Extreme ultraviolet (EUV) filters are critical components in solar space missions, ensuring the required wavelength selection and visible light attenuation. As part of the advancement for the NASA Multi-slit Solar Explorer (MUSE) mission, we present a detailed in-band EUV and out-of-band soft X-ray transmittance characterization of small samples of filter candidates representing two design choices for the mission: a baseline design based on heritage filters from past successful missions and an alternative design employing carbon nanotube (CNT) pellicles as substrates. Our aim is both to present the experimental EUV/soft X-ray transmittance measurements and, through proper data analysis, to retrieve the atomic areal densities and the inner layer thicknesses of each sample, including the quantification of the metal oxide layers for each different substrate. The measurements demonstrate the in-band transmittance performance of both heritage and CNT-based filters across EUV bands. These results provide useful input for the construction of the response functions for MUSE and similar EUV space missions. Furthermore, the methodology establishes a framework for the optimization and characterization of thin-film materials tailored as EUV entrance filters for space-based solar observations. (c) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 International License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

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. (c) 2025 Society of Photo-Optical Instrumentation Engineers (SPIE)

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 angstrom. 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 Black Hole Explorer (BHEX) mission will enable the study of the fine photon ring structure, aiming to reveal the clear universal signatures of multiple photon orbits and true tests of general relativity, while also giving astronomers access to a much greater population of black hole shadows. Spacecraft orbits can sample interferometric Fourier spacings that are inaccessible from the ground, providing unparalleled angular resolution for the most detailed spatial studies of accretion and photon orbits and better time resolution. The BHEX mission concept provides space Very Long Baseline Interferometry (VLBI) at submillimeter wavelengths measurements to study black holes in coordination with the Event Horizon Telescope and other radio telescopes. This report presents the BHEX engineering goals, objectives and TRL analysis for a selection of the BHEX subsystems. This work aims to lay some of the groundwork for a near-term Explorers class mission proposal.

Nanoflares are thought to be one of the prime candidates that can heat the solar corona to its multi-million kelvin temperature. Individual nanoflares are difficult to detect with the present generation instruments, however their presence can be inferred by comparing simulated nanoflare-heated plasma emissions with the observed emission. Using HYDRAD coronal loop simulations, we model the emission from an X-ray bright point (XBP) observed by the Marshall Grazing Incidence X-ray Spectrometer (MaGIXS), along with nearest-available observations from the Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO) and X-Ray Telescope (XRT) onboard Hinode observatory. The length and magnetic field strength of the coronal loops are derived from the linear-force-free extrapolation of the observed photospheric magnetogram by Helioseismic and Magnetic Imager (HMI) onboard SDO. Each loop is assumed to be heated by random nanoflares, whose magnitude and frequency are determined by the loop length and magnetic field strength. The simulation results are then compared and matched against the measured intensity from AIA, XRT, and MaGIXS. Our model results indicate the observed emissions from the XBP under study could be well matched by a distribution of nanoflares with average delay times 1500 s to 3000 s, which suggest that the heating is dominated by high-frequency events. Further, we demonstrate the high sensitivity of MaGIXS and XRT to diagnose the heating frequency using this method, while AIA passbands are found to be the least sensitive.

The Arcus Probe mission addresses a wide range of Astro2020 Decadal and NASA Science Mission Directorate Priority science areas, and is designed to explore astrophysical feedback across all mass scales. Arcus' three baseline science goals include: (i) Characterizing the drivers of accretion-powered feedback in supermassive black holes, (ii) Quantifying how feedback at all scales drives galaxy evolution and large-scale structure, including the tenuous cosmic web, and (iii) Analyzing stellar feedback from exoplanetary to galactic scales, including its effects on exoplanet environments targeted by current and future NASA missions. These science goals, along with a robust General Observer program, will be achieved using a mission that provides a high-sensitivity soft (10-60 angstrom) X-ray spectrometer (XRS), working simultaneously with a co-aligned UV spectrometer (UVS; 970-1580 angstrom). Arcus enables compelling baseline science and provides the broader astronomy community a revolutionary tool to characterize the full ionization range of warm and hot plasmas - including hydrogen, helium, and all abundant metals - in the Universe, from the halos of galaxies and clusters to the coronae of stars.

