JATIS thanks the reviewers who served the journal in 2023.

The Webb Space Telescope’s on-orbit performance was made possible by successful in-process optical testing and cryogenic requirement compliance certification, verification, and validation of the Webb optical components. This was accomplished by the hard work of dozens of optical metrologists, the development and qualification of multiple custom test setups, and several inventions, including 4D PhaseCam and Leica absolute distance meter. We define a set of rules for optical metrology and summarize how they were applied to the metrology tools, test setups, and processes used to characterize the Webb Space Telescope primary mirror segment assemblies.

Pathways to Discovery in Astronomy and Astrophysics for the 2020s has recommended a Great Observatory Maturation Program (GOMaP) to invest in co-maturation of mission concepts and technologies to inform an analysis of alternatives study for an ∼6 m off-axis inscribed telescope. The purpose of this telescope is to sample atmospheric spectra of around 25 potentially habitable exoplanets using ultraviolet, visible, and near-infrared wavelengths; it is planned to launch in the early 2040s with a total cost of less than $11B, including 5 years of operation. A historical review of past missions yields basic programmatic lessons learned to be considered as the community prepares to implement the Decadal Vision. First, technology development is critical for enabling missions. The robustness, breadth, and duration of concept/technology co-maturation is important for mission success. Second, NASA has never “exactly” implemented a Decadal mission as it was recommended. Third, all missions have the same basic technology challenges of mass constraints: mechanical and thermal stability to design, building a space telescope that achieves the required on-orbit performance, and verifying and validating that performance by test and model correlation. Finally, Decadal missions require sustained community support.

The James Webb Space Telescope (JWST) launched on December 25, 2021, and its optical performance in orbit has been even better than predicted pre-flight. The static wavefront error (WFE) is less than half the value specified for the requirement of having diffraction-limited image quality at 2 microns in the NIRCam shortwave channel, enabling the observatory to deliver both sharper images and higher sensitivity than anticipated. In addition to the excellent image quality, the optical stability has also exceeded expectations, both in terms of high-frequency dynamic contributions (which would be perceived as part of “static WFE”) and in terms of drifts over minutes, hours, and days. Stability over long timescales is critical for several important science cases, including exoplanet transit spectroscopy and coronagraphy. JWST’s stability success was achieved through detailed design and testing, with several important lessons learned for future observatories, especially the Habitable Worlds Observatory that is expected to need even higher levels of stability. We review the stability architecture, how it was technologically demonstrated, the ground test results and improvements, the on-orbit results, and the lessons learned.

With a long history of involvement in NASA flagship astrophysics missions, Ball Aerospace has made key contributions to Hubble, the new James Webb Space Telescope, and the upcoming Nancy Grace Roman Space Telescope. Here, we share lessons on both technical and programmatic topics gleaned from decades of productive experience in constructing system performance budgets, accelerating the initial design process, defining the philosophical approach to requirements, planning for serviceability, leveraging design complexity, accounting for subsystem interactions, breaking down barriers to cross-team collaboration, building a team capable of meeting long-term goals, integrating the customer into the enterprise, and promoting a team mindset that enables high achievement in the face of immense challenges. The teams designing and building the future Habitable Worlds Observatory can apply this knowledge to complete the next flagship mission more quickly and effectively with less expense.

The James Webb Space Telescope has extended our knowledge of the universe since the first images appeared last year with optical performance reaching the diffraction limit. The optical alignment and quality were tested prior to launch at cryogenic temperatures with a complex test configuration for this unique telescope architecture. This article reviews the strategic choices for pathfinding the integration and test processes for this telescope and the lessons learned prior to integrating and testing the flight hardware, leading to a successful integration and test campaign of the flight telescope. The biggest lesson was learning the value of investing in pathfinding integration and test activities.

The primary mirror is central to the success of the Webb Space Telescope and the product of 100s of engineers and technologists who invented technologies and processes for its manufacture and test. We summarize the Webb mirror technology development program, explain how the technology was demonstrated to be TRL-6 (including the importance of an Engineering Development Unit), and list some of the author’s personal lessons learned.

