Guest Editors Sandrine Thomas, Gelys Trancho, Elise Vernet, and Tony Travouillon introduce the Special Section on Extremely Large Telescopes.

The next generation of ground-based optical telescopes, such as the European Southern Observatory’s Extremely Large Telescope (the ELT), will have large support structures (spiders) for the secondary mirror. These spiders have the effect of segmenting the pupil. Without careful control of the wavefront, segment piston (petal modes) errors can develop. We present a flip-flop modulated/unmodulated method for the pyramid wavefront sensor (PWFS) enabling the PWFS to sense petal piston modes. This flip-flop modulation method uses a single PWFS operating in two states: a modulated state and an unmodulated state. An independent controller is used in each state; the modulated state controls the atmospheric turbulence and the unmodulated state only controls petal piston modes. In simulation, we show the flip-flop method working with the wavefront sensor in both K- and R-bands, providing an improvement of 9.9% and 13%, respectively, over a standard modulated PWFS.

The Multi-conjugate Adaptive Optics RelaY (MAORY) is one of the key adaptive optics (AO) systems on the European Southern Observatory’s Extremely Large Telescope. MAORY aims to achieve good wavefront correction over a large field of view, which involves a tomographic estimation of the three-dimensional atmospheric wavefront disturbance. Mathematically, the reconstruction of turbulent layers in the atmosphere is severely ill-posed, hence, limits the achievable reconstruction accuracy. Moreover, the reconstruction has to be performed in real time at a few hundred to one thousand hertz frame rates. Huge amounts of data have to be processed and thousands of actuators of the deformable mirrors have to be controlled by elaborated algorithms. Even with extensive parallelization and pipelining, direct solvers, such as the matrix vector multiplication method, are extremely demanding. Thus, research in recent years shifted into the direction of iterative methods. We focus on the iterative finite-element wavelet hybrid algorithm (FEWHA). The key feature of FEWHA is a matrix-free representation of all operators involved, which makes the algorithm fast and enables on-the-fly system updates whenever parameters at the telescope or in the atmosphere change. We provide a performance analysis of the method regarding quality and run-time for the MAORY instrument using the AO software package COMPASS.

Structural, thermal, and optical performance (STOP) analysis is gradually becoming a crucial tool in the development of optical systems in general and astronomical telescopes and instruments in particular. We describe the approach of the European Southern Observatory to STOP analysis at the examples of the phasing and diagnostic station and of the pre-focal station of the European Extremely Large Telescope under construction. Further, we discuss the fundamentals of the estimation of thermal effects on optical performance and how to carry it out with the in-house developed software tool Sensitizer.

Laser guide star (LGS) Shack–Hartmann (SH) wavefront sensors for next-generation Extremely Large Telescopes (ELTs) require low-noise, large format (∼1  Mpx), fast detectors to match the need for a large number of subapertures and a good sampling of the very elongated spots. One path envisaged to fulfill this need has been the adoption of complementary metal metal-oxide semiconductor detectors with a rolling shutter read-out scheme that allows low read-out noise and fast readout time at the cost of image distortion due to the detector rows exposed in different moments. Here, we analyze the impact of the rolling shutter read-out scheme when used for LGS SH wavefront sensing; in particular, we focus on the impact on the adaptive optics (AO) correction of the distortion-induced aberrations created by the rolling exposure in the case of fast varying aberrations, like the ones coming from the LGS tilt jitter due to the up-link propagation of laser beams. We show that the LGS jitter-induced aberration for an ELT can be as large as 100-nm root-mean-square, a significant term in the wavefront error budget of a typical AO system on an ELT, and we discuss possible mitigation strategies.

The maintenance concept provides the basis for overall dependability design requirements for Extremely Large Telescopes (ELTs), such as the European Southern Observatory’s ELT, the Thirty Meter Telescope (TMT), and the Giant Magellan Telescope (GMT). The maintenance concept contains the planning of the maintenance and support policy for the complete operational system of the ELTs. The way in which the maintenance concept was developed in the ELTs is shown, and the similarities and differences in the way the operational objectives will be achieved is identified. The methodology is based on explaining seven common points (maintenance philosophy/levels; maintenance types and strategies; designing for dependability: reliability, availability, and maintainability; organizational structure and responsibilities for maintenance operations; maintenance and configuration management; key performance indicators for operations; and challenges, opportunities, and lessons learned) and condensing the knowledge during the process of generation and application of the maintenance concept.

The new class of Extremely Large Telescopes (ELTs) has implemented more rigorous systems engineering processes and tools for requirements management than has been used in past observatory projects. The similarities and differences between these activities at the ESO-ELT, GMT, TMT, and NOIRLab US-ELTP projects are summarized. We show that, while the key steps of the requirements management process are common among the ELTs, each project has implemented its own variation of the processes and tools tailored to its needs.

Observatory end-to-end science operations is the overall process starting with a scientific question, represented by a proposal requesting observing time; ending with the analysis of observation data addressing that question; and including all of the intermediate steps needed to plan, schedule, obtain, and process these observations. Increasingly complex observing facilities demand a highly efficient science operations approach and at the same time must be user friendly to the astronomical user community and enable the highest possible scientific return. Therefore, this process is supported by a collection of tools. We describe the overall end-to-end process and its implementation for the three upcoming Extremely Large Telescopes (ELTs): European Southern Observatory’s ELT, the Thirty Meter Telescope, and the Giant Magellan Telescope.

