JATIS thanks the reviewers who served the journal in 2024.

Arcus is a high-resolution soft X-ray and far-ultraviolet spectroscopy mission submitted to the National Aeronautics and Space Administration’s inaugural Astrophysics Probe solicitation. Arcus makes simultaneous observations in these two critical wavelength regimes to address a broad range of science questions highlighted by the 2020 Astronomy and Astrophysics Decadal Survey, from the temperature and composition of the missing baryons in the intergalactic medium to the evolution of stars and their influence on orbiting planets. We present the science motivation for and performance of the Arcus ultraviolet spectrograph (UVS). UVS comprises a 60-cm, off-axis Cassegrain telescope feeding an imaging spectrograph operating over the 970- to 1580-Å bandpass. The instrument employs two interchangeable diffraction gratings to provide medium-resolution spectroscopy (R>20,000 in two grating modes centered at ∼1110 and 1390 Å). The spectra are recorded on an open-face, photon-counting microchannel plate detector. The instrument design achieves an end-to-end sensitivity >10 times that of the Far-Ultraviolet Spectroscopic Explorer over the key 1020- to 1150-Å range and offers arcsecond-level angular resolution spectral imaging over a 6-arcminute-long slit for observations of extended sources. We describe the example science investigations for far-ultraviolet spectroscopy on Arcus, the resultant instrument design and predicted performance, and simulated data from potential General Observer programs with Arcus.

The flight dynamics design for the Arcus Probe mission considers mission, science, and operational constraints pertaining to the launch, trajectory, science orbit, and decommissioning phases of the mission. Lunar resonance orbits offer benefits to mission operations, specifically with respect to science observations and data downlink opportunities. Post-separation implementation uses Earth phasing loops to raise and position the apogee for lunar swingby. Leveraging lunar gravity is a feature throughout the trajectory to the final science orbit, which similarly uses resonance with the Moon for maintenance. A detailed implementation approach constructs a viable solution set for the Arcus Probe mission. Trade studies assess the benefits of cis-lunar mission design for three examples of lunar resonance patterns.

The Arcus Probe mission concept has been submitted as an Astrophysics Probe Explorer candidate. It features two co-aligned high-resolution grating spectrometers: one for the soft X-ray band and one for the far ultraviolet. Together, these instruments can provide unprecedented performance to address important key questions about the structure and dynamics of our universe across a large range of length scales. The X-ray spectrometer consists of four parallel optical channels, each featuring an X-ray telescope with a fixed array of 216 lightweight, high-efficiency, blazed transmission gratings, and two charge-coupled device readout arrays. Average spectral resolving power λ/Δλ>2500 (3500 expected) across the 12 to 50 Å band and combined effective area >350 cm2 (>470 cm2 expected) near OVII wavelengths are predicted, based on the measured X-ray performance of the spectrometer prototypes and detailed ray trace modeling. We describe the optical and structural design of the grating arrays, from the macroscopic grating petals to the nanoscale gratings bars, grating fabrication, alignment, and X-ray testing. Recent X-ray diffraction efficiency results from chemically thinned grating bars are presented and show performance above mission assumptions.

Arcus is a concept for a National Aeronautics and Space Administration probe-class X-ray mission to deliver high-resolution Far Ultraviolet and X-ray spectroscopy with two separate instruments. We focus on the X-ray spectrograph (XRS). It consists of four spectral channels arranged in a double-tilted Rowland torus geometry. It combines cost-effective silicon pore optics with high-throughput critical-angle transmission gratings to achieve at least R>3000 in a bandpass from 12 to 50 Å. We present ray-tracing studies to derive performance characteristics such as the spectral resolving power and effective area and look at the best positioning of the four channels to improve the resiliency toward misalignments and reduce the overall impact of chip gaps. We study the effect of misalignments on the performance and present alignment requirements in 6 degrees of freedom for all optical elements in the XRS. We conclude that most tolerances can be achieved with mechanical means alone.

The Arcus X-ray spectrograph (XRS) will deliver high-resolution spectra in four telescope/grating modules (called “channels”) imaged onto the same set of detectors. We examine two effects (cross-dispersion from support structures in the diffraction gratings and frame transfer) that can lead to a photon being assigned to a different channel in the data reduction. Even within a channel, photons can be assigned the wrong diffraction order because orders are closely spaced in energy. Based on simulations, we show that cross-dispersion and frame transfer effects contribute <1% to the extracted signal and that order sorting is efficient enough that only very bright emission lines in a contaminating order can cause a visible signal. Finally, we describe a process to construct per-order non-X-ray background spectra for XRS simulations.

