The Sloan Digital Sky Survey V (SDSS–V) is an all-sky spectroscopic survey of <6 million objects, designed to decode the history of the Milky Way, reveal the inner workings of stars, investigate the origin of solar systems, and track the growth of supermassive black holes across the universe. The Local Volume Mapper (LVM) is a facility designed to provide a contiguous 2500 deg2 integral-field survey over a 3.5 year period from Las Campa˜nas Observatory (LCO) in Chile. The facility comprises four 0.16 m bench-mounted telescopes that feed three multiobject spectrographs with 1801 science fibres, 119 calibration fibres, and 24 sky-background fibres. The fibre cable spans approximately 20 meters from the telescope platform to the spectrograph slits. A sorting hat, located in the spectrograph room, redistributes the 1944 fibres into three 648–element bundles that terminate at the spectrograph slits. In this paper, we briefly summarize the current production progress of the integral-field units, the spectrograph slits, and the sorting hat.
This paper presents an update on the construction, testing, and commissioning of the SDSS-V Local Volume Mapper (LVM) telescope system. LVM is one of three surveys that form the fifth generation of the Sloan Digital Sky Survey, and it will employ a coordinated network of four, 16-cm telescopes feeding three fiber spectrographs at the Las Campanas Observatory. The goal is to spectrally map approximately 2500 square degrees of the Galactic plane with 37” spatial resolution and R~4000 spectral resolution over the wavelength range 360-980 nm. LVM will also target the Magellanic Clouds and other Local Group galaxies. Each of the four LVM telescopes consists of a two-mirror siderostat in alt-alt configuration feeding an optical breadboard. This produces a fixed, stable focal plane for the fiber-based Integral Field Unit (IFU). One telescope hosts the science IFU, while two others observe adjacent fields to calibrate geocoronal emission. The fourth telescope makes rapid observations of bright stars to compensate telluric absorption. The entrance slits of the spectrographs intersperse the fibers from all three types of telescope, producing truly simultaneous science and calibration exposures. We summarize the final design of the telescope system and report on its construction, alignment and testing in the laboratory. We also describe our deployment plan for commissioning at LCO, anticipated for late 2022.
The Sloan Digital Sky Survey V (SDSS-V) is an all-sky spectroscopic survey of >6 million objects, designed to decode the history of the Milky Way, reveal the inner workings of stars, investigate the origin of solar systems, and track the growth of supermassive black holes across the Universe. The Local Volume Mapper (LVM) is a facility designed to provide a contiguous 2500 deg2 integral-field survey over a 3.5 year period from Las Campanas Observatory (LCO) in Chile. The facility comprises four small (16 cm) telescopes that deliver science, calibration, and spectro-photometric light to three bench-mounted multi-object spectrographs, designed and build by Winlight Systems. All four telescopes will be equipped with a microlens array integral-field unit (IFU) to slice the focal plane into 35–arcsec large spatial elements while maintaining near-telecentric coupling at the fiber input. The science IFU comprises 1801 fibers, additional 143 fibers are allocated for sky-background and spectro-photometric calibration, totaling 1944 fibers. Each spectrograph will be fed by 648 fibers, which are reformatted into a linear array, forming the entrance slit. In this paper, we present the opto-mechanical design of the LVM-LCO fiber cable system.
The Sloan Digital Sky Survey V (SDSS-V) is an all-sky spectroscopic survey of <6 million objects, designed to decode the history of the Milky Way, reveal the inner workings of stars, investigate the origin of solar systems, and track the growth of supermassive black holes across the Universe. The Local Volume Mapper (LVM) is a facility designed to provide a contiguous 2,500 deg2 integral-field survey over a 3.5 year period from Las Campanas Observatory in Chile. In this paper we provide an overview and status update for the LVM instrument (hereafter LVM-I). Each integral-field unit’s spaxel probes linear scales that are sub-parsec (Milky Way) to ∼10 pc (Magellanic Clouds) which is accomplished with an angular diameter of 36.900. LVM’s spectral resolution is R = λ/∆λ ∼ 4, 000 which probes velocities of 33 kms−1 (1 σ) from 365 nm to 950 nm. LVM uses four 16-cm telescopes feeding three spectrographs. One telescope carries the bulk of the science load with ∼1,800 fibers coupled to the field via a pair of lenslet arrays, two telescopes are used to measure the night sky spectra in fields that flank the science field, and a fourth telescope contemporaneously monitors bright standard stars to determine atmospheric extinction. We expect LVM-I to deliver percent-level precision on important line ratios down to a few Rayleigh. The three spectrographs are being built by Winlight corporation in France based on those for the Dark Energy Spectroscopic Instrument (DESI). In this paper we present the high-level system design of LVM-I including the lenslet-coupled fiber IFUs, telescopes, guiding+acquisition system, calibration systems, enclosures, and spectrographs.
