This is a list of reviewers who served the Journal of Astronomical Telescopes, Instruments, and Systems in 2021.

Guest Editors Anna Bonaldi, Stefan J. Wijnholds, Luca Stringhetti, and Justin Jonas summarize the Special Section on the SKA Observatory.

This paper describes the design and measured performance of the band 2 (L-band, 950 MHz–1760 MHz) cryogenic receiver front-end of the Square Kilometre Array (SKA) radio telescope dish array. The system comprises a wide flare-angle axially corrugated conical horn, a dual linearly polarized orthogonal mode transduce, a noise injection directional coupler, and two amplification stages. Its compact design and cryogenic cooling allow for a very low receiver noise temperature, and it presents another step in the continuous improvement of the noise temperature performance. A Gifford–McMahon cooler physically cools the OMT with its integrated directional coupler to around 70 K, and the first stage low-noise amplifier to about 15 K. A bespoke measurement setup was designed to measure the system’s performance. The measured receiver noise is about 6 K across the frequency band.

Radio frequency interference (RFI) has historically plagued radio astronomy, worsening with the rapid spread of electronics and increasing telescope sensitivity. We present a multi-dimensional probabilistic framework for characterizing the RFI environment around a radio astronomy site that uses data from the telescope observations themselves. We apply the framework to about 1500 h of commissioning data (∼200  TB) from the MeerKAT radio telescope; producing a six-dimensional array that yields both average RFI occupancy as well as distributions around the mean as a function of key variables (time of day, frequency, baseline, azimuth, and elevation). This allows for automated alerting for significant new sources of RFI and monitoring of RFI trends. Our results provide the first detailed view of the MeerKAT RFI environment at high sensitivity. As expected, in the MeerKAT L-band, we find that the major RFI contributors are from the global positioning system satellites, flight distance measurement equipment, and the global system for mobile communications. Beyond characterizing RFI environments, our approach allows observers access to the prior probability of RFI in any combination of tracked variables, allowing for more efficient observation planning and data flagging.

The Square Kilometre Array Observatory (SKAO) will build the most sensitive radio telescopes on Earth. To address fundamental questions in astrophysics, fundamental physics, and astrobiology, it will require processing and handling complex and extremely massive data close to the exascale, hence constituting a technological challenge for the next decade. Approximately 600 Peta-bytes (PB) of calibrated data will be delivered to the network of SKA Regional Centers (SRCs) worldwide. As a world-leading scientific instrument, SKAO aims to pursue the best practices in scientific methodology. Remarkably, it includes the reproducibility of its data as a metric of success. We present the Spanish prototype of an SRC (SPSRC), which supports preparatory scientific activities for the future SKA projects. These include science with SKA precursors and pathfinders while promoting Open Science practices as a way to enable scientific reproducibility. We describe the key developments and components of the SPSRC that align with these objectives. In particular, we describe the performed work on hardware and cloud computing infrastructure, science archive, software and services, user support and training, and collaboration with other SRCs. The resulting SPSRC platform is flexible enough to host heterogeneous projects while being scalable toward the demanding SKA requirements.

An unmanned aerial vehicle (UAV) is exploited to characterize in situ the high-band antennas (HBAs) of the low-frequency array (LOFAR) CS302 station located in Exloo, The Netherlands. The size of an HBA array is about 30 m. The Fraunhofer distance (a few kilometers) is not reachable in the frequency band (120 to 240 MHz) within the flight regulation limits. Therefore, far-field patterns cannot be directly measured. The UAV, equipped with an radio frequency synthesizer and a dipole antenna, flies in the near-field region of the considered array. Measurement of three different frequencies (124, 150, and 180 MHz) is efficiently made during the same UAV flight. The near-field focusing method is exploited to validate the far-field pattern of the array under test within an angular range around the beam axis. Such a technique avoids both the time consuming λ  /  2 sampling of the aperture field and the further application of computationally heavy near-field to far-field transformations. The array beam is well reconstructed in the main lobe and first sidelobes within a 2D scan plane sampled with a radial raster. A further postprocessing technique is proposed and validated on a subarray of HBAs. It suggests efficient ways for the future characterization of regular aperture arrays for SKA-MID Phase 2.

The MeerKAT radio telescope consists of 64 Gregorian-offset antennas located in the Karoo in the Northern Cape in South Africa. The antenna system consists of multiple subsystems working collaboratively to form a cohesive instrument capable of operating in multiple modes for defined science cases. We focus on the channelizing subsystem (F-engine), the correlation subsystem (X-engine), and the beamforming subsystem (B-engine). In the wideband instrument mode, the channelizing can produce 1024, 4096, or 32,768 channels with correlation up to 64 antennas. Narrowband mode decomposes sampled bandwidth into 32,768 channels. The F-engine also performs delay compensation, equalization, quantization, and grouping and ordering. The X-engine provides both correlation and beamforming computations (independently). This document is intended to be a stand-alone entity covering the channelizing, correlation, and beamforming processes for the MeerKAT radio telescope. This includes data reception, pre- and post-processing, and data transmission.

The Square Kilometer Array (SKA) is a planned radio interferometer of unprecedented scale that will revolutionize low-frequency radio astronomy when completed. In particular, one of its core science drivers is the systematic mapping of the Cosmic Dawn and Epoch of Reionization, which mark the birth of the first stars and galaxies in the universe and their subsequent ionization of primordial intergalactic hydrogen, respectively. The SKA will offer the most sensitive view of these poorly understood epochs using the redshifted 21-cm hyperfine signal from intergalactic hydrogen. However, significant technical challenges stand in the way of realizing this scientific promise. These mainly involve the mitigation of systematics coming from astrophysical foregrounds, terrestrial radio interference, and the instrumental response. The Low Frequency Array, the Murchison Widefield Array, and the Hydrogen Epoch of Reionization Array are SKA pathfinder experiments that have developed a variety of strategies for addressing these challenges, each with unique characteristics that stem largely from their different instrumental designs. We outline these various directions, highlighting key differences and synergies, and discuss how these relate to the future of low-frequency intensity mapping with the SKA. We also briefly summarize the challenges associated with modeling the 21-cm signal and discuss the methodologies being proposed for inferring constraints on astrophysical models.