The James Webb Space Telescope is NASA’s flagship mission and successor to the highly successful Hubble Space Telescope. It is an infrared observatory featuring a cryogenic 6.6 m aperture, deployable optical telescope element with a payload of four science instruments assembled into an integrated science instrument module that provide imagery and spectroscopy in the near infrared band between 0.6 and 5 μm and in the mid-infrared band between 5 and 28 μm. JWST was successfully launched on December 25, 2021, aboard an Ariane 5 launch vehicle. All 50 major deployments were successfully completed by January 8, 2022. The observatory performed all mid-course correction maneuvers and achieved its operational mission orbit around the Sun-Earth second Lagrange Point. All commissioning and calibration activities have been completed and JWST has begun its science mission. Its present performance meets or out-performs all requirements. Launching over 20 years after its mission concept review, the JWST Observatory is a first and only of its kind of facility. This program faced many unique challenges that were not only technical in nature but also organizational and managerial. We describe the challenges faced by the JWST systems engineering team, the way the team addressed them, and make recommendations for focus areas of future flagship missions, which will likely face similar challenges. It will not explicitly address the cost challenges of the mission. We first describe the mission and its over-arching challenges. We then describe the tailoring of systems engineering processes and methods used to address these challenges and effectiveness. The events, tasks, issues, and their resolutions and the resulting specific lessons learned from the project are discussed with the over-arching recommendations for future flagship missions that derive from these lessons.

We formulate the lessons Northrop Grumman personnel have learned from their work on development of the James Webb Space Telescope. These lessons are strategic in nature and bear on the common behavior during development of all large complex systems, such as astrophysics missions, also known colloquially as Flagships. To justify the expense, a Flagship must be a large leap in scientific capability, demanding new architectures and technologies coupled with an intolerance to risk. We define “The Problem of Newness” based on our experience and data from Webb’s development. This unseen hand was present during Webb, and it is only in retrospect that we have been able to define it and present it as a lesson for the future. Future missions, Flagships in particular, should recognize the challenge of newness as a natural consequence of development and take steps to minimize its impact.

Recently a prototype for pulsar observations at low radio frequencies (RFs) (<100 MHz) using log-periodic dipole antennas (LPDAs) in the Gauribidanur Radio Observatory (≈77°E14°N) near Bangalore, India, was commissioned. The aforementioned system is currently being augmented (i) to directly digitize the RF signals from the individual antennas and (ii) with a digital beamformer to simultaneously observe different regions of the sky present within the primary “beam” of the LPDA used in the array. Our initial results indicate that co-temporal observations of a known pulsar along with the Sun using two different beams could be used to calibrate the dynamic spectrum of the solar radio transients. This is important because the calibration of the latter in observations with the conventional solar radio spectrographs is difficult.

We report on recent progress in the development of Laue lenses for applications in hard X/soft gamma-ray astronomy. Here, we focus on the realization of a sector of such a lens made of 11 bent germanium crystals and describe the technological challenges involved in their positioning and alignment with adhesive-based bonding techniques. The accurate alignment and the uniformity of the curvature of the crystals are critical for achieving optimal X-ray focusing capabilities. We assessed how the errors of misalignment with respect to the main orientation angles of the crystals affect the point spread function (PSF) of the image diffracted by a single sector. We corroborated these results with simulations carried out with our physical model of the lens, based on a Monte Carlo ray-tracing technique, adopting the geometrical configuration of the Laue sector, the observed assembly accuracy, and the measured curvatures of the crystals. An extrapolation of the performances achieved on a single sector to an entire Laue lens based on this model shows that a PSF with a half-power-diameter of 4.8 arcmin can be achieved with current technology. This has the potential to lead to a significant improvement in the sensitivity of spectroscopic and polarimetric observations in the 50 to 600 keV band.

The Roman Space Telescope Grism and Prism assemblies will allow the wide-field instrument (WFI) to perform slitless, multi-object spectroscopy across the complete field of view. These optical elements play a critical role in the High Latitude Wide Area and High Latitude Time Domain Surveys, which are designed to produce robust spectroscopic redshifts for millions of objects over the mission lifetime. To facilitate the characterization of these assemblies, a dedicated test bed was designed and utilized to perform a wide variety of spectroscopic measurements over the full range of operational wavelengths and field angles. Characterized features include, but are not limited to dispersion magnitude, dispersion clocking, encircled energy, total throughput, and bandpass edges. We present the results of this experimental campaign in which the Grism and Prism assemblies met or exceeded many of their design requirements and discuss measurement limitations.