The instruments developed for the upcoming Extremely Large Telescopes (ELTs) will need efficient adaptive optics (AO) systems to correct the effects of the atmospheric turbulence and allow imaging at the highest angular resolution. One of the most important requirements for ELT AO-assisted instruments will be to deliver diffraction-limited images in a significant part of the sky. For that, the instruments will be equipped with laser guide stars (LGSs) providing most of the information required by AO instruments. But even with LGSs, AO systems still require the use of natural guide stars (NGSs) to compensate for image motion (jitter) and some low order aberrations. These NGSs are eventually limiting the fraction of the sky that can be achieved by AO systems, the so-called sky coverage (SC). We first present the SC assessment methods used for high angular resolution monolithic optical and near-infrared integral field spectrograph (HARMONI) and multiconjugate adaptive optics relay/multi-AO imaging camera for deep observations (MAORY/MICADO), that are both instruments for the ELT of the European Southern Observatory (ESO). They are based on a semianalytical description of the main contributors in the AO error budget, allowing for a fast estimation of the residual jitter. As such, these methods are well suited for statistical estimation of the SC on multiple science fields and/or to efficiently explore the system parameter space. We then compute the SC of the two instruments in cosmological fields from the cosmic assembly near-IR deep extragalactic legacy survey catalog. The goal is to provide an insight on the possibilities given by two different types of tomographic AO systems, i.e., laser tomography AO with HARMONI and multiconjugate AO with MAORY, on the same telescope. In particular, we show that HARMONI and MAORY/MICADO are complementary, meaning that the overall SC of ESO’s ELT is much improved for applications common to both systems.

With more than a decade of design work behind them, the instrumentation program of each of the three Extremely Large Telescope (ELT)-class telescopes is now ready to lock-in the set of capabilities it will be able to offer to their respective community at first light. The Giant Magellan Telescope, the Thirty Meter Telescope, and the European Southern Observatory’s ELT have each followed a formal instrumentation prioritization process with input from the astronomical community, which resulted in extensive and complementary suites of instruments that will be divided into two generations. We present this process as well as the general scientific characteristics of every instrument currently being developed for the three observatories. We also present how these instruments will fit the operation models of the observatories.

The adaptive optics systems of future Extremely Large Telescopes (ELTs) will be assisted with laser guide stars (LGS) which will be created in the sodium layer at a height of ≈90  km above the telescopes. In a Shack–Hartmann wavefront sensor, the long elongation of LGS spots on the sub-pupils far apart from the laser beam axis constraints the design of the wavefront sensor (WFS) which must be able to fully sample the elongated spots without undersampling the non-elongated spots. To fulfill these requirements, a newly released large complementary metal oxide semiconductor sensor with 1100  ×  1600  pixels and 9  μm pixel pitch could be employed. Here, we report on the characterization of such a sensor in terms of noise and linearity, and we evaluate its performance for wavefront sensing based on the spot centroid variations. We then illustrate how this new detector can be integrated into a full LGS WFS for both the European Southern Observatory’s ELT and the Thirty Meter Telescope.

Management of equipment vibration will be a challenge for the upcoming generation of extremely large telescopes (ELTs) (GMT, TMT, and ESO’s ELT) and is being dealt with proactively by all three projects. We document the approaches, techniques, and future efforts by all three ELTs in their attempts to manage vibration in their telescopes. We detail the approaches to developing component requirements, characterizing vibration sources, simulating telescope structural movements, and approaches to mitigating source vibrations. We illustrate the iterative approach taken by the three observatories with several examples of concrete processes, measurements, and other details of use to future observatories.

The next generation of Giant Segmented Mirror Telescopes (GSMT) will have large gaps between the segments either caused by the shadow of the mechanical structure of the secondary mirror [European Extremely Large Telescope (E-ELT) and Thirty Meter Telescope (TMT)] or intrinsically by design [Giant Magellan Telescope (GMT)]. These gaps are large enough to fragment the aperture into independent segments that are separated by more than the typical Fried parameter. This creates piston and petals modes that are not well sensed by conventional wavefront sensors such as the Shack–Hartmann wavefront sensor or the pyramid wavefront sensor. We propose to use a new optical device, the holographic dispersed fringe sensor (HDFS), to sense and control these petal/piston modes. The HDFS uses a single pupil-plane hologram to interfere the segments onto different spatial locations in the focal plane. Numerical simulations show that the HDFS is very efficient and that it reaches a differential piston root-mean-square (rms) smaller than 10 nm for GMT/E-ELT/TMT for guide stars up to 13th J + H band magnitude. The HDFS has also been validated in the lab with Magellan adaptive optics extreme and high-contrast adaptive optics phasing testbed, the GMT phasing testbed. The lab experiments reached 5-nm rms piston error on the Magellan telescope aperture. The HDFS also reached 50-nm rms of piston error on a segmented GMT-like aperture while the pyramid wavefront sensor was compensating simulated atmosphere under median seeing conditions. The simulations and lab results demonstrate the HDFS as an excellent piston sensor for the GMT. We find that the combination of a pyramid slope sensor with an HDFS piston sensor is a powerful architecture for the GMT.

Laser guide star (LGS) wave-front sensing (LGSWFS) is a key element of tomographic adaptive optics system. However, when considering Extremely Large Telescope (ELT) scales, the LGS spot elongation becomes so large that it challenges the standard recipes to design LGSWFS. For classical Shack–Hartmann wave-front sensor (SHWFS), which is the current baseline for all ELT LGS-assisted instruments, a trade-off between the pupil spatial sampling [number of sub-apertures (SAs)], the SA field-of-view (FoV) and the pixel sampling within each SA is required. For ELT scales, this trade-off is also driven by strong technical constraints, especially concerning the available detectors and in particular their number of pixels. For SHWFS, a larger field of view per SA allows mitigating the LGS spot truncation, which represents a severe loss of performance due to measurement biases. For a given number of available detectors pixels, the SA FoV is competing with the proper sampling of the LGS spots, and/or the total number of SAs. We proposed a sensitivity analysis, and we explore how these parameters impacts the final performance. In particular, we introduce the concept of super resolution, which allows one to reduce the pupil sampling per WFS and opens an opportunity to propose potential LGSWFS designs providing the best performance for ELT scales.