The Arcus Probe mission concept provides high-resolution soft X-ray and ultraviolet spectroscopy to reveal feedback-driven structure and evolution throughout the universe with an agile response capability ideal for probing the physics of time-dependent phenomena. The X-ray spectrograph utilizes two nearly identical charge-coupled device (CCD) focal planes to detect and record X-ray photons from the dispersed spectra and zero-order of the critical angle transmission gratings. We describe the Arcus focal plane instrument and the CCDs, including laboratory performance results, which meet the observatory requirements.

Black hole accretion in active galactic nuclei (AGN) is coupled to the evolution of their host galaxies. Outflowing winds in AGN can play an important role in this evolution through the resulting feedback mechanism. Multi-wavelength spectroscopy is key for probing the intertwined physics of inflows and outflows in AGN. However, with the current spectrometers, crucial properties of the ionized outflows are poorly understood, such as their coupling to accretion rate, their launching mechanism, and their kinetic power. We discuss the need for simultaneous X-ray and UV high-resolution spectroscopy for tackling outstanding questions on these outflows in AGN. The instrumental requirements for achieving the scientific objectives are addressed. We demonstrate that these requirements would be facilitated by the proposed Arcus Probe mission concept. The multi-wavelength spectroscopy and timing by Arcus would enable us to establish the kinematics and ionization structure of the entire ionized outflow, extending from the vicinity of the accretion disk to the outskirts of the host galaxy. Arcus would provide key diagnostics on the origin, driving mechanism, and energetics of the outflows, which are useful benchmarks for testing various theoretical models of outflows and understanding their impact in AGN.

Arcus Probe, an X-ray and UV spectroscopy mission proposed for the 2023 NASA Astrophysics Probe call, uses two co-aligned spectrometers—the X-ray spectrometer (XRS) and the UV spectrograph (UVS). We discuss the process of aligning the XRS and UVS such that they acquire simultaneous spectra of astrophysical sources during the observatory’s operation. The definition of each instrument’s field of view and line of sight is given, informing the alignment strategy of the XRS to the UVS. Comprehensive error budgets, beginning with the internal alignment of the XRS optics and proceeding through on-orbit commissioning, are detailed. The mechanism for achieving such alignment on orbit exceeds the required alignment tolerances by ≥14x in all six degrees of freedom, lending confidence that the alignment required for successful science operations can be achieved.

We outline the scientific motivation for reducing the systematics in the image sensors used in the LSST. Some examples are described, leading to lab investigations. The CCD250 (Teledyne-e2v) and STA3900 Imaging Technology Laboratory (ITL) charge-coupled devices (CCDs) used in Rubin Observatory’s LSSTCam are tested under realistic LSST f/1.2 optical beam in a lab setup. In the past, this facility has been used to characterize these CCDs, exploring the systematic errors due to charge transport. Now, this facility is being used to optimize the clocking scheme and voltages. The effect of different clocking schemes on the on-chip systematics such as non-linear crosstalk, noise, persistence, and photon transfer is explored. The goal is to converge on an optimal configuration for the LSSTCam CCDs, which minimizes resulting dark energy science systematics.

We present a candidate sensor for future spectroscopic applications, such as a Stage-5 Spectroscopic Survey Experiment or the Habitable Worlds Observatory. This type of charge-coupled device (CCD) sensor features multiple in-line amplifiers at its output stage allowing multiple measurements of the same charge packet, either in each amplifier or in the different amplifiers. Recently, the operation of an eight-amplifier sensor has been experimentally demonstrated, and we present the operation of a 16-amplifier sensor. This new sensor enables a noise level of ∼1 erms− with a single sample per amplifier. In addition, it is shown that sub-electron noise can be achieved using multiple samples per amplifier. In addition to demonstrating the performance of the 16-amplifier sensor, we aim to create a framework for future analysis and performance optimization of this type of detectors. New models and techniques are presented to characterize specific parameters, which are absent in conventional CCDs and Skipper CCDs: charge transfer between amplifiers and independent and common noise in the amplifiers and their processing.

Scientific applications, such as astronomy or Earth observation from low-Earth orbits, often involve extreme operating conditions, such as very long wavelength or very low photon arrival rates. These present a technical challenge for infrared sensor manufacturers, in particular, dark current. The infrared detector technology at Leonardo UK is well suited to these challenges due to bandgap-engineered HgCdTe, grown by metal organic vapor phase epitaxy (MOVPE). By widening the bandgap in critical parts of the sensor, the dark current can be effectively switched off. Each diode is physically separated in a mesa process giving market-leading resolution, crosstalk, and inter-pixel capacitance. The structure also provides 100% fill factor and 100% internal quantum efficiency. We focus on astronomy because this field has the most extremely low photon flux levels (>0.01 photons/pixel/s). In particular, linear-mode avalanche photodiode arrays are vital for astronomy to amplify the single-photon response above the noise floor. The design and performance of the detectors for astronomy is a particularly good example of bandgap engineering using MOVPE. Progress in two other fields is reported. First, for future adaptive optics systems, a 512×512/24-μm sensor with frame rates over 2000 frames/s is described, aimed initially at the Extremely Large Telescope. Second, the progress on high-speed avalanche photodiode arrays mainly for free-space optical communications and light detection and ranging is summarized.