The stations of the mid-frequency aperture array (MFAA) envisaged for the second phase of the Square Kilometer Array (SKA) are expected to consist of order 10³ to 10⁴ receive paths. This will make calibration procedures based on the array covariance matrix computationally expensive. Recently, self-holography (SH) was proposed to overcome this issue. This method assumes that the signal from the chosen calibration source can be sufficiently well isolated. We study the signal-to-interference ratio (SIR) of the most suitable calibration source over a sidereal day for a representative MFAA station located on the SKA site in South Africa. We find that an SIR of 20 dB or higher is achievable over (most of) a sidereal day. This is well above the 11.5 dB needed to keep the beamformer efficiency after calibration above 99%. We therefore conclude that SH is a promising candidate for station calibration of the MFAA stations.

Systems engineering (SE) processes applied on projects are often compliance driven with a focus on completeness, rather than value and efficiency. Most projects can benefit by applying better SE process tailoring. We illustrate how the SE process was tailored for the MeerKAT project by following broadly two strategies: customizing the process for the unique properties of the project and optimizing the process by identifying the high value process elements upfront and focusing the SE effort on these high value activities.

We present the Engineering Development Array 2, which is one of two instruments built as a second generation prototype station for the future Square Kilometre Low-Frequency Array. The array is comprised of 256 dual-polarization dipole antennas that can work as a phased array or as a standalone interferometer. We describe the design of the array and the details of design changes from previous generation instruments, as well as the motivation for the changes. Using the array as an imaging interferometer, we measure the sensitivity of the array at five frequencies ranging from 70 to 320 MHz.

Stations of dipole antennas for SKA1-Low will comprise 256 elements spread over an area with a diameter of 38 m. We consider the effect of residual unsubtracted sources well outside of the main beam for differing numbers of unique station configurations, in the epoch of reionization (EoR) and the cosmic dawn (CD), both in simulation and with theoretical considerations. We find that beam sidelobes imprint power that renders the cosmological signal unobservable over a range of scales unless compact sources are subtracted beyond θ_Z  =  30  degrees from zenith, and that station apodization will likely be required to control far sidelobes. An array with N_b  =  4 unique station configurations is sufficient to reduce the contamination, with an increase to N_b  =  8 showing little improvement. Comparison with an achromatic Airy disk beam model shows that beam sidelobe level is the main contributor to excess power in the EoR window, and beam chromaticity is less relevant. In the EoR, z  =  8.5, subtracting sources above 200 mJy out to θ_Z  =  45  deg, will be required to access relevant modes of the power spectrum.

The ambitious scientific goals of the square kilometre array (SKA) require a matching capability for calibration of instrumental and atmospheric propagation contributions as functions of time, frequency, and position. The development of calibration algorithms to meet these requirements is an active field of research. We aim to characterize these, focusing on the spatial and temporal structure scales of the ionospheric effects; ultimately, these provide the guidelines for designing the optimum calibration strategy. We used empirical ionospheric measurements at the site where the SKA-Low will be built, using Murchison widefield array (MWA) phase-2 extended baseline observations and the station-based low-frequency excision of atmosphere in parallel (LEAP) calibration algorithm. We did this via direct regression analysis of the ionospheric screens and by forming the full and detrended structure functions. We found that 50% of the screens show significant nonlinear structures at scales >0.6  km that dominate at >2  km and 1% show significant subminute temporal changes, providing that there is sufficient sensitivity. Even at the moderate sensitivity and baseline lengths of MWA, nonlinear corrections are required at 88 MHz during moderate weather and at 154 MHz during poor weather or for high signal-to-noise ratio measurements. Therefore, we predict that improvements will come from correcting for higher order defocusing effects in observations with MWA phase-2 and further with new developments in MWA phase-3. Because of the giant leap in sensitivity, the correction for complex ionospheric structures will be mandatory on SKA-Low, for both imaging and the tied-array beam formation.

The details of the MeerKAT radio telescope’s time and frequency reference subsystem that enables sampling via low-jitter, low-drift microwave clock signals, and absolute timing (≤5  ns accurate) are discussed. The subsystem’s microwave and pulse per second transmission parts are now fully qualified and commissioned for the ultra high frequency (UHF) and L-bands and also provide for a 100-MHz interface and timing interfaces for S-band receivers that were installed. The subsystem includes a cable measurement system called the Karoo array timing system (KATS). Performance and differences on different bands and seasonal drift of the cable delay measurement of KATS are reported. A time scale called the Karoo Telescope Time (KTT) (which is estimated from tracking a few atomic clocks via new software) and the issuing of timing bulletins to users have been largely implemented and verified. Absolute timing calibration and linkage of KTT to the global positioning system time scale and to different UTC(k) realizations of the Coordinated Universal Time (UTC) instances are described. The subsystem uniquely enables high-fidelity sampling and stable tied array configuration. The latter configuration enables timing and transient science over time spans of 5 to 10 years. Simultaneous subarraying is supported. The backend is unique for radio telescopes in terms of being very deterministic as far as timing is concerned.

The low frequency component of the Square Kilometre Array (SKA1-Low) will be an aperture phased array located at the Murchison Radio-astronomy Observatory (MRO) site in Western Australia. It will be composed of 512 stations, each consisting of 256 log-periodic dual-polarized antennas, and will operate in the low frequency range (50 to 350 MHz) of the SKA bandwidth. The Aperture Array Verification System 2 (AAVS2), operational since late 2019, is the last full-size engineering prototype station deployed at the MRO site before the start of the SKA1-Low construction phase. The aim of this paper is to characterize the station performance through commissioning observations at six different frequencies (55, 70, 110, 160, 230, and 320 MHz) collected during its first year of activities. We describe the calibration procedure, present the resulting all-sky images and their analysis, and discuss the station calibratability and system stability. Using the difference imaging method, we also derive estimates of the SKA1-Low sensitivity for the same frequencies and compare them with those obtained through electromagnetic simulations across the entire telescope bandwidth, finding good agreement (within 13%). Moreover, our estimates exceed the SKA1-Low requirements at all considered frequencies by up to a factor of ∼2.3. Our results are very promising and allow for an initial validation of the AAVS2 prototype station performance, which is an important step toward the coming SKA1-Low telescope construction and science.