This paper describes specification and early design of back end signal processing subsystems for the Black Hole Explorer (BHEX) Very Long Baseline Interferometry (VLBI) space telescope. The "back end" consists of two subsystems. First, the block downconverter (BDC) is a heterodyne system that performs a frequency translation of the analog signal from IF to baseband and amplifies and filters it for digitization. Second, the digital back end (DBE) samples the analog signal with an analog-to-digital converters (ADC) and digitally processes the data stream formatting them to the VLBI "VDIF" standard and converting to Ethernet packets for 100 gigabit-per-second (Gb/s) Ethernet transport to the optical downlink system. Both the BDC and the DBE for BHEX support eight channels of 4.096 GHz bandwidth each, for a total processed bandwidth of 32.768 GHz. The BHEX back end benefits from mature terrestrial back end heritage, described in some detail. The BHEX back end itself is in the early stages of design, with requirements, interface specifications, and component trade studies well advanced. The aim is to build a prototype using terrestrial grade parts which are available in functionally identical space grade equivalents, and to use this prototype to advance the back end Technology Readiness Level (TRL) preparing for a Small Explorer (SMEX) proposal in 2025.

We present the Black Hole Explorer (BHEX), a mission that will produce the sharpest images in the history of astronomy by extending submillimeter Very-Long-Baseline Interferometry (VLBI) to space. BHEX will discover and measure the bright and narrow "photon ring" that is predicted to exist in images of black holes, produced from light that has orbited the black hole before escaping. This discovery will expose universal features of a black hole's spacetime that are distinct from the complex astrophysics of the emitting plasma, allowing the first direct measurements of a supermassive black hole's spin. In addition to studying the properties of the nearby supermassive black holes M87* and Sgr A*, BHEX will measure the properties of dozens of additional supermassive black holes, providing crucial insights into the processes that drive their creation and growth. BHEX will also connect these supermassive black holes to their relativistic jets, elucidating the power source for the brightest and most efficient engines in the universe. BHEX will address fundamental open questions in the physics and astrophysics of black holes that cannot be answered without submillimeter space VLBI. The mission is enabled by recent technological breakthroughs, including the development of ultra-high-speed downlink using laser communications, and it leverages billions of dollars of existing ground infrastructure. We present the motivation for BHEX, its science goals and associated requirements, and the pathway to launch within the next decade.

Arcus is a concept for a probe class mission to deliver high-resolution FUV and X-ray spectroscopy. For X-rays, it combines cost-effective silicon pore optics (SPO) with high-throughput critical-angle transmission (CAT) gratings to achieve R> 3000 in a bandpass from 12-50 Angstroem. We show in detail how the X-ray and the UV spectrographs (XRS and UVS) on Arcus will be aligned to each other. For XRS we present ray-tracing studies to derive performance characteristics such as the spectral resolving power and effective area, study the effect of misalignments on the performance, and conclude that most tolerances can be achieved with mechanical means alone. We also present an estimate of the expected on-orbit background.

The Advanced X-ray Imaging Satellite (AXIS) is a probe-class mission concept with a large collecting area with a point-spread-function of order 1 to 2 arcsec. We describe a possible X-ray grating spectrometer (XGS) that could be added to AXIS with minimal design changes to the telescope itself and costs a small fraction of the total mission budget. The XGS would be based on critical-angle transmission (CAT) gratings, a technology already matured for Arcus and Lynx. Using detailed ray-tracing, we investigate several options for subaperturing that provide a trade-off between effective area and spectral resolving power. Depending on how much of the full aperture is covered with gratings (e.g., 17% to 100%), we find a high spectral resolving power up to lambda/Delta lambda = 4000 can be achieved with effective area of 1500 cm(2) in the 1.2 to 2.8 nm range or lambda/Delta lambda = 6000 with effective area 500 cm(2). An important benefit of CAT gratings is that they are mostly transparent at high energies, and thus hard x-rays can still be used for simultaneous imaging spectroscopy. We study different grating sizes and other enhancements, but even in the basic configuration an XGS can be added to AXIS to provide high-resolution spectral capabilities, opening a range of new science investigations. Our ray-tracing shows that this concept is mature and can be added to AXIS with minimal impact on other instruments. We discuss one exemplary science case that would be enabled by the XGS. (c) 2023 Society of Photo-Optical Instrumentation Engineers (SPIE)