During electro-optical testing of the camera for the upcoming Vera C. Rubin Observatory Legacy Survey of Space and Time, a unique low-signal pattern was found in differenced pairs of flat images used to create photon transfer curves, with peak-to-peak variations of a factor of 10−3. A turbulent pattern of this amplitude was apparent in many differenced flat-fielded images. The pattern changes from image to image and shares similarities with atmospheric “weather” turbulence patterns. We applied several strategies to determine the source of the turbulent pattern and found that it is representative of the mixing of the air and index of refraction variations caused by the internal camera purge system displacing air, which we are sensitive to due to our flat field project setup. Characterizing this changing environment with 2D correlation functions of the weather patterns provides evidence that the images reflect the changes in the camera environment due to the internal camera purge system. Simulations of the full optical system using the galsim and galsim codes show that the weather pattern affects the dispersion of the camera point-spread function at only one part in 10−4 level.

In the pursuit of observing fainter astronomical sources and phenomena, a significant challenge in detector development lies in ensuring that these devices can detect each individual photon they receive. By amplifying each incoming photon by several orders of magnitude, electron-multiplying charge-coupled devices (EMCCDs) offer a promising solution to meet this challenge. Although these detectors boast impressive potential, they can be intricate, requiring precise optimization and fine-tuning of their parameters to unlock their full capabilities in the photon-starved regime. The Faint Intergalactic-medium Redshifted Emission Balloon (FIREBall-2) is a stratospheric project that aims to detect and map the low surface brightness environment of galaxies in the ultraviolet (UV) at z∼0.7. As a technology demonstrator for photon-starved astronomy and to advance the technology readiness level of UV EMCCDs, the instrument uses a Teledyne-e2v (T-e2v) EMCCD delta-doped by the Jet Propulsion Laboratory, combined with a NüVü controller. To analyze the detector data and retrieve the device noise contributions, we developed a comprehensive EMCCD model along with DS9 analysis tools to compare the model with the actual data under diverse operating conditions. This allowed us to examine the current performance and limitations of these devices both on the ground and in the stratospheric environment, to unravel the intricacies of these detectors. In addition, we will discuss the development and implementation of an exposure time calculator designed to optimize the end-to-end signal-to-noise ratio under diverse conditions and analyze the different trade-offs associated with such devices. This will be used to explore some EMCCD-related issues encountered on FIREBall-2 and present some recent and potential future upgrade strategies (controller upgrade, red-blocking filter, over-spill register implementation, etc.) to mitigate them.

Astrophysics demands higher precision in measurements across photometry, spectroscopy, and astrometry. Several science cases necessitate not only precision but also a high level of accuracy. We highlight the challenges involved, particularly in achieving spectral fidelity, which refers to our ability to accurately replicate the input spectrum of an astrophysical source. Beyond wavelength calibration, this encompasses correcting observed spectra for atmosphere, telescope, and instrumental signatures. Elevating spectral fidelity opens avenues for addressing fundamental questions in physics and astrophysics. We delve into specific science cases, critically analyzing the prerequisites for conducting crucial observations. Special attention is given to the requirements for spectrograph detectors, their calibrations, and data reduction. Importantly, these considerations align closely with the needs of photometry and astrometry.

Image persistence in HAWAII-2RG HgCdTe detectors has been observed by multiple parties. Also known as latent signal, this effect occurs when sensor images following an illumination show a decayed form of the illuminated image even though the source has been removed and the detector has been reset. Using data from an engineering grade detector array delivered for SPHEREx testing illuminated with a wide range of fluxes, we demonstrate an interpretation and a working model from which the decaying signal can be estimated, providing the ability to flag pixels subject to excess persistence current beyond a specified threshold. Simulated persistence images allow validation of the module and prediction of its effect in flight data.