Accurate infrared magnetic system (AIMS) is a ground-based solar telescope with the effective aperture of 1 m. The system has complex optical path and contains multiple aspherical mirrors. Since some mirrors are anisotropic in space, parallel light undergoes complex spatial reflection after passing through the optical pupil. It is also required that part of the optical axis coincides with the mechanical rotation axis. The system is difficult to align. This article proposes two innovative alignment methods. First, a modularized alignment method is presented. Each module is individually assembled with optical reference reserved. System integration can be completed through optical reference of each module. Second, computer-aided alignment technology is adopted to achieve perfect wavefront. By perturbing the secondary mirror (M2), the influence of M2 position on the wavefront is measured and the mathematical relationship is obtained. Based on the measured wavefront data, the least squares method is used to calculate the M2 alignment and multiple adjustments have been made to M2. The final system wavefront has reached RMS=0.12 λ@632.8 nm. Through observations of stars and sunspots, it has been demonstrated that the optical system has good wavefront quality. The observed sunspot is clear with the penumbral and umbra discernible. The proposed method has been verified and provides an effective alignment solution for complex off-axis telescope with large aperture.

A primary scientific goal of the future Habitable Worlds Observatory will be the direct detection and characterization of Earth-like planets. Estimates of the exoplanet yields for this concept will help guide mission design through detailed trade studies. It is therefore critical that yield estimation codes optimally adapt observations to the mission’s performance parameters to ensure accurate trade studies. To aid in this, we implement wavelength optimization in yield calculations for the first time, allowing the yield code to determine the ideal detection and characterization bandpasses. We use this capability to confirm the observational wavelength assumptions made for the large UV/Optical/IR surveyor, design B (LUVOIR-B) study, namely that the optimum detection wavelength is 500 nm for the majority of targets and the optimum wavelength to detect water is near 1000 nm, given LUVOIR-B’s assumed instrument performance parameters. We show that including the wavelength-dependent albedo of an Earth twin as a prior provides no significant benefit to the yields of exoEarth candidates and caution against tuning observations to modern Earth twins. We also show that coronagraphs whose inner working angles are similar to step functions may benefit from wavelength optimization and demonstrate how wavelength-dependent instrument performance can impact the optimum wavelengths for detection and characterization. The optimization methods we implement automate wavelength selection and remove uncertainties regarding these choices, helping to adapt the observations to the instrument’s performance parameters.

The Line Emission Mapper X-ray Probe-class mission concept is based on a microcalorimeter array tuned to energies in the range 0.1 to 2 keV. The study of cosmic ecosystems defines the directed portion of the Line Emission Mapper (LEM) mission, thus LEM has been optimized for observations of diffuse X-ray-emitting gas, largely with very low surface brightness. To broaden the range of targets that general observers can study with LEM, we have investigated the particular needs for UV/optical bright stars and solar-system objects. X-ray microcalorimeters are susceptible to degraded energy resolution that can result from thermal noise from residual UV, optical, and IR radiation. Using the present baseline design of the microcalorimeter thermal filters, we compute the UV-IR loading expected from bright stars over the effective temperature range 3500 to 39,000 K and from solar-system objects. The dominant leak of out-of-band energy is in the far-UV around 1500 Å, with a secondary peak of throughput around 4000 Å. For stars with magnitudes V<10 and for all solar-system planets as well as the Moon, the loading is significant, indicating that additional UV/optical blocking is essential if bright objects are to be observed. We have investigated the efficacy of several filter options for optical-blocking filters on the LEM filter wheel, demonstrating that new technology development is not necessary to open up many of these classes of objects to investigation with the high spectral resolution of LEM.

Current scalar coronagraph focal plane mask designs are performance-limited by chromaticity. We investigate the effects of adding central Roddier and dual zone phase dimples to scalar masks to improve broadband performance by suppressing the chromatic stellar leakage. We present hybrid designs with radial phase dimples integrated with the sawtooth vortex, wrapped vortex, and cosine phase mask. We show that, using these dimples, it is possible to substantially improve the broadband contrast performance of scalar phase masks. We also show that, although adding a phase dimple does not increase the sensitivity to low-order aberrations, suppressing the central leakage of scalar vortex coronagraphs does not restore the aberration sensitivities to their notional state.