Square Kilometer Array (SKA)-Low is the radio telescope operating in the lowest frequency band of the SKA, from 50 up to 350 MHz. It consists of 512 stations, each composed of 256 dual-polarization log-periodic antennas for a total of 262,144 independent signal paths. The low-frequency aperture array (LFAA) is the portion of the SKA-Low telescope including the antennas and the related electronics. Signal processing is hosted in a temperature controlled and shielded facility: the central processing facility (CPF), for all the core stations, or remote processing facilities (RPF), for stations in the array arms, to limit the maximum fiber length. Such a geographically distributed and interconnected radio telescope, spanning ∼65  km in diameter, requires that frequency and timing reference signals are distributed to the processing facilities with high stability and precision to ensure the required system performances. We present the realization of the clock and pulse per second distribution network inside the LFAA signal processing cabinet where subracks containing signal acquisition boards are housed. We describe the different parts of the chain, and we report on the total jitter introduced by this structure.

The signal reception chain is an essential element for achieving the square kilometer array-low (SKA-low) system requirements in terms of high sensitivity and dynamic range. The balance between gain, linearity, and low power consumption, as well as the cost, are fundamental parameters that influence the selection of the most suitable technology for SKA-low. Further factors, such as low self-generated radio frequency (RF) interference, high reliability, robustness under extreme environment, and last but not least, the distance between the antennas and the acquisition systems, have impacts on the selection for both architecture and receiver system design. The selected technology for the SKA-low RF signal transportation is RF-over-fiber systems, where the preamplified RF signal picked up by the antennas is carried via analogue modulation over optical fiber. The rationales behind the selection are reported, along with descriptions on the development of the receiver prototypes. The prototypes were deployed and installed on the demonstrator arrays at the selected SKA-low site in Western Australian. Particular attention has been put on the thermal characterization of the receiver system under the actual operating temperature on site, especially when both transmitting part and the optical medium are subjected to external ambient temperature variations. Performance issues encountered in the demonstrator arrays are also discussed along with some proposals for future activities.

We summarize the activities conducted since 2019 in the numerical electromagnetic analysis of one prototype station of the SKA-Low telescope. Working closely with the SKA Observatory, two teams based in Australia and Italy have collaborated effectively in modeling and analyzing AAVS2, which is the most recent prototype of an SKA-Low station installed in Western Australia. A comprehensive overview of the main electromagnetic parameters at element and array level obtained with two different commercial solvers is presented. Results for scattering parameters, individual element patterns, and station beams are shown; all these fully incorporate mutual coupling effects. Sensitivity of the station is addressed, as the cross-polarization performance. Finally, we also address some lessons learned and their impact on the project.

The Square Kilometre Array Low is a next generation radio telescope, consisting of 512 antenna stations spread over 65 km, to be built in Western Australia. The correlator and beamformer (CBF) design is central to the telescope signal processing. CBF receives 6 Tera-bits-per-second (Tbps) of station data continuously and processes it in real time with a compute load of 2 Peta-operations-per-second (Pops). The correlator calculates up to 22 million cross products between all pairs of stations, whereas the beamformers (BFs) coherently sum station data to form more than 500 beams. The output of the correlator is up to 7 Tbps, and the BF 2 Tbps. The design philosophy, called “Atomic COTS,” is based on commercial off-the-shelf (COTS) hardware. Data routing is implemented in network switches programmed using the Programming Protocol-Independent Packet Processors (P4) language and the signal processing occurs in COTS field-programmable gate array (FPGA) cards. The P4 language allows routing to be determined from the metadata in the Ethernet packets from the stations. That is, metadata describing the contents of the packet determines the routing. Each FPGA card inputs a fraction of the overall bandwidth for all stations and then implements the processing needed to generate complete science data products. Generation of complete science products in a single FPGA is named here as Atomic processing. A Tango distributed control system configures the multitude of processing modes as well as maintaining the overall health of the CBF system hardware. The resulting 6 Tbps in and 9 Tbps out, 2 Pops Atomic COTS network attached accelerator occupies five racks and consumes 60 kW.

The Hydrogen Intensity and Real-time Analysis Experiment (HIRAX) is a radio interferometer array currently in development, with an initial 256-element array to be deployed at the South African Radio Astronomy Observatory Square Kilometer Array site in South Africa. Each of the 6 m, f  /  0.23 dishes will be instrumented with dual-polarization feeds operating over a frequency range of 400 to 800 MHz. Through intensity mapping of the 21 cm emission line of neutral hydrogen, HIRAX will provide a cosmological survey of the distribution of large-scale structure over the redshift range of 0.775  <  z  <  2.55 over ∼15,000 square degrees of the southern sky. The statistical power of such a survey is sufficient to produce ∼7  %   constraints on the dark energy equation of state parameter when combined with measurements from the Planck satellite. Additionally, HIRAX will provide a highly competitive platform for radio transient and HI absorber science while enabling a multitude of cross-correlation studies. We describe the science goals of the experiment, overview of the design and status of the subcomponents of the telescope system, and describe the expected performance of the initial 256-element array as well as the planned future expansion to the final, 1024-element array.

We present the main performance aspects of the sensitivity and dynamic range of the Square Kilometre Array (SKA). The sensitivity and dynamic range of a radio astronomy interferometer are affected by different sources of errors and noises. A general description of these effects is given, focusing on the direction-independent effects.