The Keck Planet Finder (KPF) is a fiber-fed, high-resolution, high-stability spectrometer in development at the UC Berkeley Space Sciences Laboratory for the W.M. Keck Observatory. KPF is designed to characterize exoplanets via Doppler spectroscopy with a goal of a single measurement precision of 0.3 m s-1 or better, however its resolution and stability will enable a wide variety of astrophysical pursuits. Here we provide post-preliminary design review design updates for several subsystems, including: the main spectrometer, the fabrication of the Zerodur optical bench; the data reduction pipeline; fiber agitator; fiber cable design; fiber scrambler; VPH testing results and the exposure meter.
KEYWORDS: Spectrographs, New and emerging technologies, Velocity measurements, Single mode fibers, Adaptive optics, Near infrared, Cameras, Sensors, Optical design, Relays
Small diameter single-mode fiber (SMF) allows for the design of compact spectrographs that operate at the diffraction limit. The small instrument scale, in turn, allows cost-effective configuration flexibility to use the instrument as a testbed for infrared SMF spectrograph technologies. The same base instrument could be coupled to different adaptive optics (AO) and non-AO SMF feeds. We present the build for such a spectrograph, Iranti, which works in the near-infrared (NIR) range and incorporates novel techniques. Our implementation of this instrument has sufficient cross dispersion to allow testing of a range of input fiber links, including multiple fibers or multi-core fibers (MCFs); the camera optics and detector can also be swapped out easily for different wavelength ranges. The base system uses a white pupil design that relays a slow beam between the collimator, an R6 echelle grating with 13.33 lines/mm and a volume phase holographic grating (VPH) as a cross disperser. In Iranti, we also address mechanical and thermal considerations to improve stability in the instrument. We configure the instrument for ranges in 800 to 1300 nm and characterize system efficiency and stability.
The Sloan Digital Sky Survey V (SDSS-V) is an all-sky spectroscopic survey of <6 million objects, designed to decode the history of the Milky Way, reveal the inner workings of stars, investigate the origin of solar systems, and track the growth of supermassive black holes across the Universe. The Local Volume Mapper (LVM) is one of three surveys that form SDSS-V. LVM will employ a coordinated system of four telescopes feeding three fiber spectrographs at Las Campanas Observatory in Chile. The goal is to map approximately 2500 square degrees of the Galactic plane over the wavelength range 360-980 nm with R~4000 spectral resolution. These observations will reveal for the first time how distinct gaseous environments within the Galaxy interact with each other and with the stellar population, producing the large-scale interstellar medium that we observe. Accurately mapping and calibrating a substantial portion of the sky at this spatial resolution requires a unique type of telescope system. Each of the four LVM telescopes has a diameter of 16 cm, making them considerably smaller and lighter than the instruments they feed. One telescope will host the science IFU containing ~1800 fibers arranged in a close-packed hexagon. Two additional Calibration telescopes will observe fields adjacent to the science IFU, in order to calibrate out terrestrial airglow and other geo-coronal emission. The fourth, Spectrophotometric telescope will make rapid observations of bright stars (typically 12 during a single IFU / Calibration exposure) to correct for telluric absorption lines and overall extinction. The fibers from all three types of telescope will be interspersed in the entrance slits of the spectrographs, allowing for simultaneous science and calibration exposures. Although considerably smaller than the next generation of giants, the LVM telescopes must also operate close to the limits of physical optics, and the geometry and scope of the LVM survey present unique challenges. For example, with this type of telescope at the Las Campanas site, the effects of optical aberrations, diffraction, seeing, and (uncorrected) atmospheric dispersion are all of comparable scale. This, coupled with the need for repeated and reliable measurements over years, leads to some unconventional design choices. This paper presents the preliminary design of the LVM telescope system and discusses the requirements and tradeoffs that led to the baseline choices.
