B. A. Mazin et al.,
“MKIDs for direct imaging of exoplanets,”
AAS/Div. Extreme Solar Syst. Abstr., 3 104.07
(2015). Google Scholar
B. A. Mazin et al.,
“Science with KRAKENS,”
(2015). Google Scholar
T. Cook et al.,
“Planetary imaging concept testbed using a recoverable experiment-coronagraph (picture c),”
J. Astron. Telesc. Instrum. Syst., 1
(4), 044001
(2015). https://doi.org/10.1117/1.JATIS.1.4.044001 Google Scholar
M. Calvo et al.,
“Development of Kinetic Inductance Detectors for Cosmic Microwave Background experiments,”
Exp. Astron., 28 185
–194
(2010). https://doi.org/10.1007/s10686-010-9197-y Google Scholar
K. Karatsu et al.,
“Development of 1000 arrays MKID camera for the CMB observation,”
Proc. SPIE, 8452 84520Q
(2012). https://doi.org/10.1117/12.925775 PSISDG 0277-786X Google Scholar
H. McCarrick et al.,
“Horn-coupled, commercially-fabricated aluminum lumped-element kinetic inductance detectors for millimeter wavelengths,”
Rev. Sci. Instrum., 85
(12), 123117
(2014). https://doi.org/10.1063/1.4903855 Google Scholar
A. E. Lowitz et al.,
“Design, fabrication, and testing of lumped element kinetic inductance detectors for 3 mm CMB Observations,”
Proc. SPIE, 9153 91532R
(2014). https://doi.org/10.1117/12.2057102 PSISDG 0277-786X Google Scholar
S. Oguri et al.,
“Groundbird: observing cosmic microwave polarization at large angular scale with kinetic inductance detectors and high-speed rotating telescope,”
J. Low Temp. Phys., 184
(3-4), 786
–792
(2016). https://doi.org/10.1007/s10909-015-1420-9 Google Scholar
P. J. de Visser et al.,
“Fluctuations in the electron system of a superconductor exposed to a photon flux,”
Nat. Commun., 5 3130
(2014). https://doi.org/10.1038/ncomms4130 Google Scholar
M. Griffin et al.,
“SPACEKIDS: kinetic inductance detectors for space applications,”
Proc. SPIE, 9914 991407
(2016). https://doi.org/10.1117/12.2231100 PSISDG 0277-786X Google Scholar
A. Monfardini et al.,
“Lumped element kinetic inductance detectors for space applications,”
Proc. SPIE, 9914 99140N
(2016). https://doi.org/10.1117/12.2231758 PSISDG 0277-786X Google Scholar
M. D. Shaw et al.,
“Quantum capacitance detector: a pair-breaking radiation detector based on the single Cooper-pair box,”
Phys. Rev. B, 79 144511
(2009). https://doi.org/10.1103/PhysRevB.79.144511 Google Scholar
J. Bueno et al.,
“Proof of concept of the quantum capacitance detector,”
Appl. Phys. Lett., 96 103503
(2010). https://doi.org/10.1063/1.3339163 Google Scholar
J. Bueno et al.,
“Optical characterization of the quantum capacitance detector at ,”
Appl. Phys. Lett., 99 173503
(2011). https://doi.org/10.1063/1.3651277 Google Scholar
K. Stone et al.,
“Real time quasiparticle tunneling measurements on an illuminated quantum capacitance detector,”
Appl. Phys. Lett., 100
(26), 263509
(2012). https://doi.org/10.1063/1.4731880 Google Scholar
P. M. Echternach et al.,
“Photon shot noise limited detection of terahertz radiation using a quantum capacitance detector,”
Appl. Phys. Lett., 103 053510
(2013). https://doi.org/10.1063/1.4817585 Google Scholar
P. M. Echternach et al.,
“Single photon detection of 1.5 THz radiation with the quantum capacitance detector,”
Nat. Astron., 2 90
–97
(2018). https://doi.org/10.1038/s41550-017-0294-y Google Scholar
R. Ramaswami, K. Sivarajan and G. Sasaki, Optical Networks: A Practical Perspective, 3rd ed.Morgan Kaufmann Publishers Inc., San Francisco, California
(2009). Google Scholar
D. K. Sparacin et al.,
“Trimming of microring resonators by photo-oxidation of a plasma-polymerized organosilane cladding material,”
Opt. Lett., 30 2251
–2253
(2005). https://doi.org/10.1364/OL.30.002251 Google Scholar
J. Schrauwen, D. van Thourhout and R. Baets,
“Trimming of silicon ring resonator by electron beam induced compaction and strain,”
Opt. Express, 16 3738
(2008). https://doi.org/10.1364/OE.16.003738 Google Scholar
A. H. Atabaki et al.,
“Accurate post-fabrication trimming of ultra-compact resonators on silicon,”