The Giant Magellan Telescope (GMT) design consists of seven circular 8.4-m diameter mirrors, together forming a single 25.4-m diameter primary mirror. This large aperture and collecting area can help extreme adaptive optics (ExAO) systems such as GMT’s GMagAO-X achieve the small angular resolutions and contrasts required to image habitable zone earth-like planets around late type stars and possibly lead to the discovery of life outside of our solar system. However, the GMT primary mirror segments are separated by large >30  cm gaps, creating the possibility of fluctuations in optical path differences (piston) due to flexure, segment vibrations, wind buffeting, temperature effects, and atmospheric seeing. To utilize the full diffraction-limited aperture of the GMT for high-contrast, natural guide star-adaptive optics science, the seven mirror segments must be co-phased to well within a fraction of a wavelength. The current design of the GMT involves seven adaptive secondary mirrors, a slow (∼0.03  Hz) off-axis dispersed fringe sensor (part of the acquisition guiding and wavefront sensing system’s active optics off-axis guider), and a pyramid wavefront sensor [PyWFS; part of the natural guide star wavefront sensor (NGWS) adaptive optics] to measure and correct the total path length between segment pairs, but these methods have yet to be tested “end-to-end” in a lab environment. We present the design and working prototype of a “GMT high contrast adaptive optics phasing testbed” that leverages the existing MagAO-X ExAO instrument to demonstrate segment phase sensing and simultaneous AO-control for high-contrast GMT natural guide star science [i.e., testing the NGWS wavefront sensor (WFS) architecture]. We present the first test results of closed-loop piston control with one GMT segment using MagAO-X’s PyWFS with and without simulated atmospheric turbulence. We show that the PyWFS was able to successfully control segment piston without turbulence within 12- to 33-nm RMS for 0 λ  /  D to 5 λ  /  D modulation, but was unsuccessful at controlling segment piston with generated ∼0.6  arcsec (median seeing conditions at the GMT site) and ∼1.2  arcsec seeing turbulence due to nonlinear modal cross-talk and poor pixel sampling of the segment gaps on the PyWFS detector. These results suggest that a PyWFS alone is not an ideal piston sensor for the GMT (and likely the TMT and ELT). Hence, a dedicated “second channel” piston sensor is required. We report the success of an alternate solution to control piston using a holographic dispersed fringe sensor (HDFS) while controlling all other modes with the PyWFS purely as a slope sensor (piston mode removed). This “second channel” WFS method worked well to control segment piston to within 50 nm RMS and ±10  μm dynamic range under simulated 0.6 arcsec atmospheric seeing (median seeing conditions at the GMT site). These results led to the inclusion of the HDFS as the official second channel piston sensor for the GMT NGWS WFS. This HDFS + PyWFS architecture should also work well to control piston petal modes on the ELT and TMT telescopes.

The precision thermal control (PTC) project was a multiyear effort initiated in fiscal year 2017 to mature the technology readiness level (TRL) of technologies required to enable ultra-thermally stable ultraviolet/optical/infrared space telescope primary-mirror assemblies for ultra-high-contrast observations of exoplanets. PTC had three objectives: (1) validate thermal optical performance models, (2) derive thermal system stability specifications, and (3) demonstrate multi-zonal active thermal control. PTC successfully achieved its objectives and matured active thermal control technology to at least TRL-5. PTC’s key accomplishments are a demonstration of better than 2-mK root-mean-square stable thermal control of the 1.5-m ultra-low-expansion (ULE^®) Advanced Mirror Technology Development-2 (AMTD-2) mirror when exposed to thermal disturbances in a relevant thermal/vacuum environment, and the ability to shape the 1.5-m AMTD-2 mirror to picometer precision. Additionally, an analysis approach is demonstrated for quantifying thermally induced mid-spatial frequency error which can cause speckle noise in the coronagraph dark hole.

A wide-field zenith-looking telescope operating in a mode similar to time-delay-integration (TDI) or drift scan imaging can perform an infrared sky survey without active pointing control, but it requires a high-speed, low-noise infrared detector. Operating from a hosted payload platform on the International Space Station (ISS), the Emu space telescope employs the paradigm-changing properties of the Leonardo SAPHIRA electron avalanche photodiode array to provide powerful new observations of cool stars at the critical water absorption wavelength (1.4  μm) largely inaccessible to ground-based telescopes due to the Earth’s own atmosphere. Cool stars, especially those of spectral-type M, are important probes across contemporary astrophysics, from the formation history of the Galaxy to the formation of rocky exoplanets. Main sequence M-dwarf stars are the most abundant stars in the Galaxy and evolved M-giant stars are some of the most distant stars that can be individually observed. The Emu sky survey will deliver critical stellar properties of these cool stars by inferring oxygen abundances via measurement of the water absorption band strength at 1.4  μm. Here, we present the TDI-like imaging capability of Emu mission, its science objectives, instrument details, and simulation results.

The Imaging X-ray Polarimetry Explorer, a NASA small explorer mission, will be the first mission dedicated to x-ray polarimetry. The payload consists of three identical telescopes, each comprising a mirror module assembly (MMA) with a polarization-sensitive detector at its focus. We describe all aspects of the MMA, from initial optical and mechanical design considerations to meet program requirements through mirror shell fabrication, mirror shell integration and module assembly, environmental testing, x-ray calibration, and on-ground and on-orbit alignment.

We present the results of a digital calibration technique applied to an Atacama Large Millimeter/submillimeter Array sideband separating wideband astronomical receiver of 275 to 500 GHz radio frequency (RF) and 3 to 22 GHz intermediate frequency bandwidth. The calibration technique consists of computing the magnitude ratio and the phase difference of the receiver output, and then applying correction constants to the digitized signals. Two analog-digital converters are used to digitize the signals and an field-programmable gate array for the processing. No modification in the analog receiver is required to apply the calibration, as it works directly on upper sideband/lower sideband signals. The technique improved the receiver temperature compared with the double sideband case by increasing the sideband rejection ratio by around 30 dB on average. It is shown that even more rejection can be obtained with more careful control of the RF calibration input power.

We propose an optical system of an extremely large telescope for ground-based or planetary use. The system comprises a segmented spherical mirror with a diameter of 100 m and f-number of f  /  1. There are three annular zones on the primary mirror, which corresponds to three annular telescopes (ATs) with f-numbers f  /  2, f  /  3.2, and f  /  5.2, all using a concave cardioidal secondary mirror with a maximum diameter of 3.18 m. This two-mirror system satisfies Fermat’s principle and the Abbe’s sine condition. The central zone of the primary mirror with a diameter of 23.8 m is used for the central three-mirror telescope, which is based on an afocal two-mirror system with a convex aspheric secondary mirror with a diameter of 3 m. Four possible configurations are presented for the central telescope, which makes it possible to vary the f-number in a wide range with design examples given for f  /  1, f  /  4.2, f  /  14, and f  /  33 systems. The ATs form three coherent images of the same astronomical object, which offers possibilities of simultaneous observations at three different wavelengths or image processing of a combined image with enhanced angular resolution. The main goal of the paper is to investigate the properties of new optical systems for ground-based and space telescopes with a fast spherical primary mirror for which aberration correction is achieved with a minimum number of auxiliary aspheric mirrors near the prime focus.