In the new era of Extremely Large Telescopes (ELTs) currently under construction, challenging requirements drive spectrograph designs towards techniques that efficiently use a facility's light collection power. Operating in the single-mode (SM) regime, close to the diffraction limit, reduces the footprint of the instrument compared to a conventional high-resolving power spectrograph. The custom built injection fiber system with 3D-printed microlenses on top of it for the replicable high-resolution exoplanet and asteroseismology spectrograph (RHEA) at Subaru in combination with extreme adaptive optics of SCExAO, proved its high efficiency in a lab environment, manifesting up to ~77% of the theoretical predicted performance.
Precise wavelength calibration is a critical issue for high-resolution spectroscopic observations. The ideal calibration source should be able to provide a very stable and dense grid of evenly distributed spectral lines of constant intensity. A new method which satisfies all mentioned conditions has been developed by our group. The approach is to actively measure the exact position of a single spectral line of a Fabry-Perot etalon with very high precision with a wavelength-tuneable laser and compare it to an extremely stable wavelength standard. The ideal choice of standard is the D2 absorption line of Rubidium, which has been used as an optical frequency standard for decades. With this technique, the problem of stable wavelength calibration of spectrographs becomes a problem of how reliably we can measure and anchor one etalon line to the Rb transition. In this work we present our self-built module for Rb saturated absorption spectroscopy and discuss its stability.
Ever more precise radial velocity instruments are needed to observe potential earth-like exoplanet targets that are beyond the range of current generation high resolution echelle spectrographs. Meanwhile, extreme adaptive optics systems at 8 meter class facilities have made ground based observations possible at the diffraction limit. In the field of Doppler spectroscopy, one way to take advantage of these AO capabilities is by the development of ultra-stable single mode fiber fed spectrographs.1 Coupling the light efficiently into SMFs with an extreme adaptive optics system offers significant advantage in removing modal noise, reducing instrument size, enabling superior environmental control and curbing cost. We report the design and challenges in building an ultra-stable spectrograph for the near infrared range. The design wavelength range is 650 to 1500 nm.
Precise wavelength calibration is a persistent problem for highest precision Doppler spectroscopy. The ideal calibrator provides an extremely stable spectrum of equidistant, narrow lines over a wide bandwidth, is reliable over timescales of years, and is simple to operate. Unlike traditional hollow cathode lamps, etalons provide an engineered spectrum with adjustable line distance and width and can cover a very broad spectral bandwidth. We have shown that laser locked etalons provide the necessary stability with an ideal spectral format for calibrating precision Echelle spectrographs, in a cost-effective and robust package. Anchoring the etalon spectrum to a very precisely known hyperfine transition of rubidium delivers cm/s-level stability over timescales of years. We have engineered a fieldable system which is currently being constructed as calibrator for the MAROON-X, HERMES, KPF, FIES and iLocater spectrographs.
RHEA is a compact high-resolution single-mode spectrograph that can easily be produced in larger quantities as budgets allow. The instrument will be housed in a temperature-stabilized vacuum chamber which is surrounded by several layers of thermal shielding. The optical design employs cost-effective commercially available compo- nents, a cooled CMOS detector, and a double-fiber input which permits simultaneous wavelength calibration.
RHEA is a single-mode ´echelle spectrograph designed to be a replicable and cost effective method of undertaking precision radial velocity measurements. The instrument has a novel fiber feed with an integral field unit injecting into a grid of single-mode fibers reformatted to form a pseudo-slit, increasing throughput and enabling highspatial resolution observations when operating behind Subaru and the SCExAO adaptive optics system. The past 18 months have seen a replacement cable constructed for the instrument to address modal noise caused by closely packed fibers with similar path lengths. Here we detail the cable fabrication procedure, design improvements, increased precision in meeting the required sub-micron optical tolerances, throughput gains, and known remaining issues.