The Palomar Radial Velocity Instrument (PARVI) is a J & H band, high resolution (R∼80,000) spectrograph on the Hale 5.08-m telescope at Palomar Observatory. PARVI is a stabilized, single-mode fiber-fed spectrometer designed to search for small rocky planets in the habitable zones of late-type stars. PARVI is one of the first radial velocity (RV) instruments to employ single-mode fibers (SMF) instead of the more common multi-mode fibers (MMF). SMFs provide a number of advantages over MMFs. In particular, they only allow one spatial mode of light to propagate, resulting in a time-invariant, near-Gaussian output beam that eliminates speckle noise present in MMFs. A challenge with SMFs is that the single spatial mode is composed of two polarization states (PS) and the output PS is completely decoupled from the input PS. During the commissioning of PARVI, we discovered a source of polarization noise that manifested as a time-dependent, wavelength-dependent, PSF displacement responsible for RV errors on the order of 10 ms−1 that we call the accordion effect. We set up a warm testbed to test the polarization response of the PARVI optics and determined the cross-dispersing prism and variable PS of injected light were responsible for the polarization noise. We used raytracing software to simulate birefringence in the prism and found it to be consistent with the observed effect. We designed a number of tests with the PARVI spectrometer operating cold and under vacuum to evaluate how modifications to the prism impacted the effect. After mitigating stress-induced birefringence in the prism and installing fiber polarization scramblers before light is injected into the spectrograph, we are able to demonstrate an instrument RV precision of <1 ms−1.

We have successfully demonstrated three 4×2 hot electron bolometer (HEB) mixer arrays, designed to operate between 4.2 and 5.5 K, with local oscillator (LO) frequencies of 1.4, 1.9, and 4.7 THz, respectively. These arrays consist of spiral antenna coupled NbN HEB mixers combined with elliptical lenses. These are to date the highest pixel count arrays using a quasi-optical coupling scheme at supra-THz frequencies. At 1.4 THz, we obtained an average double sideband mixer noise temperature of 330 K, a mixer conversion loss of 5.2 dB, and an optimum LO power of 210 nW. The array at 1.9 THz has an average mixer noise temperature of 425 K, a mixer conversion loss of 6.4 dB, and an optimum LO power of 190 nW. For the array at 4.7 THz, we obtained an average mixer noise temperature of 715 K, a mixer conversion loss of 8.9 dB, and an optimum LO power of 240 nW. We found the arrays to be uniform regarding the mixer noise temperature with a standard deviation of 3% to 4%, the conversion loss with a standard deviation of 8% to 11%, and optimum LO power with a standard deviation of 5% to 6%. The noise bandwidth was also measured, being 3.5 GHz for the three arrays. These performances are comparable to previously reported values in the literature for single pixels and also other detector arrays at similar frequencies. Our arrays met the instrument requirements and were employed in the Galactic/Extra-Galactic ULDB Spectroscopic Terahertz Observatory (GUSTO), a NASA balloon-borne observatory. GUSTO launched from Antarctica on the 31st of December 2023 having a successful flight of 57 days, the longest ever recorded by NASA for such a mission profile.

With the recent observational confirmation of accretion bursts in high-mass protostars, high-mass star formation has entered the discipline of time-domain astronomy. These bursts of accretion cause variations in the radio continuum emission and radio frequency maser emission emitted by these protostars, but the causes and mechanisms by which these variations arise have yet to be explored exhaustively. The associated rising demand for high-cadence observations calls for the development of observational facilities that can effectively monitor the radio frequency continuum and maser emission in a manner that provides high detection sensitivity and can be highly automated. We have initiated the Interferometer for Variable Astrophysical Radio Sources (IVARS) project, which comprises the development of an 800-m single-baseline radio interferometer, along with highly automated observation and data processing infrastructure, to monitor a sample of 30 high-mass protostars. We describe the project background and automation tools developed as part of the IVARS project.

The nonlinear curvature wavefront sensor (nlCWFS) uses multiple (typically four) out-of-focus images to reconstruct the phase and amplitude of a propagating light beam. Because these images are located between the pupil and focal planes, they contain tip/tilt information. Rather than using a separate sensor to measure image locations, it would be beneficial to extract tip/tilt information directly and routinely as part of the reconstruction process. In the presence of atmospheric turbulence, recovering precise centroid offsets for each out-of-focus image becomes a dynamic process as image structure is altered by changing aberrations. We examine several tip/tilt extraction methods and compare their precision and accuracy using numerical simulations. With knowledge of the z-distance to each plane, we find that applying a best-fit linear model to multiple image centroid locations can offer fast and accurate tip/tilt (jitter) mode retrieval. We also develop a nonlinear tip/tilt extraction method that self-consistently uses the speckle field to compensate for jitter but find that algorithm latency precludes practical implementation. Simulations of closed-loop adaptive optics correction demonstrate that centroiding precisions may reach the diffraction limit despite using images with a finite region of interest that is purposefully defocused.