We describe the optimum telescope focal ratio for a two-element, three-surface, telecentric image-transfer microlens-to-fiber coupled integral field unit within the constraints imposed by microoptics fabrication and optical aberrations. We create a generalized analytical description of the microoptics optical parameters from first principles. We find that the optical performance, including all aberrations, of a design constrained by an analytic model considering only spherical aberration and diffraction matches within   ±  4  %   of a design optimized by ray-tracing software such as Zemax. The analytical model does not require any compromise on the available clear aperture; about 90% mechanical aperture of hexagonal microlens is available for light collection. The optimum telescope f-ratio for a 200-μm core fiber-fed at f  /  3.5 is between f  /  7 and f  /  12. We find the optimum telescope focal ratio changes as a function of fiber core diameter and fiber input beam speed. A telescope focal ratio of f  /  8 would support the largest range of fiber diameters (100 to 500  μm) and fiber injection speeds (between f  /  3 and f  /  5). The optimization of the telescope and lenslet-coupled fibers is relevant for the design of high-efficiency dedicated survey telescopes, and for retrofitting existing facilities via introducing focal macro-optics to match the instrument input requirements.

Beam combiners are important components of an optical/infrared astrophysical interferometer, with many variants as to how to optimally combine two or more beams of light to fringe-track and obtain the complex fringe visibility. One such method is the use of an integrated optics chip that can instantaneously provide the measurement of visibility without temporal or spatial modulation of the optical path. Current asymmetric planar designs are complex, resulting in a throughput penalty, and so here, we present developments into a three-dimensional triangular tricoupler that can provide the required interferometric information with a simple design and only three outputs. Such a beam combiner is planned to be integrated into the upcoming Pyxis interferometer, where it can serve as a high-throughput beam combiner with a low size footprint. Results into the characterization of such a coupler are presented, highlighting a throughput of 85  ±  7  %   and a flux splitting ratio between 33:33:33 and 52:31:17 over a 20% bandpass. We also show the response of the chip to changes in the optical path, obtaining an instantaneous complex visibility and group delay estimate at each input delay.

The Nancy Grace Roman Space Telescope will carry a coronagraph instrument (CGI) that will serve as a demonstrator for technologies needed for future high-contrast imaging missions in space, including deformable mirrors (DMs) to correct high-order wavefront errors that would otherwise limit the achievable contrast. The CGI has three baselined interchangeable observing configurations, one of which is a bowtie shaped pupil coronagraph for high-contrast spectroscopy. We present the flight designs for two closely related mask configurations of the bowtie shaped pupil coronagraph: a baseline 0-deg mask configuration for the technology demonstration and a 60-deg mask configuration contributed by the NASA Exoplanet Exploration Program. The shaped pupil mask and Lyot stop for each mask configuration result from an iterative process that maximizes the core throughput subject to constraints on other performance metrics, such as the contrast: a linear program optimizes the shaped pupil mask for a given Lyot stop, and the optimization repeats for various Lyot stops until the highest-throughput combination is identifiable. The flight designs for the baseline and rotated mask configurations have core throughputs of 4.50% and 3.89%, respectively, at 4 λD and are robust to conservative estimates of potential pupil errors such as misalignments and manufacturing errors. If these estimates are exceeded in flight, the DMs can be used to mitigate the effects of the excess error.

We performed wave-optics-based numerical simulations at mid-infrared wavelengths to investigate how the presence or absence of entrance slits and optical aberrations affect the spectral resolving power R of a compact, high-spectral-resolving-power spectrometer containing an immersion-echelle grating. We tested three cases of telescope aberration (aberration-free, astigmatism, and spherical aberration), assuming the aberration budget of the Space Infrared Telescope for Cosmology and Astrophysics, which has a 20     μm wavelength diffraction limit. In cases with a slit, we found that the value of R at around 10 to 20  μm is approximately independent of the assumed aberrations, which is significantly different from the prediction of geometrical optics. Our results also indicate that diffraction from the slit improves R by enlarging the effective illuminated area on the grating window and that this improvement decreases at short wavelengths. For the slit-less cases, we found that the impact of aberrations on R can be roughly estimated using the Strehl ratio.

Commercially manufactured complementary metal–oxide–semiconductor (CMOS) sensors have demonstrated competitive x-ray spectral imaging performance to the charge-coupled devices flown on the Suzaku and Chandra missions without the cooling demands required of these sensors. This performance, in combination with their reduced costs, warrants regarding CMOS sensors as promising candidates for low-Earth orbit (LEO) x-ray small satellites. We investigate the radiation tolerance of these devices to the anticipated total ionizing dose (TID) radiation expected in LEO. We expose a backside-illuminated Sony IMX290LLR CMOS sensor to up to 12 krad of TID from Cs137 gamma-ray radiation. We find an increase in the abundance of noisy pixels with increasing dosage, but no discernible increase in the average dark signal or RMS noise. Measurements of the x-ray spectrum from a Fe55 source indicate no change in spectral resolution and only minor gain degradation with TID.

Launched on 2021 December 9, the Imaging X-ray Polarimetry Explorer (IXPE) is a NASA Small Explorer Mission in collaboration with the Italian Space Agency (ASI). The mission will open a new window of investigation—imaging x-ray polarimetry. The observatory features three identical telescopes, each consisting of a mirror module assembly with a polarization-sensitive imaging x-ray detector at the focus. A coilable boom, deployed on orbit, provides the necessary 4-m focal length. The observatory utilizes a three-axis-stabilized spacecraft, which provides services such as power, attitude determination and control, commanding, and telemetry to the ground. During its 2-year baseline mission, IXPE will conduct precise polarimetry for samples of multiple categories of x-ray sources, with follow-on observations of selected targets.