Asgard/NOTT (previously Hi-5) is a European Research Council (ERC)-funded project hosted at KU Leuven and a new visitor instrument for the Very Large Telescope Interferometer (VLTI). Its primary goal is to image the snow line region around young stars using nulling interferometry in the L′-band (3.5 to 4.0) μm, where the contrast between exoplanets and their host stars is advantageous. The breakthrough is the use of a photonic beam combiner, which only recently allowed the required theoretical raw contrast of 10−3 in this spectral range. Nulling interferometry observations of exoplanets also require a high degree of balancing between the four pupils of the VLTI in terms of intensity, phase, and polarization. The injection into the beam combiner and the requirements of nulling interferometry are driving the design of the warm optics and the injection system. The optical design up to the beam combiner is presented. It offers a technical solution to efficiently couple the light from the VLTI into the beam combiner. During the coupling, the objective is to limit throughput losses to 5% of the best expected efficiency for the injection. To achieve this, a list of different loss sources is considered with their respective impact on the injection efficiency. Solutions are also proposed to meet the requirements of beam balancing for intensity, phase, and polarization. The different properties of the design are listed, including the optics used, their alignment and tolerances, and their impact on the instrumental performances in terms of throughput and null depth. The performance evaluation gives an expected throughput loss <6.4% of the best efficiency for the injection and a null depth of ∼2.10−3, mainly from optical path delay errors outside the scope of this work.

We present extreme ultraviolet (EUV) transmission measurements of two thin-film filters designed to obtain improved images of plasmaspheric He+, and the first global images of O+/O++ in the dense oxygen torus. Compared to previous He+ 30.4 nm imaging that used an Al filter, we show that a combined Al+C filter achieves superior rejection of 58.4 nm background from neutral helium (He I). We show that an indium filter provides both the required transmission at 83.4 nm and adequate rejection of brighter He I (58.4 nm) and H I (121.6 nm) background emissions. We find that the In transmission at 58.4 nm is a factor of ∼16 lower than predicted based on optical constants that rely on interpolations at λ<68 nm. We show that the observed lower 58.4 nm transmission is consistent with alternative optical constants derived from previous lab measurements.

Integral field spectroscopy (IFS) is an observational method for obtaining spatially resolved spectra over a specific field of view (FoV) in a single exposure. In recent years, near-infrared IFS has gained importance in observing objects with strong dust attenuation or at a high redshift. One limitation of existing near-infrared IFS instruments is their relatively small FoV, less than 100 arcsec2, compared with optical instruments. Therefore, we developed a near-infrared (0.9 to 2.5 μm) image-slicer type integral field unit (IFU) with a larger FoV of 13.5×10.4 arcsec2 by matching a slice width to a typical seeing size of 0.4 arcsec. The IFU has a compact optical design utilizing off-axis ellipsoidal mirrors to reduce aberrations. Complex optical elements were fabricated using an ultra-precision cutting machine to achieve root mean square surface roughness of less than 10 nm and a P-V shape error of less than 300 nm. The ultra-precision machining can also simplify the alignment procedures. The on-sky performance evaluation confirmed that the image quality and the throughput of the IFU were as designed. In conclusion, we successfully developed a compact IFU utilizing an ultra-precision cutting technique, almost fulfilling the requirements.

We report a development status of a focal plane detector for the GEO-X (GEOspace X-ray imager) mission that will perform soft X-ray (≤2 keV) imaging spectroscopy of Earth’s magnetosphere from a micro satellite. The mission instrument consists of a microelectromechanical systems (MEMS) X-ray mirror and a focal plane detector. A sensor with fine positional resolution and moderate energy resolution in the energy band of 0.3 to 2 keV is required. Because the observing target is the magnetosphere around the day-side Earth, the visible-light background must be decreased by shortening the integration time for readout. To satisfy the above requirements, a high-speed X-ray CMOS sensor is being evaluated as a primary candidate for the detector. Irradiating the flight candidate sensor with monochromatic X-rays, we obtained the energy resolution of 205 eV (FWHM) at 6 keV by cooling the devices to −15°C. Radiation tolerance of the sensor, especially in terms of total dose effect, is investigated with 100 MeV proton. The amount of degradation of energy resolution is <50 eV up to 10 krad, which ensures that we will be able to track and calibrate the change of the line width in orbit.