The MUlti-slit Solar Explorer (MUSE) is a NASA medium-class explorer mission that is currently in phase B and scheduled for launch no earlier than 2027. The MUSE science investigation aims to use high-resolution and high-cadence spectroscopic and imaging EUV observations of the solar atmosphere to understand the multi-scale physical processes that heat the multi-million-degree solar corona, drive the source of the solar wind, and cause solar activity (flares and coronal mass ejections) that lead to space weather that impacts Earth. MUSE will consist of an EUV context imager and an EUV spectrograph, both requiring normal incidence mirrors with a very high level of polishing and figuring, in order to allow high-resolution imaging and spectroscopy. The mission is led by Lockheed Martin Solar and Astrophysics Laboratory (LMSAL). The payload is being developed by LMSAL and the Center for Astrophysics (CfA) at the Harvard Smithsonian Astrophysical Observatory, while INAF-OAB will produce the focusing mirrors with the financial support of the Italian Space Agency (ASI). In this paper, we describe the first steps that are being taken in the procurement of the focusing mirrors in Zerodur, the work plan with the ion beam figuring and the pitch tool aimed at bringing the surface defects within the specification. Additionally, we describe the metrology system that we are setting up to detect the residual deviation to the final shape.

The Arcus Probe is designed to measure the feedback cycle of material into and out of galaxies, and the inter-relation between these flows and the central black holes that drive many of these processes. Arcus consists of a high-resolution x-ray spectrometer (led by the Smithsonian Astrophysics Observatory; SAO) with a companion medium resolution (R ~ 24,500) far-ultraviolet imaging spectrograph covering the 970 - 1580 Å bandpass. The Arcus Ultraviolet Spectrograph (UVS) is designed in part to be a sucessor to the successful FUSE mission, with more than five-times the sensitivity in the essential Lyman UV, including rest-frame O VI 1032 ˚A, than any previous medium resolution spectroscopic instrument. The instrument consists of a 60 cm off-axis Cassegrain telescope feeding a two-channel spectrograph, with the spectra recorded on an open-face microchannel plate detector. The channels each consist of a medium resolution grating mounted to a grating selector: the G110M (970 - 1280 Å, optimized for 1000 - 1280 Å) and the G140M (1195 - 1580 Å). The Arcus UVS is led by the University of Colorado Laboratory for Atmospheric and Space Physics (LASP) and incorporates several technologies developed in the more than two decades since F USE, and matured on previous CU-LASP flight programs, including enhanced lithium fluoride protected aluminum mirror coatings (eLiF) and large-format borosilicate glass MCPs. We describe the recent development and TRL advancement of these enabling technologies, and then outline the UVS instrument and projected performance.

The Marshall Grazing Incidence X-ray Spectrometer (MaGIXS) is a sounding rocket mission that completed a successful flight from the White Sands Missile Range on July 30, 2021. MaGIXS captured spatially resolved soft X-ray spectra from portions of two solar active regions during its roughly 5-minute flight. The instrument was originally designed as a grazing incidence slit spectrograph but flew in a slit-less configuration that produced overlapping spectroheliograms. For the second flight, MaGIXS-2, the instrument has been reconfigured to a more simplified optical layout that reuses the Wolter-I telescope and blazed varied-line space reflective grating. The field stop at the telescope focal plane and the finite conjugate spectrometer mirror pair have been removed – the telescope now directly feeds the grating. Additionally, an identical but new 2k x 1k CCD camera has been built for this flight. The MaGIXS-2 data product will again be overlapping spectroheliograms of at least one solar active region, but with improved resolution, a larger field of view and increased effective area. Here we present the updated instrument layout, the expected performance, the integration and calibration approach, and proposed future improvements, including the implementation of additional complimentary spectral diagnostics.

On 2017 August 21, the Airborne Infrared Spectrometer (AIR-Spec) observed the total solar eclipse at an altitude of 14 km from aboard the NSF/NCAR Gulfstream V research aircraft. The instrument successfully observed the five coronal emission lines that it was designed to measure: Si x 1.431 μm, S xi 1.921 μm, Fe ix 2.853 μm, Mg viii 3.028 μm, and Si ix 3.935 μm. Characterizing these magnetically sensitive emission lines is an important first step in designing future instruments to monitor the coronal magnetic field, which drives space weather events, as well as coronal heating, structure, and dynamics. The AIR-Spec instrument includes an image stabilization system, feed telescope, grating spectrometer, and slit-jaw imager. This paper details the instrument design, optical alignment method, image processing, and data calibration approach. The eclipse observations are described and the available data are summarized.