Next-generation x-ray observatories, such as the Lynx X-ray Observatory Mission Concept or other similar concepts in the coming decade, will require detectors with high quantum efficiency (QE) across the soft x-ray band to observe the faint objects that drive their mission science objectives. Hybrid CMOS detectors (HCDs), a form of active-pixel sensor, are promising candidates for use on these missions because of their fast read-out, low power consumption, and intrinsic radiation hardness. We present QE measurements of a Teledyne H2RG HCD, performed using a gas-flow proportional counter as a reference detector. We find that this detector achieves high QE across the soft x-ray band, with an effective QE of 94.6  ±  1.1  %   at the Mn Kα  /  Kβ energies (5.90/6.49 keV), 98.3  ±  1.9  %   at the Al Kα energy (1.49 keV), 85.6  ±  2.8  %   at the O Kα energy (0.52 keV), and 61.3  ±  1.1  %   at the C Kα energy (0.28 keV). These values are in good agreement with our model, based on the absorption of detector layers. We find similar results in a more restrictive analysis considering only high-quality events, with only somewhat reduced QE at lower energies.

Complementary metal-oxide semiconductor (CMOS) detectors offer many advantages over charge-coupled devices (CCDs) for optical and ultraviolet (UV) astronomical applications, especially in space where high radiation tolerance is required. However, astronomical instruments are most often designed for low light-level observations demanding low dark current and read noise, good linearity, and high dynamic range, characteristics that have not been widely demonstrated for CMOS imagers. We report the performance, over temperatures from 140 to 240 K, of a radiation hardened SRI 4k  ×  2k back-side illuminated CMOS image sensor with surface treatments that make it highly sensitive in blue and UV bands. After suppressing emission from glow sites resulting from defects in the engineering grade device examined, a 0.077  me⁻  /  s dark current floor is reached at 160 K, rising to 1  me⁻  /  s at 184 K, rivaling that of the best CCDs. We examine the trade-off between readout speed and read noise, finding that 1.43  e⁻ median read noise is achieved using line-wise digital correlated double sampling at 700  kpix  /  s  /  ch corresponding to a 1.5 s readout time. The 15  ke⁻ well capacity in high gain mode extends to 120  ke⁻ in dual gain mode. Continued collection of photogenerated charge during readout enables a further dynamic range extension beyond 10⁶ e⁻ effective well capacity with only 1% loss of exposure efficiency by combining short and long exposures. A quadratic fit to correct for non-linearity reduces gain correction residuals from 1.5% to 0.2% in low gain mode and to 0.4% in high gain mode. Cross-talk to adjacent pixels is only 0.4% vertically, 0.6% horizontally, and 0.1% diagonally. These characteristics plus the relatively large (10  μm) pixel size, quasi 4-side buttability, electronic shutter, and sub-array readout make this sensor an excellent choice for wide field astronomical imaging in space, even at far-UV wavelengths where sky background is very low.

The broad energy response, low electronic read noise, and good energy resolution have made x-ray charge-coupled devices (CCDs) an obvious choice for developing soft x-ray astronomical instruments over the last half-century. They also come in large array formats with small pixel sizes, which make them a potential candidate for the next-generation astronomical x-ray missions. However, the next-generation x-ray telescopic experiments propose for significantly larger collecting area compared with the existing observatories to explore the low luminosity and high redshift x-ray universe that requires these detectors to have an order of magnitude faster readout. In this context, Stanford University (SU) in collaboration with the Massachusetts Institute of Technology has initiated the development of fast readout electronics for x-ray CCDs. At SU, we have designed and developed a fast and low noise readout module with the goal of achieving a readout speed of 5  Mpixel  /  s. We successfully ran a prototype CCD matrix of 512  ×  512  pixels at 4  Mpixels  /  s. In this paper, we describe the details of the readout electronics and report the performance of the detectors at these readout speeds in terms of read noise and energy resolution. In the future, we plan to continue to improve the performance of the readout module and eventually converge to a dedicated application-specific integrated circuit or ASIC-based readout system to enable parallel readout of large array multinode CCD devices.

We present an evaluation of an on-chip charge detector, called the single electron sensitive read out (SiSeRO), for charge-coupled device image sensor applications. It uses a p-channel metal-oxide-semiconductor field-effect transistor (p-MOSFET) transistor at the output stage with a depleted internal gate beneath the p-MOSFET. Charge transferred to the internal gate modulates the source-drain current of the transistor. We have developed a drain current readout module to characterize the detector. The prototype sensor achieves a charge/current conversion gain of 700 pA per electron, an equivalent noise charge (ENC) of 15 electrons (e⁻) root mean square, and a full width half maximum of 230 eV at 5.9 keV. Further, we discuss the SiSeRO working principle, the readout module developed at Stanford, and the first characterization test results of the SiSeRO prototypes. While at present only a proof-of-concept experiment, in the near future we plan to use next generation sensors with improved noise performance and an enhanced readout module. In particular, we are developing a readout module enabling repetitive non-destructive readout of the charge, which can in principle yield subelectron ENC performance. With these developments, we eventually plan to build a matrix of SiSeRO amplifiers to develop an active pixel sensor with an on-chip application specific integrated circuit-based readout system. Such a system, with fast readout speeds and subelectron noise, could be effectively utilized in scientific applications requiring fast and low-noise spectro-imagers.

We have been developing the monolithic active pixel detector XRPIX onboard the future x-ray astronomical satellite FORCE. XRPIX is composed of complementary metal-oxide-semiconductor pixel circuits, SiO₂ insulator, and Si sensor by utilizing the silicon-on-insulator (SOI) technology. When the semiconductor detector is operated in orbit, it suffers from radiation damage due to x-rays emitted from celestial objects as well as cosmic rays. From previous studies, positive charges trapped in the SiO₂ insulator are known to cause degradation of the detector performance. To improve the radiation hardness, we developed XRPIX equipped with a double-SOI (D-SOI) structure, introducing an additional silicon layer in the SiO₂ insulator. This structure is aimed at compensating for the effect of the trapped positive charges. Although the radiation hardness of the D-SOI detectors to cosmic rays has been evaluated, the radiation effect due to x-ray irradiation has not been evaluated. Thus, we then conduct an x-ray irradiation experiment using an x-ray generator with a total dose of 10 krad at the SiO₂ insulator, equivalent to 7 years in orbit. As a result of this experiment, the energy resolution in full-width half maximum for the 5.9 keV x-ray degrades by 17.8  %    ±  2.8  %   and the dark current increases by 89  %    ±  13  %  . We also investigate the physical mechanism of the increase in the dark current due to x-ray irradiation using technology computer-aided design simulation. It is found that the increase in the dark current can be explained by the increase in the interface state density at the Si  /  SiO₂ interface.