The Square Kilometre Array Observatory (SKAO) will construct two radio telescopes: SKA-Low in Australia and SKA-Mid in South Africa. When completed, the Square Kilometer Array (SKA) will be the largest radio telescope on Earth, with unprecedented sensitivity and scientific capability. The first phase of SKA-Mid (called SKA1-Mid) includes an array of 197 dish antennas incorporating the recently completed MeerKAT dishes to cover the frequency range of 350 MHz to 15.4 GHz. The 19  Tb  /  s digitized data stream is transported from the dishes in the remote Karoo to Cape Town where data are correlated and processed through high-performance computing systems. The demanding scientific performance requires extremely accurate timing and synchronization of the data measured by the distributed dishes. The combination of large-scale deployment, significant real-time processing, geographic distribution, and limited budget poses significant challenges for the physical, control, and processing architectures. We present the architectural highlights of the SKA1-Mid Telescope baseline design, for which its Critical Design Review was completed in 2019 and construction was started in July 2021.

The Square Kilometre Array (SKA) is a next-generation radio astronomy facility that will revolutionize our understanding of the Universe and the laws of fundamental physics. To achieve the intended objectives, it needs a stable reference frequency and accurate timing signals at each digitizer. These references are used for digitizing astronomical signals received from the receptors. The stability and accuracy of these references are highly important for coherently sampling the astronomical data. They are distributed using long-distance fibers that are susceptible to environmental perturbations, which makes meeting the requirements a challenge. The system overcomes these perturbations by actively stabilizing the noise during fiber transmission to achieve the required reference signal stability and sub-nanosecond level of timing accuracy. We collect together summary descriptions of the sub-systems designed for distributing the reference frequency and timing signals for each telescope, to provide an overview of the whole timing and frequency system for the SKA.

One of the advantages of arrays with aperiodic distributed elements is their ability to mitigate the detrimental mutual coupling effects on the radiation pattern. However, we show that the mutual coupling inside a random array can still generate undesired structures in the frequency response although the single antenna features a spectral smooth response. For small subsets (a couple of SKALA4.1 antennas and a 16-element array) of a low-frequency instrument station of the Square Kilometre Array, the combination of large mutual coupling and antenna geometry creates systematic distortions in the element frequency responses. This phenomenon compromises the station spectral smoothness response versus frequency. However, we demonstrate that it is possible to partially mitigate these frequency structures by reconfiguring the antenna distribution based on exclusion zones.

The Square Kilometre Array (SKA) is an ambitious project to build the world’s largest radio telescope to revolutionize our understanding of the Universe and the laws of fundamental physics. Geographically distributed between three host countries, and with more than a dozen member nations, the SKA is composed of two radio telescopes (SKA1-LOW and SKA1-MID) and a Headquarters facility. The SKA is now moving toward the start of the procurement phase and construction activities and is about to become a reality on the ground. Here, we focus on the SKA1-LOW and present the architectural highlights of what will be the most sensitive aperture array telescope on the earth, operating between 50 and 350 MHz.

The third-generation South Pole Telescope camera (SPT-3G) improves upon its predecessor (SPTpol) by an order of magnitude increase in detectors on the focal plane. The technology used to read out and control these detectors, digital frequency-domain multiplexing (DfMUX), is conceptually the same as used for SPTpol, but extended to accommodate more detectors. A nearly 5× expansion in the readout operating bandwidth has enabled the use of this large focal plane, and SPT-3G performance meets the forecasting targets relevant to its science objectives. However, the electrical dynamics of the higher-bandwidth readout differ from predictions based on models of the SPTpol system due to the higher frequencies used and parasitic impedances associated with new cryogenic electronic architecture. To address this, we present an updated derivation for electrical crosstalk in higher-bandwidth DfMUX systems and identify two previously uncharacterized contributions to readout noise, which become dominant at high bias frequency. The updated crosstalk and noise models successfully describe the measured crosstalk and readout noise performance of SPT-3G. These results also suggest specific changes to warm electronics component values, wire-harness properties, and SQUID parameters, to improve the readout system for future experiments using DfMUX, such as the LiteBIRD space telescope.

The Spitzer Space Telescope operated for over 16 years in an Earth-trailing solar orbit, returning not only a wealth of scientific data but also, as a by-product, spacecraft and instrument engineering data that will be of interest to future mission planners. These data will be particularly useful because Spitzer operated in an environment essentially identical to that at the L2 LaGrange point, where many future astrophysics missions will operate. In particular, the radiative cooling demonstrated by Spitzer has been adopted by other infrared space missions, from JWST to SPHEREx. We aim to facilitate the utility of the Spitzer engineering data by collecting the more unique and potentially useful portions into a single, readily accessible publication. We avoid discussion of less unique systems, such as the telecom, flight software, and electronics systems, and do not address the innovations in mission and science operations that the Spitzer team initiated. These and other items of potential interest are addressed in references supplied in an appendix to this paper.

NASA’s return to the Moon coincides with explosive growth in exoplanet discovery. Missions are being formulated to search for habitable planets orbiting other stars, making this the ideal time to deploy an instrument suite to the lunar surface to help us recognize a habitable exoplanet when we see it. We present EarthShine, a technically mature, three-instrument suite to observe the whole Earth from the Moon as an exoplanet proxy. EarthShine data will validate and improve models critical for designing missions to image and characterize exoplanets, thus informing observing strategies for flagship missions to directly image exoplanets. EarthShine will answer interconnected questions in Earth and lunar science, exoplanets, and astrobiology, related to the credo “follow the water.” EarthShine can take advantage of current NASA programs to conduct science from the Moon with low-cost, mature space hardware to reduce risk and assure success. Like the 1968 Apollo Earthrise image of our home planet, lonely in the black sky, the appeal of EarthShine to a multidisciplinary array of researchers in Earth Science, Planetary Science, and astrophysics will maximize both its scientific impact and its impact on the general public.

Ground-based observatories’ capabilities can be greatly enhanced by working with satellites in astrostationary orbits. Satellites inertially aligned with ground-based observatories can help mitigate atmospheric effects in astronomical, solar, and planetary observations. We present a method for developing and modeling the trajectory of potential astrostationary orbits as seen from the ground to understand whether they meet defined astrostationary conditions such as the location in the field of interest and the observation time. It discovers an L2 orbit family and shows that it can meet astrostationary conditions. It then presents an example mission, Orbiting Configurable Artificial Star (ORCAS), in which a satellite aligns with a ground-based observatory to provide near-diffraction limited observations in the visible wavelength. A highly elliptical orbit family which can meet the ORCAS requirements is studied, and we go on to show how multiple observations can be made during a single-orbit period and how specific configurations that increase the observation time can be found. Finally, other potential astrostationary families are presented, as well as additional applications that could benefit from spacecraft and ground-based observatories working together, and future work which can be done to move forward in the field is discussed.