To detect exoplanets and study their formation and evolution, several exoplanet space missions, such as Kepler, TESS, GAIA, and CHEOPS, have been successfully developed and fully operated in space. However, China has not yet had its own exoplanet space mission. The Earth 2.0 (ET) space mission is being developed in China aiming at detecting and characterizing exoplanets, especially extra-terrestrial like planets. ET will carry six transit telescopes pointing to the same sky region and a gravitational microlensing telescope, with the goal of finding habitable Earth like planets (Earth 2.0s) around solar type stars and measure its occurrence rate. In order to detect Earth 2.0s, ultrahigh-precision photometry of ∼30 ppm is required, which places tight constrain on camera performance, such as high-speed readout, low readout noise, mosaic detectors, and radiation tolerance. As of now, a prototype camera utilizing a CCD250-82 detector from Teledyne e2v has been developed and its performance has been tested. At a readout rate of 2 M pixels/s, the readout noise of 10.96 e− RMS and the pixel response nonuniformity of 0.66% at 600 nm have been achieved. After receiving radiation doses of 5 krad (Si) and 13.43 krad (Si), the dark current of the CCD increased by 30% and 126%, respectively. The camera’s key performance meets the basic requirements for the ET space mission, except for its high cooling power consumption.

The existence of kilometer size objects in the Oort cloud has been conjectured but not observed as these objects are too small and too far away to be detected by reflected sunlight with current telescopes. However, these objects could be detected by serendipitous occultations of stars. We outline a pilot study for a planned occultation survey that will mount high frame-rate sCMOS cameras in the otherwise unused areas in the focal planes of large, ground-based telescopes. Because gigabytes of imaging data are produced every second, making storage of the imaging impractical for long-term observing campaigns, the image processing must be done in real time. We describe a data system that processes up to 330 frames/s of 3200×3200 images in real time and a procedure to analyze the recorded photometry for candidate occultation events by outer solar system objects. Although we did not detect any occultations, we describe successfully trialing our system taking continuous data on 7 nights of sCMOS observations of nearby open clusters with the 2.1 m telescope at McDonald Observatory, Texas.

We demonstrate so-called repetitive non-destructive readout (RNDR) for the first time on a single electron sensitive readout (SiSeRO) device. SiSeRO is a novel on-chip charge detector output stage for charge-coupled device image sensors, developed at MIT Lincoln Laboratory. This technology uses a p-MOSFET transistor with a depleted internal gate beneath the transistor channel. The transistor source-drain current is modulated by the transfer of charge into the internal gate. RNDR was realized by transferring the signal charge non-destructively between the internal gate and the summing well (SW), which is the last serial register. The advantage of the non-destructive charge transfer is that the signal charge for each pixel can be measured at the end of each transfer cycle, and by averaging for a large number of measurements (Ncycle), the total noise can be reduced by a factor of 1/Ncycle. In our experiments with a prototype SiSeRO device, we implemented nine (Ncycle=9) RNDR cycles, achieving around two electron readout noise (equivalent noise charge or ENC) with a spectral resolution close to the fano limit for silicon at 5.9 keV. These first results are extremely encouraging, demonstrating successful implementation of the RNDR technique in SiSeROs. They also lay the foundation for future experiments with more optimized test stands (better temperature control, larger number of RNDR cycles, and RNDR-optimized SiSeRO devices), which should be capable of achieving sub-electron noise sensitivities. This new device class presents an exciting technology for next generation astronomical X-ray telescopes requiring very low-noise spectroscopic imagers. The sub-electron sensitivity also adds the capability to conduct in-situ absolute calibration, enabling unprecedented characterization of the low energy instrument response.

The Astrophysics with Italian Replicating Technology Mirrors (ASTRI) Mini-Array is an international collaboration led by the Italian National Institute for Astrophysics (INAF) and devoted to imaging atmospheric Cherenkov light for very-high γ-ray astrophysics, detection of cosmic-rays, and stellar Hambury-Brown intensity interferometry. The project is deploying an array of nine dual-mirror aplanatic imaging atmospheric Cherenkov telescopes of 4-m class at the Teide Observatory on Tenerife in the Canary Islands. Based on SiPM sensors, the focal plane camera covers an unprecedented field of view of 10.5 deg in diameter. The array is most sensitive to γ-ray radiation above 1 up to 200 TeV, with an angular resolution of 3 arcmin, better than the current particle arrays, such as LHAASO and HAWC. We describe the overall software architecture of the ASTRI Mini-Array and the software engineering approach for its development. The software covers the entire life cycle of the Mini-Array, from scheduling to remote operations, data acquisition, and processing until data dissemination. The on-site control software allows remote array operations from different locations, including automated reactions to critical conditions. All data are collected every night, and the array trigger is managed post facto. The high-speed networking connection between the observatory site and the Data Center in Rome allows for ready data availability for stereoscopic event reconstruction, data processing, and almost real-time science products generation.

