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.

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.