The Cherenkov Telescope Array (CTA) is the next ground-based gamma-ray observatory in the TeV γ-ray spectral region operating with the Imaging Atmospheric Cherenkov Technique. It is based on almost 70 telescopes of different class diameters—Large-Sized Telescope, Medium-Sized Telescope (MST), and Small-Sized Telescope (SST) of 23, 12, and 4 m, respectively—to be installed in two sites in the two hemispheres (at La Palma, Canary Islands, and near Paranal, Chile). Several thousands of reflecting mirror tiles larger than 1  m² will be produced for realizing the segmented primary mirrors of a so large number of telescopes. Almost in parallel, the ASTRI mini-array (MA) is being implemented in Tenerife (Canary Islands), composed of nine 4-m diameter dual-mirror Cherenkov telescopes (very similar to the SSTs). We completed the mirror production for all nine telescopes of the ASTRI MA and two MSTs (400 segments in total) using the cold glass slumping replication technology. The results related to the quality achieved with a large-scale production are presented, also discussing the adopted testing methods and approaches. They will be very useful for the adoption and optimization of the quality assurance process for the huge production (almost 3000  m² of reflecting surface) of the MST and SST CTA telescopes.

The Extreme-ultraviolet Stellar Characterization for Atmospheric Physics and Evolution (ESCAPE) mission is an astrophysics Small Explorer employing ultraviolet spectroscopy (EUV: 80 to 825 Å and FUV: 1280 to 1650 Å) to explore the high-energy radiation environment in the habitable zones around nearby stars. ESCAPE provides the first comprehensive study of the stellar EUV and coronal mass ejection environments that directly impact the habitability of rocky exoplanets. In a 20-month science mission, ESCAPE will provide the essential stellar characterization to identify exoplanetary systems most conducive to habitability and provide a roadmap for NASA’s future life-finder missions. ESCAPE accomplishes this goal with roughly two-order-of-magnitude gains in EUV efficiency over previous missions. ESCAPE employs a grazing incidence telescope that feeds an EUV and FUV spectrograph. The ESCAPE science instrument builds on previous ultraviolet and x-ray instrumentation, grazing incidence optical systems, and photon-counting ultraviolet detectors used on NASA astrophysics, heliophysics, and planetary science missions. The ESCAPE spacecraft bus is the versatile and high-heritage Ball Aerospace BCP-Small spacecraft. Data archives will be housed at the Mikulski Archive for Space Telescopes.

The prototype Schwarzschild-Couder Telescope (pSCT) is a candidate for a medium-sized telescope in the Cherenkov Telescope Array. The pSCT is based on a dual-mirror optics design that reduces the plate scale and allows for the use of silicon photomultipliers as photodetectors. The prototype pSCT camera currently has only the central sector instrumented with 25 camera modules (1600 pixels), providing a 2.68-deg field of view (FoV). The camera electronics are based on custom TARGET (TeV array readout with GSa/s sampling and event trigger) application-specific integrated circuits. Field programmable gate arrays sample incoming signals at a gigasample per second. A single backplane provides camera-wide triggers. An upgrade of the pSCT camera that will fully populate the focal plane is in progress. This will increase the number of pixels to 11,328, the number of backplanes to 9, and the FoV to 8.04 deg. Here, we give a detailed description of the pSCT camera, including the basic concept, mechanical design, detectors, electronics, current status, and first light.

The polarization modulator unit for the low-frequency telescope in LiteBIRD employs an achromatic half-wave plate (AHWP). It consists of five layers of a-cut sapphire plate, which are stacked based on a Pancharatnam recipe. In this way, the retardance of the AHWP is a half-wave over a bandwidth of 34 to 161 GHz. The diameter of a single sapphire plate is about 500 mm and the thickness is 5 mm. When a large-diameter AHWP is used for a space mission, it is important for the AHWP to survive launch vibration. A preliminary study indicates that the five-layer stacked HWP has a risk of breakage at the launch unless the five layers are glued together and mechanically treated as one disk. We report our investigation using a sodium silicate solution that can bond the sapphire plates. This technique has been previously investigated as a candidate of cryogenic bonding for a mirror material, including sapphire, of the gravitational wave experiments: LIGO, VIRGO, and KAGRA. We experimentally studied the mechanical strength of the bonded interface for two different surface conditions: polished and unpolished. We demonstrated that the tensile and shear strength >20  MPa for samples with a polished surface. This satisfied the requirement of 5.5 MPa derived from the mechanical simulation assuming a launch load of 30 G. We identified that samples glued on a polished surface exhibit higher strength than unpolished ones by a factor of 2 for tensile and 18 for shear strength. We measured the millimeter-wave transmittance between 90 and 140 GHz using sapphire plates with a diameter of 50 mm before and after bonding. We did not find any optical effects caused by the bonded interface within 2% error in transmittance, which originates from the measurement system.

We describe a method by which the metrology system of the Nuclear Spectroscopic Telescope Array (NuSTAR) X-ray space observatory, which uses two lasers to characterize the relative motion of the optics and focal plane benches, can be approximated should one laser fail. The two benches are separated by a 10-m-long rigid mast that undergoes small amounts of thermal flexing that needs to be compensated for to produce a nonblurred image. We analyze the trends of mast motion by archival observation parameters to discover whether the mast motion in future observations can be predicted. We find that, using the solar aspect angle, observation date, and orbital phase, we can simulate the motion of one laser by translating the track produced by the other and applying modifications to the resulting mast aspect solution, allowing for the reconstruction of a minimally distorted point spread function in most cases. We will implement the generation of simulated mast files along with the usual NuSTAR data reduction pipeline for contingency purposes. This work has implications for reducing the risk of implementing laser metrology systems on future missions that use deployable masts to achieve the long focal lengths required in high-energy astronomy by mitigating the impact of a metrology laser failure in the extended phase of a mission.