The Maunakea Spectroscopic Explorer (MSE) project will provide multi-object spectroscopy in the optical and near-infrared bands using an 11.25-m aperture telescope, repurposing the original Canada–France–Hawaii Telescope site. MSE will observe 4332 objects per single exposure with a field of view of 1.5 square degrees, utilizing two spectrographs with low-moderate (R∼3000, 6000) and high (R≈30,000) spectral resolution. In general, an exposure time calculator (ETC) is used to estimate the performance of an observing system by calculating the signal- to-noise ratio (S/N) and exposure time. We present the design of the MSE ETC, which has four calculation modes (S/N, exposure time, S/N trend with wavelength, and S/N trend with magnitude) and incorporates the MSE system requirements as specified in the conceptual design. The MSE ETC currently allows for user-defined inputs of the target AB magnitude, water vapor, air mass, and sky brightness AB magnitude (additional user inputs can be provided depending on the computational mode). The ETC is built using Python 3.7 and features a graphical user interface that allows for cross-platform use. The development process of the ETC software follows an Agile methodology and utilizes the unified modeling language diagrams to visualize the software architecture. We also describe the testing and verification of the MSE ETC.

X-ray detectors for space astrophysics missions are susceptible to noise caused by photons with energies outside the operating energy range; for this reason, efficient external optical blocking filters are required to shield the detector from the out-of-band radiation. These filters play a crucial role in meeting the scientific requirements of the X-ray detectors, and their proper operation over the life of the mission is essential for the success of the experimental activity. We studied thin sandwich membranes made of silicon nitride and aluminum as optical blocking filters for high-energy detectors in space missions. Here, we report the results of a multi-technique characterization of SiN membranes with thicknesses in the range from 40 nm to 145 nm coated with few tens of nanometers of aluminum on both sides. In particular, we have measured the X-ray transmission at synchrotron radiation beamlines, the rejection of ultraviolet, visible, and near-infrared radiation, the amount of native oxide on the aluminum surfaces by X-ray photoelectron spectroscopy, the morphology of the sample surfaces by atomic force microscopy, and the aging effects under proton irradiation.

We present an advanced system for calibrating the detector gain responsivity with a chopped thermal source for POLARBEAR-2a, which is the first receiver system of a cosmic microwave background (CMB) polarimetry experiment: the Simons Array. Intensity-to-polarization leakage due to calibration errors between detectors can be a significant source of systematic error for a polarization-sensitive experiment. To suppress this systematic uncertainty, POLARBEAR-2a calibrates the detector gain responsivities by observing a chopped thermal source before and after each period of science observations. The system includes a high-temperature ceramic heater that emits blackbody radiation covering a wide frequency range and an optical chopper to modulate the radiation signal. We discuss the experimental requirements of gain calibration and system design to calibrate POLARBEAR-2a. We evaluate the performance of our system during the early commissioning of the receiver system. This calibration system is promising for the future generation of CMB ground-based polarization observations.

Optical surfaces of space instruments usually need to be blackened to minimize adverse effects affecting their performance in photometric, spectrometric, and imaging applications. Blackening is often obtained by application of coatings that strongly absorb the incoming photon flux and diffusively scatter the incident photons. We discuss reflectance measurements and a phenomenological model of the bidirectional reflectance distribution function (BRDF) for the Magic Black™ coating, which is a commercial product supplied by the Acktar company. The coating has a vast satellite-instrument heritage and is planned to be used in the GLOWS photometer onboard the upcoming NASA Interstellar Mapping and Acceleration Probe nmission. The reflectance measurements were conducted at ∼121.6 nm, corresponding to the Lyman-α line for hydrogen, which is important in astrophysics. This line is commonly considered a crossover between the far ultraviolet and extreme ultraviolet spectral ranges. To generate radiation in this range, a laser-plasma source based on the gas-puff target was used. Six samples coated with Acktar Magic Black™ were studied in an optical system with a back-illuminated CCD camera as a detector. The measurements were used to derive the phenomenological BRDF model based on a series of analytic fits to the measurements, which makes it easily applicable in both numerical simulations and manual calculations. The formulas provide an approximation in the full hemispheric domain, i.e., both for the in-specular-plane and out-of-specular-plane behaviors of the BRDF for the coating. A similar fit-based phenomenological model is also described for the visible range (the wavelength of 532 nm) as a byproduct of our analysis for the UV range.