Opt. Express, 21 14139
(2013). https://doi.org/10.1364/OE.21.014139 Google Scholar
R. Klein et al.,
“FIFI LS: the far-infrared integral field spectrometer for SOFIA,”
Proc. SPIE, 6269 62691F
(2006). https://doi.org/10.1117/12.671505 PSISDG 0277-786X Google Scholar
A. Poglitsch et al.,
“The MPE/UCB far-infrared imaging Fabry-Perot Interferometer (FIFI),”
Int. J. Infrared Millimeter Waves, 12
(8), 859
–884
(1991). https://doi.org/10.1007/BF01009647 Google Scholar
J. Zmuidzinas,
“Thermal noise and correlations in photon detection,”
Appl. Opt., 42 4989
–5008
(2003). https://doi.org/10.1364/AO.42.004989 APOPAI 0003-6935 Google Scholar
S. C. Parshley et al.,
“A miniature cryogenic scanning Fabry–Perot interferometer for mid-IR to submm astronomical observations,”
Proc. SPIE, 9147 914745
(2014). https://doi.org/10.1117/12.2057169 PSISDG 0277-786X Google Scholar
A. Kovács et al.,
“SuperSpec: design concept and circuit simulations,”
Proc. SPIE, 8452 84522G
(2012). https://doi.org/10.1117/12.927160 PSISDG 0277-786X Google Scholar
J. Wheeler et al.,
“SuperSpec: development towards a full-scale filter bank,”
Proc. SPIE, 9914 99143K
(2016). https://doi.org/10.1117/12.2233798 PSISDG 0277-786X Google Scholar
E. Shirokoff et al.,
“MKID development for SuperSpec: an on-chip, mm-wave, filter-bank spectrometer,”
Proc. SPIE, 8452 84520R
(2012). https://doi.org/10.1117/12.927070 PSISDG 0277-786X Google Scholar
S. Hailey-Dunsheath et al.,
“Status of SuperSpec: a broadband, on-chip millimeter-wave spectrometer,”
Proc. SPIE, 9153 91530M
(2014). https://doi.org/10.1117/12.2057229 PSISDG 0277-786X Google Scholar
S. Bryan et al.,
“WSPEC: a waveguide filter-bank focal plane array spectrometer for millimeter wave astronomy and cosmology,”
J. Low Temp. Phys., 184 114
–122
(2016). https://doi.org/10.1007/s10909-015-1396-5 Google Scholar
R. Schieder et al.,
“The potential of IR-heterodyne spectroscopy,”
The Power of Optical/IR Interferometry: Recent Scientific Results and 2nd Generation, 465
–471 2008). Google Scholar
P. F. Goldsmith,
“Sub-millimeter heterodyne focal-plane arrays for high-resolution astronomical spectroscopy,”
URSI Radio Sci. Bull., 362 53
–73
(2017). https://doi.org/10.23919/URSIRSB.2017.8267373 Google Scholar
T. M. Klapwijk and A. V. Semenov,
“Engineering physics of superconducting hot-electron bolometer mixers,”
IEEE Trans. Terahertz Sci. Technol., 7 627
–648
(2017). https://doi.org/10.1109/TTHZ.2017.2758267 Google Scholar
T. de Graauw et al.,
“The Herschel-heterodyne instrument for the far-infrared (HIFI),”
Astron. Astrophys., 518 L6
(2010). https://doi.org/10.1051/0004-6361/201014698 AAEJAF 0004-6361 Google Scholar
P. R. Roelfsema et al.,
“In-orbit performance of Herschel-HIFI,”
Astron. Astrophys., 537 A17
(2012). https://doi.org/10.1051/0004-6361/201015120 AAEJAF 0004-6361 Google Scholar
D. Rigopoulou et al.,
“The far infrared spectroscopic explorer (FIRSPEX): probing the lifecycle of the ISM in the universe,”
Proc. SPIE, 9904 99042K
(2016). https://doi.org/10.1117/12.2233593 PSISDG 0277-786X Google Scholar
P. F. Goldsmith,
“Radio telescopes and measurements at radio wavelengths,”
Single-Dish Radio Astronomy: Techniques and Applications, 278 45
–79 2002). Google Scholar
C. Groppi et al.,
“Test and integration results from SuperCam: a 64-pixel array receiver for the 350 GHz atmospheric window,”
Proc. SPIE, 7741 77410X
(2010). https://doi.org/10.1117/12.857504 PSISDG 0277-786X Google Scholar
D. J. Hayton et al.,
“A 4.7 THz heterodyne receiver for a balloon borne telescope,”
Proc. SPIE, 9153 91531R
(2014). https://doi.org/10.1117/12.2055790 PSISDG 0277-786X Google Scholar
H. Richter et al.,
“4.7-thz local oscillator for the great heterodyne spectrometer on sofia,”
IEEE Trans. Terahertz Sci. Technol., 5
(4), 539
–545
(2015). https://doi.org/10.1109/TTHZ.2015.2442155 Google Scholar
H. Richter et al.,