Starshades are large, precisely shaped occulting masks flown in space and positioned between a telescope and its target star. The function of the starshade is to block direct starlight so that any exoplanets orbiting the target star may be observed. Formation flying with meter-level accuracy—needed to keep the starshade aligned between the telescope and the star—has been demonstrated at Technology Readiness Level 5. With the starlight largely blocked by the starshade, a second problem is to keep the telescope accurately pointed at the star, in the face of disturbances. The feasibility of measuring both telescope pointing and starshade position simultaneously from the residual starlight reaching a pupil-plane camera within the telescope beam train is investigated. Three sensing approaches, combining two types of sensors and two types of image-processing algorithms, are compared. It is shown that the required measurement accuracy can be achieved both for starshade position and telescope pointing, with little impact on scientific observations and with no additional hardware. Overall, differences in accuracy, speed, and memory requirements lead to a preferred architecture.

Zero-gravity surface figure testing is important for space optical elements. We propose the three orientations method (TOM) to achieve this goal. The optical element is rotated according to the proposed degree of freedom, and we calculate its zero-gravity surface figure by any three rotation angles (θ₁, θ₂, and θ₃) and their corresponding surface figures (S₁,  S₂, and   S₃). The algorithm of TOM can also be applied to the face-up/face-down test and horizontal rotation test. Theoretically, TOM can get an absolute zero-gravity surface figure; however, a rotation angle error can result in a zero-gravity surface figure error. The surface figure model (SFM) and gravity-included angle model (GAM) are proposed to analyze the relationship between the rotation angle error and zero-gravity surface figure error. For numerical results, SFM is equivalent to GAM. In addition, GAM addresses the zero-gravity surface figure error, which is induced by the rotation angle error, also being regarded as a surface figure, which is caused by a certain amount of gravity. For all models, we provide both theoretical derivations and numerical examples.

Infrared space interferometers can surpass the spatial resolution limitations of single-dish space telescopes. However, stellar interferometers from space have not been realized because of technical difficulties. Two beams coming from individual satellites separated by more than a few tens of meters should precisely interfere such that the optical-path and angular differences between the two beams are reduced at the wavelength level. Herein, we propose a unique beam combiner for space interferometers that records the spectrally resolved interferometric fringes using the densified pupil spectroscopic technique. As the detector plane is optically conjugated to a plane, on which the two beams interfere, we can directly measure the relative phase difference between the two beams. Additionally, when an object within the field of view is obtained with a modest signal-to-noise ratio, we can extract the true complex amplitude from a continuous broadband fringe (i.e., one exposure measurement), without scanning a delay line and chopping interferometry. We discovered that this spectral imaging method is validated for observing the solar system objects by simulating the reflected light from Europa with a small stellar interferometer. However, because the structure of the object spectrum may cause a systematic error in the measurement, this method may be limited in extracting the true complex amplitude for other astronomical objects. Applying this spectral imaging method to general astrophysics will facilitate further research. The beam combiner and spectral imaging method are applied to a formation flying stellar interferometer with multiple small satellites in a Sun-synchronous orbit, named Space Experiment of InfraRed Interferometric Observation Satellite (SEIRIOS), for observation of the solar system objects in visible and near-infrared. We present an overview of SEIRIOS and the optimized optical design for a limited-volume spacecraft.

The optical design of an integral field unit (IFU) based on image-slicers represents an interesting design exercise because of the small size of the components and the critical parameters to reduce optical aberrations. When the IFU to be designed is also intended to substitute an IFU prototype that is already integrated into a spectropolarimeter, it becomes even more challenging. The main reason to upgrade the IFU prototype is to cover a larger field of view (FOV). One of the work packages of the European project “Getting Ready for the European Solar Telescope” (GREST) had as its objective to find the best option to design such an IFU. After analyzing the requirements and the design considerations, the optical design pointed toward an IFU that doubles the FOV, reaching a value of 40.5  arcsec². The designed IFU is composed of two image-slicers coupled together, each one with eight mirrors of 1.8  mm  ×  100  μm that feed two slits to the spectrograph.

The requirements for a coronagraph instrument to image and obtain spectra of rocky planets around bright stars from space are tight. Indeed, the goal of imaging an Earth-like planet requires a starlight suppression system that cancels light to a level of ^{10  −  10} with sufficient stability and robustness to errors. Furthermore, the key science questions necessitate an adequate sample size; consequently, the throughput of the coronagraph drives the achievable yield of a given mission. The trade among achievable raw contrast, sensitivity to wavefront errors, and throughput poses a challenging problem in coronagraph design. The complexity of this problem drives us toward the simultaneous solving of all optical elements. We present a set of methods to optimize the design of a coronagraph. We implement these for the case of the hybrid Lyot coronagraph in the context of the Nancy Grace Roman Space Telescope Coronagraph Instrument. We discuss our findings in terms of coronagraph instrument design, and optical subsystems, and performance interplay.

An indium-gallium-arsenide (InGaAs) detector is tested for use on the new Dynamic REd All-sky Monitoring Survey (DREAMS) 0.5-m telescope. DREAMS is novel for its use of InGaAs as a higher-noise and lower-cost alternative to mercury-cadmium-telluride. The Princeton Infrared Technologies 1280SCICAM, which has one of the smallest pitches and largest focal planes of any commercially available InGaAs detector, is extensively characterized to determine the viability of InGaAs detectors for astronomy. We find the 1280SCICAM to have the one of the lowest dark currents (67e⁻  /  s) of any commercially available InGaAs focal plane array, and also confirm no fringing or non-linearity is present. Given its low noise, we conclude that DREAMS will be sufficiently background limited with InGaAs, and by extension, InGaAs is well-suited for application on low-angular-resolution NIR instruments.