Arcus, a Medium Explorer (MIDEX) mission, was selected by NASA for a Phase A study in August 2017. The observatory provides high-resolution soft X-ray spectroscopy in the 12-50Å bandpass with unprecedented sensitivity: effective areas of >450 cm2 and spectral resolution >2500. The Arcus key science goals are (1) to measure the effects of structure formation imprinted upon the hot baryons that are predicted to lie in extended halos around galaxies, groups, and clusters, (2) to trace the propagation of outflowing mass, energy, and momentum from the vicinity of the black hole to extragalactic scales as a measure of their feedback and (3) to explore how stars, circumstellar disks and exoplanet atmospheres form and evolve. Arcus relies upon the same 12m focal length grazing-incidence silicon pore X-ray optics (SPO) that ESA has developed for the Athena mission; the focal length is achieved on orbit via an extendable optical bench. The focused X-rays from these optics are diffracted by high-efficiency Critical-Angle Transmission (CAT) gratings, and the results are imaged with flight-proven CCD detectors and electronics. The power and telemetry requirements on the spacecraft are modest. Mission operations are straightforward, as most observations will be long (~100 ksec), uninterrupted, and pre-planned, although there will be capabilities to observe sources such as tidal disruption events or supernovae with a ~3 day turnaround. Following the 2nd year of operation, Arcus will transition to a proposal-driven guest observatory facility.

The Airborne Infrared Spectrometer (AIR-Spec) was commissioned during the 2017 total solar eclipse, when it observed five infrared coronal emission lines from a Gulfstream V research jet owned by the National Science Foundation and operated by the National Center for Atmospheric Research. The second AIR-Spec research flight took place during the 2019 July 2 total solar eclipse across the south Pacific. The 2019 eclipse flight resulted in seven minutes of observations, during which the instrument measured all four of its target emission lines: S xi 1.393 mu m, Si x 1.431 mu m, S xi 1.921 mu m, and Fe ix 2.853 mu m. The 1.393 mu m S xi line was detected for the first time, and probable first detections were made of Si xi 1.934 mu m and Fe x 1.947 mu m. The 2017 AIR-Spec detection of Fe ix was confirmed and the first observations were made of the Fe ix line intensity as a function of solar radius. Telluric absorption features were used to calibrate the wavelength mapping, instrumental broadening, and throughput of the instrument. AIR-Spec underwent significant upgrades in preparation for the 2019 eclipse observation. The thermal background was reduced by a factor of 30, providing a 5.5x improvement in signal-to-noise ratio, and the postprocessed pointing stability was improved by a factor of 5 to <10 '' rms. In addition, two imaging artifacts were identified and resolved, improving the spectral resolution and making the 2019 data easier to interpret.

The Event Horizon Explorer (EHE) is a mission concept to extend the Event Horizon Telescope via an additional space-based node. We provide highlights and overview of a concept study to explore the feasibility of such a mission. We present science goals and objectives, which include studying the immediate environment around supermassive black holes, and focus on critical enabling technologies and engineering challenges. We provide an assessment of their technological readiness and overall suitability for a NASA Medium Explorer (MIDEX) class mission.

The Marshall Grazing Incidence X-ray Spectrometer (MaGIXS) sounding rocket experiment launched on 2021 July 30 from the White Sands Missile Range in New Mexico. MaGIXS is a unique solar observing telescope developed to capture X-ray spectral images of coronal active regions in the 6–24 Å wavelength range. Its novel design takes advantage of recent technological advances related to fabricating and optimizing X-ray optical systems, as well as breakthroughs in inversion methodologies necessary to create spectrally pure maps from overlapping spectral images. MaGIXS is the first instrument of its kind to provide spatially resolved soft X-ray spectra across a wide field of view. The plasma diagnostics available in this spectral regime make this instrument a powerful tool for probing solar coronal heating. This paper presents details from the first MaGIXS flight, the captured observations, the data processing and inversion techniques, and the first science results.