“Performance of the 4.7thz local oscillator with quantum cascade laser on board sofia,”
in 40th Int. Conf. Infrared, Millimeter, and Terahertz Waves (IRMMW-THz),
1
(2015). Google Scholar
A. Poglitsch et al.,
“The MPE/UCB far-infrared imaging Fabry–Perot interferometer (FIFI),”
Int. J. Infrared Millimeter Waves, 12 859
–884
(1991). https://doi.org/10.1007/BF01009647 Google Scholar
G. J. Stacey et al.,
“KWIC: a widefield mid-infrared array camera/spectrometer for the KAO,”
Proc. SPIE, 1946 238
–248
(1993). https://doi.org/10.1117/12.158676 PSISDG 0277-786X Google Scholar
T. de Graauw et al.,
“Observing with the ISO Short-Wavelength Spectrometer,”
Astron. Astrophys., 315 L49
–L54
(1996). AAEJAF 0004-6361 Google Scholar
P. E. Clegg et al.,
“The ISO long-wavelength spectrometer,”
Astron. Astrophys., 315 L38
–L42
(1996). AAEJAF 0004-6361 Google Scholar
C. M. Bradford et al.,
“SPIFI: a direct-detection imaging spectrometer for submillimeter wavelengths,”
Appl. Opt., 41 2561
–2574
(2002). https://doi.org/10.1364/AO.41.002561 APOPAI 0003-6935 Google Scholar
F. A. Pepe et al.,
“Liquid-helium cooled scanning far-infrared Fabry-Perot interferometer for astronomical observations with a balloon-borne telescope,”
Infrared Phys. Technol., 35 863
–871
(1994). https://doi.org/10.1016/1350-4495(94)90053-1 Google Scholar
N. Ismail et al.,
“Fabry-perot resonator: spectral line shapes, generic and related airy distributions, linewidths, finesses, and performance at low or frequency-dependent reflectivity,”
Opt. Express, 24 16366
–16389
(2016). https://doi.org/10.1364/OE.24.016366 OPEXFF 1094-4087 Google Scholar
T. G. Hawarden et al.,
“Optimised radiative cooling of infrared space telescopes and applications to possible missions,”
Space Sci. Rev., 61 113
–144
(1992). https://doi.org/10.1007/BF00212480 SPSRA4 0038-6308 Google Scholar
Jr. H. A. Thronson et al.,
“The Edison infrared space observatory,”
Space Sci. Rev., 74 139
–144
(1995). https://doi.org/10.1007/BF00751262 SPSRA4 0038-6308 Google Scholar
R. G. Ross, D. L. Johnson,
“NASA’s advanced cryocooler technology development program (ACTDP),”
Advances in Cryogenic Engineering: Transactions of the Cryogenic Engineering Conference, 823 607
–614 2006). Google Scholar
P. J. Shirron et al.,
“Design and on-orbit operation of the adiabatic demagnetization refrigerator on the Hitomi Soft X-ray Spectrometer instrument,”
Proc. SPIE, 9905 99053O
(2016). https://doi.org/10.1117/12.2231301 PSISDG 0277-786X Google Scholar
R. G. Ross, R. F. Boyle and P. Kittel,
“NASA space cryocooler programs—a 2003 overview,”
Am. Inst. Phys. Conf. Ser., 710 1197
–1204 2004). Google Scholar
P. Shirron et al.,
“A compact, high-performance continuous magnetic refrigerator for space missions,”
Cryogenics, 41
(11), 789
–795
(2001). https://doi.org/10.1016/S0011-2275(01)00164-3 CRYOAX 0011-2275 Google Scholar
J. Tuttle et al.,
“Development of a space-flight ADR providing continuous cooling at 50 mK with heat rejection at 10 K,”
Mater. Sci. Eng. Conf. Ser., 278 012009
(2017). https://doi.org/10.1088/1757-899X/278/1/012009 Google Scholar
C. K. Walker et al.,
“10 meter sub-orbital large balloon reflector (LBR),”
in IEEE Aerosp. Conf.,
1
–7
(2014). Google Scholar
D. Lesser et al.,
“10 meter sub-orbital large balloon reflector (LBR),”
in 40th Int. Conf. Infrared, Millimeter, and Terahertz Waves (IRMMW-THz),
1
–2
(2015). Google Scholar
G. Cortes-Medellin et al.,
“Optical design for the large balloon reflector,”
Proc. SPIE, 9906 99061Y
(2016). https://doi.org/10.1117/12.2233861 PSISDG 0277-786X Google Scholar
J. P. Maillard et al.,
“Integral wide-field spectroscopy in astronomy: the Imaging FTS solution,”
Exp. Astron., 35 527
–559
(2013). https://doi.org/10.1007/s10686-013-9330-9 Google Scholar
F. Boulanger et al.,
“The molecular hydrogen explorer H2EX,”
Exp. Astron., 23 277
–302
(2009). https://doi.org/10.1007/s10686-008-9108-7 Google Scholar
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