High precision Doppler observations of bright stars can be made efficiently with small aperture telescopes. We are constructing a high resolution echelle spectrograph for the new 0.6 m telescope at Central Washington University. The spectrograph is fed by a multimode fiber and operates in the visible wavelength range of 380-670 nm. The spectrograph uses a white pupil design with 100 mm beam diameter and a monolithic R4 echelle grating.
The Keck Planet Finder (KPF) is a fiber-fed, high-resolution, high-stability spectrometer in development for the W.M. Keck Observatory. The instrument recently passed its preliminary design review and is currently in the detailed design phase. KPF is designed to characterize exoplanets using Doppler spectroscopy with a single measurement precision of 0.5 m s−1 or better; however, its resolution and stability will enable a wide variety of other astrophysical pursuits. KPF will have a 200 mm collimated beam diameter and a resolving power greater than 80,000. The design includes a green channel (445 nm to 600 nm) and red channel (600 nm to 870 nm). A novel design aspect of KPF is the use of a Zerodur optical bench, and Zerodur optics with integral mounts, to provide stability against thermal expansion and contraction effects.
The RHEA Spectrograph is a single-mode echelle spectrograph designed to be a replicable and cost effective method of undertaking precision radial velocity measurements. Two versions of RHEA currently exist, one located at the Australian National University in Canberra, Australia (450 - 600nm wavelength range), and another located at the Subaru Telescope in Hawaii, USA (600 - 800 nm wavelength range). Both instruments have a novel fibre feed consisting of an integral field unit injecting light into a 2D grid of single mode fibres. This grid of fibres is then reformatted into a 1D array at the input of the spectrograph (consisting of the science fibres and a reference fibre capable of receiving a white-light or xenon reference source for simultaneous calibration). The use of single mode fibres frees RHEA from the issue of modal noise and significantly reduces the size of the optics used. In addition to increasing the overall light throughput of the system, the integral field unit allows for cutting edge science goals to be achieved when operating behind the 8.2m Subaru Telescope and the SCExAO adaptive optics system. These include, but are not limited to: resolved stellar photospheres; resolved protoplanetary disk structures; resolved Mira shocks, dust and winds; and sub-arcsecond companions. We present details and results of early tests of RHEA@Subaru and progress towards the stated science goals.
The Replicable High-resolution Exoplanet and Asteroseismology (RHEA) spectrograph is being developed to serve as a basis for multiple copies across a network of small robotic telescopes. The spectrograph operates at the diffraction-limit by using a single-mode fiber input, resulting in a compact and modal-noise-free unit. The optical design is mainly based on off-the-shelf available components and comprises a near-Littrow configuration with prism cross-disperser. The échelle format covers a wavelength range of 430-650 nm at R=75,000 resolving power. In this paper we briefly summarize the current status of the instrument and present preliminary results from the first on-sky demonstration of the prototype using a fully automated 16" telescope, where we observe stable and semi-variable stars up to V=3.5 magnitude. Future steps to enhance the efficiency and passive stability of RHEA are discussed in detail. For example, we show the concept of using a multi-fiber injection unit, akin to a photonic lantern, which not only enables increased throughput but also offers simultaneous wavelength calibration.
SCExAO is the premier high-contrast imaging platform for the Subaru Telescope. It offers high Strehl ratios at near-IR wavelengths (y-K band) with stable pointing and coronagraphs with extremely small inner working angles, optimized for imaging faint companions very close to the host. In the visible, it has several interferometric imagers which offer polarimetric and spectroscopic capabilities. A recent addition is the RHEA spectrograph enabling spatially resolved high resolution spectroscopy of the surfaces of giant stars, for example. New capabilities on the horizon include post-coronagraphic spectroscopy, spectral differential imaging, nulling interferometry as well as an integral field spectrograph and an MKID array. Here we present the new modules of SCExAO, give an overview of the current commissioning status of each of the modules and present preliminary results.