A multiple-order diffractive engineered surface (MODE) lens is an optical component that is ideally suited for photon-starved astronomical applications. For example, the Nautilus Array, a proposed space observatory for high-fidelity exoplanet detection and characterization, will utilize an array of MODE lenses to achieve unprecedented light collection area. Despite this potential, work is necessary to advance MODE manufacturing capabilities from current laboratory prototypes to production levels that are required for successful technology insertion. Accordingly, we present the first formalized production-level MODE manufacturing process and first analytically validated model of MODE production. Specifically, an integrated model for the MODE production cost, schedule, and risk was constructed utilizing Monte Carlo simulation. This model was utilized to simulate and analyze the production of 35 MODE lenses—the Nautilus baseline. We applied queuing theory to the Monte Carlo model to improve MODE production through optimizing manufacturing variables. Future system architects, engineers, and managers should utilize this integrated model and optimization methodology to shape MODE production. Overall, this research serves two consensus civil space industry priorities for the 2020s—exoplanet detection missions and advancing programmatic and technical readiness in tandem—through improving MODE actualization and offering a general methodology applicable to emerging optical technology production.

We present a unique implementation of Python coding in an asynchronous object-oriented programming (OOP) framework to fully automate the process of collecting data with the George Mason University (GMU) Observatory’s 0.8-meter telescope. The goal of this project is to perform automated follow-up observations for the Transiting Exoplanet Survey Satellite (TESS) mission, while still allowing for human control, monitoring, and adjustments. Prior to our implementation, the facility was computer-controlled by a human observer through a combination of webcams, TheSkyX, Astronomy Common Object Model Dome, MaxIm DL, and a weather station. We automate slews and dome movements, charge-coupled device exposures, saving flexible image transfer system images and metadata, initial focusing, guiding on the target, using the ambient temperature to adjust the focus as the telescope cools through the night, taking calibration images (darks and flats), and monitoring local weather data. The automated weather monitor periodically checks various weather data from multiple sources to automate the decision to close the observatory during adverse conditions. We organize the OOP code structure so that each hardware device or important higher-level process is categorized as its own object class or “module” with associated attributes and methods, with inherited common methods across modules for code reusability. To allow actions to be performed simultaneously across different modules, we implement a multithreaded approach in which each module is given its own CPU thread on which to operate concurrently with all other threads. After the initial few modules (camera, telescope, dome, and data I/O) were developed, further development of the code was carried out in tandem with testing on sky on clear nights. We achieve our goal of a fully automated nightly observation process. The code, in its current state, was tested and used for observations on 171 nights, with more planned usage and feature additions in the future.

The optimization of the elevation rotation structure (ERS) is one of the critical problems in the design of a large submillimeter telescope (LST). Here, combining the super element model with topology optimization method and genetic algorithm (SEMTOMGA) is proposed for the ERS design of an LST. The SEMTOMGA has three key steps: (1) the super element model is applied to condensing all the degrees of freedom of the large structure elements, which needs no topology optimization, except for the connecting nodes and the objective structure elements; (2) the topology method is applied to optimizing the objective structure; (3) based on the optimization results of the second step, the further whole structure optimization with multiobjective genetic algorithm(GA) is performed. The SEMTOMGA, which exploits the complementary merits of the super element model, topology optimization method, and GA, solves the problem of the ERS design effectively. As an application, a 60-m submillimeter telescope is designed and optimized by SEMTOMGA. The results have shown that the SEMTOMGA not only obtains a lightweight design of the ERS but also has sufficient stiffness. Moreover, the performance of the whole structure has been improved, and the residual half-path length errors of the main reflector have declined from 263.7 to 135.6  μm, which is about half of the original 263.7  μm of the initial design.

Teledyne’s H4RG, H2RG, and H1RG near-infrared array detectors provide reference pixels embedded in their data streams. Although they do not respond to light, the reference pixels electronically mimic normal pixels and track correlated read noise. We describe how the reference pixels can be used with linear algebra and training data to optimally reduce correlated read noise. Simple improved reference subtraction (SIRS) works with common detector clocking patterns and, when applicable, relies only on postprocessing existing data so long as the reference pixels are available. The resulting reference correction is optimal, in a least squares sense, when the embedded reference pixels are the only references and the reference columns on the left and right are treated as two reference streams. We demonstrate SIRS using H4RG ground test data from the Nancy Grace Roman Space Telescope Project. The Julia language SIRS software is freely available for download from the NASA GitHub. The package includes a python-3 “backend” that can be used to apply SIRS corrections if a SIRS calibration file has been provided by the instrument builders.

Charge-coupled devices (CCDs) have been the detector of choice for large-scale space missions for many years. Although dominant in this field, the charge transfer performance of the technology degrades over time due to the radiation-harsh space environment. Charge transfer performance can be optimized; however, it is often time consuming and expensive due to the many operating modes of the CCD, especially considering the ever-increasing needs of detector performance. A technique that uses measurements of the trap landscape present in a CCD to predict changes in charge transfer inefficiency as a function of different experimental variables is presented and developed. Using this technique, it is possible to focus experimental lab testing on key device parameters, potentially saving many months of laboratory effort. Due to the generality of the method, it can be used to optimize the charge transfer performance of any CCD and, as such, has many uses across a wide range of fields and space missions. Future CCD variants that will be used in potential space missions (EMCCD and p-channel CCDs) can use this technique to provide feedback of the key device performance to the wider mission consortium before devices are available for experimental testing.