Dichroic beamsplitters, or dichroics, rely on the optical interference that occurs within thin-film layers to ensure the separation of the transmission and reflection of selective wavelengths of an incident beam of light at a given angle of incidence. Utilized within the optical systems of numerous space telescopes, they act to separate the incoming light spectrally and spatially into various channels. As space missions increasingly demand simultaneous observations across wavebands spanning extreme wavelength ranges, the necessity for exceedingly complex broadband dichroics has emerged. Subsequently, the uncertainties pertaining to their optical performance have also become more intricate. We use transmission line modeling to evaluate the spectral performance of multilayer coatings deposited on a substrate material for given thicknesses, materials, angles of incidence, and polarization. A dichroic recipe in line with the typical specifications and requirements of a dichroic is designed with the aid of a Monte Carlo simulation. The tolerances of the coating performance to systematic and random uncertainties from the manufacturing process, as well as from environmental changes in space, are studied. With the aid of accurate manufacturing recipes and uncertainty amplitudes from commercial manufacturers, this tool can predict variations in the optical performance that result from the propagation of each of these uncertainties for various hypothetical scenarios and systematic effects.

Direct imaging of Earth-like exoplanets is one of the most prominent scientific drivers of the next generation of ground-based telescopes. Typically, Earth-like exoplanets are located at small angular separations from their host stars, making their detection difficult. Consequently, the adaptive optics (AO) system’s control algorithm must be carefully designed to distinguish the exoplanet from the residual light produced by the host star. A promising avenue of research to improve AO control builds on data-driven control methods, such as reinforcement learning (RL). RL is an active branch of the machine learning research field, where control of a system is learned through interaction with the environment. Thus, RL can be seen as an automated approach to AO control, where its usage is entirely a turnkey operation. In particular, model-based RL has been shown to cope with temporal and misregistration errors. Similarly, it has been demonstrated to adapt to nonlinear wavefront sensing while being efficient in training and execution. In this work, we implement and adapt an RL method called policy optimization for AO (PO4AO) to the GPU-based high-order adaptive optics testbench (GHOST) test bench at ESO headquarters, where we demonstrate a strong performance of the method in a laboratory environment. Our implementation allows the training to be performed parallel to inference, which is crucial for on-sky operation. In particular, we study the predictive and self-calibrating aspects of the method. The new implementation on GHOST running PyTorch introduces only around 700 μs of in addition to hardware, pipeline, and Python interface latency. We open-source well-documented code for the implementation and specify the requirements for the RTC pipeline. We also discuss the important hyperparameters of the method and how they affect the method. Further, the paper discusses the source of the latency and the possible paths for a lower latency implementation.

The Prototype Segmented Mirror Telescope is a 1.3 m aperture, seven segment telescope, being developed as a technology demonstrator for India’s large optical-IR telescope project. For this segmented mirror telescope, a Shack Hartmann sensor based alignment device has been designed and developed. The device not only precisely captures the segment misalignment but also measures the segment focus error with an accuracy of a few microns and hence helps in the tip-tilt correction and co-focusing of the mirror segments. The device is designed to work primarily in two different modes: the Shack–Hartmann mode and the imaging mode. After completion of the alignment procedure, the final image quality can be checked in the imaging mode. The device is designed in such a way that it also has a provision to conduct the Keck kind of phasing experiment with one pair of mirror segments. To make the device cost effective, only off-the-shelf components are used. The optical design and opto-mechanical analysis of the device were carried out using Zemax and SolidWorks software. Then, the device was realized and its extensive testing was carried out in the laboratory. Here, we have presented the details of the opto-mechanical design and analysis as well as the preliminary results of performance tests conducted in the laboratory.

Erratum corrects errors on Figs. 4 and 5.