The modulation transfer function (MTF) is a useful measure in image quality analysis and performance budget determination. Sensitive long wavelength infrared (LWIR) detectors for astronomical space telescopes require slight modifications to the existing MTF measurement methods due to the increased prevalence of high dark current pixels. Presented here are the specifics of a modified slanted edge method to determine the MTF in λ_c  >  10  μm HgCdTe detectors to be used with the planned Near-Earth Object Surveyor Mission. The measured MTF at Nyquist using 6  μm light is 0.22  ±  0.02 and is 0.25  ±  0.02 using 10  μm light for both 250 and 350 mV of applied reverse bias. These measurements are from edge spread functions with median signal values around 50% of the well depth, as the MTF is expected to change with signal value due to two brighter-fatter type effects. The expected trends caused by the influences of these two effects and the expected trends with wavelength of absorbed photons are all observed.

The European Space Agency’s Gaia spacecraft has been operating in L2 ever since its launch in December 2013 with a payload that includes 106 scientific charge-coupled devices (CCDs). Due to the predicted radiation environment at the pre-flight testing stage in addition to the high level of accuracy demanded by the science objectives, the non-ionizing energy loss (NIEL) damage on the detectors was identified as a major factor that could affect the science goals of the mission. Here, we present the analysis of an extended set of charge calibration data, taken up to almost six years after launch. It is found that the rate of radiation damage accumulation by the CCDs has not differed significantly from previous results. While the parallel and serial CTI measure an increase in time, the trap defect landscape is still dominated by the pre-flight defects rather than the radiation-induced traps. CCD devices that were predicted to have a lower NIEL dose measure comparatively larger rates of CTI increase. In addition to this, thicker devices have been measured to have lower serial CTI values compared to thinner devices. The initial parallel CTI values have also been found to be dependent on manufacture year.

Future high-resolution imaging x-ray observatories may require detectors with both fine spatial resolution and high quantum efficiency at relatively high x-ray energies (E  ≥  5  keV). A silicon imaging detector meeting these requirements will have a ratio of detector thickness to pixel size of six or more, roughly twice that of legacy imaging sensors. The larger aspect ratio of such a sensor’s detection volume implies greater diffusion of x-ray-produced charge packets. We investigate consequences of this fact for sensor performance, reporting charge diffusion measurements in a fully depleted back-illuminated CCD with a thickness of 50  μm and pixel size of 8  μm. We are able to measure the size distributions of charge packets produced by 5.9 and 1.25 keV x-rays in this device. We find that individual charge packets exhibit a Gaussian spatial distribution and determine the frequency distribution of event widths for a range of detector bias (and thus internal electric field strength) levels. At the largest bias, we find a standard deviation for the largest charge packets (produced by x-ray interactions closest to the entrance surface of the device) of 3.9  μm. We show that the shape of the event width distribution provides a clear indicator of full depletion and use a previously developed technique to infer the relationship between event width and interaction depth. We compare measured width distributions to simulations. Although we can obtain good agreement for a given detector bias, with our current simulation, we are unable to fit the data for the full range of bias levels with a single set of simulation parameters. We compare traditional, “sum-above-threshold” algorithms for individual event amplitude determination to Gaussian fitting of individual events and find that better spectroscopic performance is obtained with the former for 5.9 keV events, whereas the two methods provide comparable results at 1.25 keV. The reasons for this difference are discussed. We point out the importance of read-noise driven charge detection thresholds in degrading spectral resolution, and note that the derived read noise requirements for mission concepts such as AXIS and Lynx are probably too lax to assure that spectral resolution requirements can be met. While the measurements reported here were made with a CCD, we note that they have implications for the performance of high aspect-ratio silicon active pixel sensors as well.

The wide field survey telescope (WFST) is a new generation survey telescope that is being built in China. Its optical design is a primary-focus system, and its camera is a mosaic charge-coupled device (CCD) camera composed of nine 9  K  ×  9  K CCD290-99 chips for scientific imaging. A verification platform to test the CCD290-99 chips is designed. The test platform includes a light source system, CCD controller, vacuum Dewar, and refrigerator for cooling the CCD. The CCD controller is a prototype design of the WFST camera that has a high performance, including low readout noise, flexible readout rate configuration, low power dissipation, etc. The digital double correlated sample method is used for video sampling of the CCD’s 16 channels. The specifications of the CCD detector system using a CCD290, such as gain, noise linearity, and crosstalk, are tested using this platform. The test results show that the CCD test platform meets the requirement of the CCD test and the design of CCD controller meets the scientific imaging requirements for the WFST camera.

A prototype for dedicated observations of pulsars and other astrophysical transients in the frequency range of 50 to 80 MHz has been recently commissioned in the Gauribidanur Radio Observatory near Bangalore in India. The antenna setup, the analog and digital receiver systems, and the initial observations are presented.

A laboratory-based reflectometer designed for characterizing the reflectivity of optical coatings in 30- to 200-nm wavelength range was recently developed at IPOE. An RF-produced gas-discharge light source is applied to generate characteristic lines. The light source is mounted on a grazing incident monochromator with a 146-deg deviation angle between the incident and diffracted arms. By precisely adjusting the toroidal grating inside the monochromator chamber, monochromatic lights are acquired through the exit slit. A collimator mirror and two sets of collimation slits with 2  mm  ×  2  mm dimension are utilized for reducing the divergence of the beam incident on the sample. A high-precision six-axis translation stage, which allows a heavy sample with a maximum diameter of 100 mm, is used to control positions of the samples and the detector. A chopper disk used both for incident light intensity monitor and signal modulation is placed with an incidence angle of 70 deg relative to the incident light beam. The configuration, adjustment process, and test results of the reflectometer are presented in detail. The experimental reflectivity results for Al  /  LiF  /  MgF₂ film obtained from our laboratory and BESSY-II Synchrotron as well as Hefei Synchrotron Light Source are given and compared for demonstrating the reliability of the system.

Exoplanet imaging missions utilizing an external occulter (starshade) for starlight suppression require precise alignment between the telescope and starshade, necessitating maintenance of the starshade orbit during observations. Differential lateral acceleration between the two spacecraft serves as a proxy for fuel use and number of required thruster firings, which may interrupt the observation. Comparison against results from high fidelity simulations of stationkeeping validates the use of this easy-to-compute proxy. Among starshade positions constrained to the surface of a sphere centered about the telescope, minima of differential lateral acceleration lie on a great circle and its corresponding poles. We present a closed expression for telescope to star vectors requiring minimal stationkeeping for observation from a telescope at an arbitrary position. This proxy metric along with analytical knowledge of optimal observations facilitates computationally efficient design of exoplanet imaging missions employing a starshade.