We present an analysis of the long-term performance of the W. M. Keck observatory laser guide star adaptive optics (LGS-AO) system and explore factors that influence the overall AO performance most strongly. Astronomical surveys can take years or decades to finish, so it is worthwhile to characterize the AO performance on such timescales in order to better understand future results. The Keck telescopes have two of the longest-running LGS-AO systems in use today, and as such they represent an excellent test-bed for processing large amounts of AO data. We use a Keck-II near infrared camera 2 (NIRC2) LGSAO surve of the Galactic Center (GC) from 2005 to 2019 for our analysis, combining image metrics with AO telemetry files, multiaperture scintillation sense/differential imaging motion monitor turbulence profiles, seeing information, weather data, and temperature readings in a compiled dataset to highlight areas of potential performance improvement. We find that image quality trends downward over time, despite multiple improvements made to Keck-II and its AO system, resulting in a 9 mas increase in the average full width at half maximum (FWHM) and a 3% decrease in the average Strehl ratio over the course of the survey. Image quality also trends upward with ambient temperature, possibly indicating the presence of uncorrected turbulence in the beam path. Using nine basic features from our dataset, we train a simple machine learning (ML) algorithm to predict the delivered image quality of NIRC2 given current atmospheric conditions, which could eventually be used for real-time observation planning and exposure time adjustments. A random forest algorithm trained on this data can predict the Strehl ratio of an image to within 18% and the FWHM to within 7%, which is a solid baseline for future applications involving more advanced ML techniques. The assembled dataset and coding tools are released to the public as a resource for testing new predictive control and point spread function-reconstruction algorithms.

The next technological breakthrough in millimeter–submillimeter astronomy is three-dimensional imaging spectrometry with wide instantaneous spectral bandwidths and wide fields of view. The total optimization of the focal-plane instrument, the telescope, the observing strategy, and the signal-processing software must enable efficient removal of foreground emission from the Earth’s atmosphere, which is time-dependent and highly nonlinear in frequency. Here, we present Time-dependent End-to-end Model for Post-process Optimization (TiEMPO) of the DEep Spectroscopic HIgh-redshift MApper (DESHIMA) spectrometer. TiEMPO utilizes a dynamical model of the atmosphere and parameterized models of the astronomical source, the telescope, the instrument, and the detector. The output of TiEMPO is a time stream of sky brightness temperature and detected power, which can be analyzed by standard signal-processing software. We first compare TiEMPO simulations with an on-sky measurement by the wideband DESHIMA spectrometer, and find good agreement in the noise and sensitivity. We then use TiEMPO to simulate the detection of the line emission spectrum of a high-redshift galaxy using the DESHIMA 2.0 spectrometer in development. The TiEMPO model is open source. Its modular and parametrized design enables users to adapt it to optimize the end-to-end performance of spectroscopic and photometric instruments on existing and future telescopes.

Superconducting circuit elements used in millimeter-submillimeter (mm-submm) astronomy would greatly benefit from deposited dielectrics with small dielectric loss and noise. This will enable the use of multilayer circuit elements and thereby increase the efficiency of mm-submm filters and allow for a miniaturization of microwave kinetic inductance detectors (MKIDs). Amorphous dielectrics introduce excess loss and noise compared with their crystalline counterparts, due to two-level system defects of unknown microscopic origin. We deposited hydrogenated amorphous silicon films using plasma-enhanced chemical vapor deposition, at substrate temperatures of 100°C, 250°C, and 350°C. The measured void volume fraction, hydrogen content, microstructure parameter, and bond-angle disorder are negatively correlated with the substrate temperature. All three films have a loss tangent below ^{10  −  5} for a resonator energy of 10⁵ photons, at 120 mK and 4 to 7 GHz. This makes these films promising for MKIDs and on-chip mm-submm filters.

To address the fundamental questions of exoplanetary science, future space-based observatories will have to obtain quality spectra of a large enough set of Earth-like planets around main-sequence stars. Although coronagraph instruments provide the necessary observational efficiency to probe many systems with respect to a starshade observation, they typically suffer from a limited achievable bandwidth at the necessary contrast and relatively poor throughput to off-axis sources. This is mainly due to the fact that the starlight is suppressed within the optical system, so the quasi-static aberrations from optical imperfections are the dominant term and need to be dealt with deformable mirrors (DMs). The DMs have limited capabilities to achieve large bandwidths, and their high stroke after corrections is highly detrimental to the Strehl ratio of off-axis sources. A technological path to overcome these issues is the use of single-mode fibers (SMFs). Coupling the planet light into an SMF to feed a high-resolution spectrograph has been shown to improve the final signal-to-noise ratio. Furthermore, it has been shown that it is more favorable to do broadband wavefront control with SMFs when exploiting their modal selectivity; the DMs have to work less so the bandwidth is improved and the off-axis throughput is better. Here, we demonstrate the potential of this technology by performing wavefront control through an SMF in a two-step process: first, by digging a small dark hole around the position of the SMF, and second, performing an innovative version of the electric field conjugation algorithm modified for SMFs. We perform these experiments with 20% bandwidth light at the high-contrast spectroscopy testbed achieving 2.5  ×  10^−  8 raw contrast.

The classical approaches for generating and studying a sodium guide star, direct imaging and light detection and ranging (LIDAR) technology are limited by the spatial resolution and equipment requirements. Therefore, the well-established technique to modulate continuous wave laser based on a pseudorandom binary sequence (PRBS) used in the field of radio detection and ranging and LIDAR has been extended to adaptive optics (AO). To monitor the sodium layer for applications such as AO and atmospheric studies, the technique was tested at the Calar Alto Observatory in 1999, Large Zenith Telescope in 2014, and by the University of Science and Technology of China in 2019. Based on these experiments, the PRBS modulation technique proved to be promising for the AO field. However, for this technique to be implemented in AO systems, it must be validated at different modulation strengths and modulation frequencies that satisfy the system requirements. Therefore, we aim to experimentally verify the PRBS modulation technique at a laser guide star facility installed at the 1.8 m Electro Optics Systems Telescope, Mount Stromlo, near Canberra, Australia. Numerical simulations are conducted with parameters at Mount Stromlo before the hardware implementation. The simulations show that the centroid error varies between 68 and 5 m for a low sodium column density. Additional numerical simulations are performed to verify the potential of the PRBS modulation technique to fulfill the laser guide star AO requirements of large telescopes. We show that with the implemented of PRBS modulation technique in one of the laser guide star (LGS) to monitor sodium layer can meet the LGS AO requirements of Giant Magellan Telescope even at low sodium column densities. Further, we discuss the computed results from the perspective of AO requirements, for the current and future implementation of the PRBS technique in the field of AO.