We present the design for a high resolution near-infrared spectrograph. It is fed by a single-mode fiber coupled to a high performance adaptive optics system, leading to an extremely stable instrument with high total efficiency. The optical design is a cross-dispersed Echelle spectrograph based on a white pupil layout. The instrument uses a R6 Echelle grating with 13.3 grooves per mm, enabling very high resolution with a small beam diameter. The optical design is diffraction limited to enable optimal performance; this leads to subtle differences compared to spectrographs with large input slits.
We present a stable, inexpensive wavelength reference, based on a white-light interferometer for the use on current and future (arrays of) diffraction-limited radial velocity (RV) spectrographs. The primary aim of using an interferometer is to obtain a dense sinusoidal wavelength reference with spectral coverage between 450-650 nm. Its basic setup consists of an unbalanced fiber Mach-Zehnder interferometer (FMZI) that creates an interference pattern in the spectral domain due to superposition of phase delayed light, set by a fixed optical path-length difference (OPD). To achieve long-term stability, the interferometer is actively locked to a stable atomic line. The system operates in closed-loop using a thermo-optic modulator as the phase feedback component. We conducted stability measurements by superimposing the wavelength reference with thorium-argon (ThAr) emission lines
and found the differential RMS shift to be ~5 m s-1 within 30 minute bins in an experiment lasting 5 hours.
We present the opto-mechanical design and the characterization of the Replicable High-resolution Exoplanet and Asteroseismology (RHEA) spectrograph. RHEA is an ultra-compact fiber-fed echelle spectrograph designed to be used at 0.2-0.4 m class robotic telescopes where long term dedicated projects are possible. The instrument will be primarily used for radial velocity (RV) studies of low to intermediate-mass giant stars for the purpose of searching for hot Jupiters and using asteroseismology to simultaneously measure the host star parameters and de-correlate stellar pulsations. The optical design comprises a double-pass (i.e. near Littrow) configuration with
prism cross-disperser and single-mode fiber (SMF) input. The spectrograph has a resolving power of R>70,000 and operates at 430–670 nm with minimum order separation of ~180 μm. This separation allows a 1x6 photonic
lantern integration at a later stage which is currently under development. The current design is built with the aim of creating an inexpensive and replicable unit. The spectrograph is optimised for long-baseline RV observations through careful temperature stabilisation and simultaneous wavelength calibration. As a further improvement the echelle grating is housed in a vacuum chamber to maintain pressure stability. The performance of the current prototype is currently being tested on a 0.4 m telescope at the Macquarie University Observatory.
The Echelle spectrograph FOCES1 is currently located at the laboratories of Munich University Observatories
under pressure and temperature stabilized conditions. It is being used as a test bed for a number of different
stability issues related to high precision radial velocity spectroscopy.
We utilize FOCES to study spectrograph stability, illumination stability and fiber transport stability. With
this work we continue the series of papers that present our efforts to obtain temperature and pressure stabilization
in the spectrograph environment. In particular we present first optical measurement results and compare them
to simulations previously published. We show the movement of the image on the CCD with changes of pressure
and temperature and the stability of the spot positions in the stabilized system using measurements done by a
ThAr gas discharge source.
To improve our understanding of fiber scrambling properties a test bed where fiber near-field and far-field can be measured simultaneously is described. A variety of measurements has been conducted with a selection of fibers from different vendors, including state-of-the-art octagonal and hexagonal fibers. After characterization of the test bench with respect to stability and resolution, scrambling measurements have been conducted using LEDs with central wavelengths ranging between 465-635 nm. The dependence on wavelength regarding to photometrical scrambling has been initially demonstrated. Moreover, two mechanical combined fiber cables have been analyzed that were made from octagonal-circular and hexagonal-octagonal fiber sections. In this context an apparatus for focal ratio degradation (FRD) measurements was assembled to compare different shaped fibers and fiber combinations. Finally, all these preliminary investigations will help in choosing a fiber with good radial scrambling performance for the next generation fiber-link of the fiber optic coupled Cassegrain echelle spectrograph FOCES intended to be operated at the 2.0m Fraunhofer Telescope at the Wendelstein Observatory.
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