We present the concept of using an orbiting laser as a coherent optical reference to phase a several kilometer diameter array of ground-based lasers designed to accelerate interstellar nano-spacecraft to 20% light-speed using laser propulsion. We investigate the geometrical and temporal constraints for the initial case of the target star Proxima b in the Alpha Centauri system using a laser ground site in the southern hemisphere. Based on these constraints, we detail requirements for the mission architecture for an orbiting laser to be used as an optical reference. We then present two orbits that can meet all given requirements and represent a range of engagement times and days between engagements. We also present a range of orbits with periods from 3 to 4 days and engagement times from 660 to 800 s. If desired, the orbit can be matched to the sidereal day, so each orbit period, the beacon can align with the ground station and the same target star without maneuvers. A discussion of the tradeoff between the Earth-based site latitude, time on engagement, and days between engagements is presented.

The Wide Field Imager (WFI) flying on Athena will usher in the next era of studying the hot and energetic Universe. Among Athena’s ambitious science programs are observations of faint, diffuse sources limited by statistical and systematic uncertainty in the background produced by high-energy cosmic ray particles. These particles produce easily identified “cosmic-ray tracks” along with less easily identified signals produced by secondary photons or x-rays generated by particle interactions with the instrument. Such secondaries produce identical signals to the x-rays focused by the optics and cannot be filtered without also eliminating these precious photons. As part of a larger effort to estimate the level of unrejected background and mitigate its effects, we here present results from a study of background-reduction techniques that exploit the spatial correlation between cosmic-ray particle tracks and secondary events. We use Geant4 simulations to generate a realistic particle background signal, sort this into simulated WFI frames, and process those frames in a similar way to the expected flight and ground software to produce a realistic WFI observation containing only particle background. The technique under study, self-anti-coincidence (SAC), then selectively filters regions of the detector around particle tracks, turning the WFI into its own anti-coincidence detector. We show that SAC is effective at improving the systematic uncertainty for observations of faint, diffuse sources, but at the cost of statistical uncertainty due to a reduction in signal. If sufficient pixel pulse-height information is telemetered to the ground for each frame, then this technique can be applied selectively based on the science goals, providing flexibility without affecting the data quality for other science. The results presented here are relevant for any future silicon-based pixelated x-ray imaging detector and could allow the WFI and similar instruments to probe to truly faint x-ray surface brightness.

Compressive sensing (CS) is a unique mathematical technique for simultaneous data acquisition and compression. This technique is particularly apt for time-series photometric measurements; we apply CS to time-series photometric measurements specifically obtained due to gravitational microlensing events. We show the error sensitivity in detecting microlensing event parameters through simulation modeling. Particularly, we show the relation of both the amount of error and its impact on the microlensing parameters of interest. We derive statistical error bounds to apply those as a baseline for analyzing the effectiveness of CS application. Our results of single and binary microlensing events conclude that we can obtain error less than 1% over a three-pixel radius of the center of the microlensing star by using 25% Nyquist rate measurements.

The contrast performance of current extreme adaptive optics (XAO) systems can be improved by adding a second AO correction stage featuring its own wavefront sensor (WFS), deformable mirror (DM), and real-time controller. We develop a dynamical model for such a cascade adaptive optics (CAO) system with two stages each controlled by a standard integrator and study its control properties. We study how such a configuration can improve an existing system without modifying the first stage. We analyze the CAO architecture in general and show how part of the disturbance is transferred from low to high temporal frequencies with a nefarious effect of the second stage integrator overshoot and suggest possible ways to mitigate this. We also carry out numerical simulations of the particular case of a first stage AO using a Shack–Hartmann WFS and a second stage AO with a smaller DM running at a higher framerate to reduce temporal error. In this case, we demonstrate that the second stage improves imaging contrast by one order of magnitude and shortens the decorrelation time of atmospheric turbulence speckles by even a greater factor. The results show that CAO presents a promising and relatively simple way to upgrade some existing XAO systems and achieve improved imaging contrasts fostering a large number of science case including the direct imaging of exoplanets.

We propose an efficient approach for focal-plane wavefront estimation for high-contrast imaging. It is designed for very low photon flux regimes with the Roman Space Telescope Coronagraph Instrument in mind. During days-long exposures of Roman, the starlight wavefront will drift due to the thermal instabilities of the optical tube assembly (OTA) and deformable mirror (DM) actuators. To correct the drift in high-order spatial wavefront modes, Roman will periodically be pointed to a bright reference star to sense the speckles in the dark hole (the high-contrast region of the image) and correct them. However, this maneuver decreases observation time by several hours a day and may lead to wavefront instability by causing structural and thermal stresses on the OTA. Instead, we propose maintaining the dark hole using the DM to modulate (probe) the speckles while observing the target star. Our proposed algorithm can then be used to directly estimate the DM commands that would create such speckles, rather than first estimating the electric field of the speckles. Applying the opposite commands on the DM reduces the intensity of the speckles without slewing to the reference star. Further, we analyze this dark-hole maintenance approach using data from Roman Observing Scenario 9 (OS9) simulations while assuming that the high-order wavefront control on Roman will be carried out from the Earth (ground-in-the-loop). In our simulations of Roman’s Hybrid Lyot Coronagraph, the high-order wavefront errors were sensed and corrected while observing a dim target, 47 UMa. Despite the DM probes, the photon flux in the dark hole remained close to that in OS9 and well below detector noise. In terms of planet-detectability, dark hole maintenance kept the post-processing contrast within 10% of OS9’s, even after pointing the telescope at the target star for 6 days. The OTA stability that dark hole maintenance provides compared to slewing the telescope potentially outweighs the small negative impact it has on the contrast.

This erratum corrects errors in two figures.