We used telemetry data from the Gemini North ALTAIR adaptive optics system to investigate how well the commands for wavefront correction (both tip/tilt and high-order turbulence) can be forecasted to reduce lag error (due to wavefront sensor averaging and computational delays) and improve delivered image quality. We showed that a high level of reduction (∼5 for tip-tilt and ∼2 for high-order modes) in the RMS wavefront error was achieved using a “forecasting filter” based on a linear autoregressive model with only a few coefficients (∼30 for tip-tilt and ∼5 for high-order modes) to complement the existing integral servo-controller. Updating this filter to adapt to evolving observing conditions is computationally inexpensive and requires <10  s of telemetry data. We also used several machine learning models (long-short term memory and dilated convolutional models) to evaluate whether further improvements could be achieved with a more sophisticated nonlinear model. Our attempts showed no perceptible improvements over linear autoregressive predictions, even for large lags in which residuals from the linear models are high, suggesting that nonlinear wavefront distortions for ALTAIR at the Gemini North telescope may not be forecasted with the current setup.

Next-generation x-ray observatories require lightweight, high throughput optics that maintain a <0.5  arcsec resolution to probe the physics of black holes and gain understanding of the early universe. One potential type of x-ray mirror consists of a 400-μm thick curved Corning EAGLE XG^® glass substrate with a Cr/Ir x-ray mirror coating deposited on the front (concave) side and an array of radio frequency sputtered Pb_0.995  (  Zr_0.52Ti_0.48)_0.99Nb_0.01O₃ piezoelectric thin film actuators on the back (convex) side to enable correction of figure errors. A stress-balancing process was developed to correct the figure distortion arising from thin film stresses in the actuator layers. Compressively stressed SiO₂ films were deposited on the convex side of the mirror to balance the tensile integrated stress of the actuator array while also matching the film thickness distribution. Finite-element methods were used to assess the impact of film thickness distributions on the convex and concave substrate surfaces. The resulting models show peak-to-valley figure errors of 105 nm, well within the 1-μm peak-to-valley dynamic range of the piezoelectric adjusters. In contrast, when stress compensation was done with an iridium mirror film deposited on the front side, the mismatched thickness distribution results in peak-to-valley figure errors over 3  μm.

The Mid-infrared ELT Imager and Spectrograph (METIS) will be equipped with a single conjugate adaptive optics (SCAO) system comprising the deformable mirror M4, the tip–tilt mirror M5, a wavefront sensor, and a modal controller implemented on real-time computers. To prevent any damage to M4 during operation and windup phenomena due to internal checks of M4, its absolute shape, time-discrete change of shape, and inter-actuator strokes are limited. Hence, we present a strategy to consider M4’s Cartesian constraints in the modal METIS-SCAO controller within this conceptual study. This strategy modifies the modal control error before it is fed into the SCAO controller considering the spatio-temporal and segmented characteristics of M4. Additionally, we present three different algorithms realizing this strategy. Furthermore, the presented strategy features the following characteristics, among others: add-on to a previously designed METIS-SCAO controller, no permanent trade-off between performance and constraint compliance, and application of numerically cheap approximating models. Moreover, we verify the functionality of the presented algorithms via standalone and closed loop simulations of the METIS-SCAO system. The simulations show that all presented algorithms work as intended and successfully enforce M4’s constraints. Therefore, the presented strategy and the three corresponding algorithms are applicable to the METIS-SCAO system.

The behavior of an adaptive optics (AO) system for ground-based high contrast imaging dictates the achievable contrast of the instrument. In conditions where the coherence time of the atmosphere is short compared with the speed of the AO system, the servo-lag error can become the dominant error term of the AO system. While the AO system measures the wavefront error and subsequently applies a correction (typically taking a total of one or a few milliseconds), the atmospheric turbulence above the telescope has changed resulting in the servo-lag error. In addition to reducing the Strehl ratio, the servo-lag error causes a build-up of speckles along the direction of the dominant wind vector in the coronagraphic image, severely limiting the contrast at small angular separations. One strategy to mitigate this problem is to predict the evolution of the turbulence over the delay time. Our predictive wavefront control algorithm minimizes, in a mean square sense, the wavefront error over the delay and has been implemented on the Keck II AO bench. We report on the latest results of our algorithm and discuss updates to the algorithm itself. We explore how to tune various filter parameters based on both daytime laboratory tests and on-sky tests. We show a reduction in residual-mean-square wavefront error for the predictor compared with the leaky integrator (the standard controller for Keck) implemented on Keck for three separate nights. Finally, we present contrast improvements for daytime and on-sky tests for the first time. Using the L-band vortex coronagraph for Keck’s NIRC2 instrument, we find a contrast gain of up to 2 at a separation of 3 λ  /  D and up to 3 for larger separations (3  −  7 λ  /  D).

This publication presents an active compensation system for the position of the secondary mirror of a small Ritchey–Chrétien telescope system. The goal is to maintain the optical imaging quality under varying gravitational and thermal influences, by compensation for the relative position deviations between the primary and secondary mirrors. An extensive analysis concerning the feasibility of such a system for a commercial off-the-shelf small telescope is performed and used as a basis for the design of the precision measurement and positioning system. The developed prototype uses dimensional metrology to capture relative position errors of the secondary mirror. A newly designed actuator with three degrees of freedom for the secondary mirror allows us to compensate for these deviations in a closed-loop control manner and ensures optimal positions of the two mirrors at all times. The support structure design requirements are reduced, allowing the utilization of more lightweight structures, as the artificial stiffness of the compensation system takes care of keeping the telescope mirrors in place. Furthermore, the measurement principle requires no light from the telescope, thus providing 100% of the collected light for the observation. The developed actuation and measurement principles are designed for simple scalability to larger representatives of small telescopes. The implemented setup is evaluated in various poses and temperature influences, successfully demonstrating that the calculated Strehl ratio is kept well above the diffraction limit of 80% for the used telescope system.