This is a list of reviewers who served the Journal of Astronomical Telescopes, Instruments, and Systems in 2019.

Pyramid wavefront sensors are planned to be a part of many instruments that are currently under development for the extremely large telescopes (ELT). The unprecedented scales of the upcoming ELT-era instruments are inevitably connected with serious challenges for wavefront reconstruction and control algorithms. Apart from the huge number of correcting elements to be controlled in real-time, real-life features such as the segmentation of the telescope pupil, the low wind effect, the nonlinearity of the pyramid sensor, and the noncommon path aberrations will have a significantly larger impact on the imaging quality in the ELT framework than they ever had before. We summarize various kinds of wavefront reconstruction algorithms for the pyramid wavefront sensor. Based on several forward models, different algorithms were developed in the last decades for linear and nonlinear wavefront correction. The core ideas of the algorithms are presented, and a detailed comparison of the presented methods with respect to underlying pyramid sensor models, computational complexities, and reconstruction qualities is given. In addition, we review the existing and possible solutions for the above-named real-life phenomena. At the same time, directions for further investigations are sketched.

Deployable optics promise a revolution in the capability of observing the universe by delivering drastically reduced mass and volume needs for a desired level of performance compared to their conventional counterparts. However, this places new demands on the mechanical and thermal designs of new telescopes, essentially trading mass and volume for structural and control complexity. We compile the thermomechanical challenges that should be taken into consideration when designing optical space systems, as well as summarize 14 projects proposed to address them. Stringent deployment repeatability requirements demand low hysteresis, whereas stability requirements require high stiffness, proper thermal management, and active optics.

This guest editorial summarizes the Special Section on Detectors for Astronomy and Cosmology.

A wave of precision radial velocity (RV) instruments will open the door to exploring the populations of companions of low-mass stars. The Palomar Radial Velocity Instrument (PARVI) will be optimized to detect RV signals of cool K and M stars with an instrument precision floor of 30  cm  /  s. PARVI will operate in the λ  =  1.2- to 1.8-μm-wavelength range with a spectral resolution of λ  /  Δλ  ∼  100,000. It will operate on the Palomar 5.1-m Hale telescope and use Palomar’s PALM-3000 adaptive optics system, single-mode fibers, and an H-band laser frequency comb to probe and characterize the population of planets around cool, red stars. We describe the performance of the PARVI guide camera: a C-RED 2 from First Light Advanced Imagery. The C-RED 2 will be used in a tip-tilt loop, which requires fast readout at low noise levels to eliminate any residual guide errors and ensure the target starlight stays centered on the fiber. At −40  °  C and a frame rate of 400 frames per second in nondestructive read mode, the C-RED 2 has a combined dark and background current of 493 e⁻  /  s. Using up-the-ramp sampling, we are able to reduce the read noise to 21.2 e⁻. With the C-RED 2, PARVI will be able to guide using targets as faint as 14.6 H magnitude.

Hawaii x Reference Guide (HxRG) detectors have crosstalk between amplifier channels at a scientifically relevant level. In principle, crosstalk signals can be fully calibrated and removed from data but only if a full crosstalk matrix is measured for the detector. We present a fast method of crosstalk characterization that can be performed with most instrument calibration units. It requires only a flat-field illumination and window programming in the HxRG detectors. We show the crosstalk matrices obtained with this method for both fast and slow modes in an H2RG detector and provide examples of how this data can be used to tune detector operation parameters, for feedback into the electronics design for the cryogenic preamplifiers, and in the data pipeline to remove crosstalk signals from scientific data.

The University of Rochester infrared detector group is working together with Teledyne Imaging Sensors to develop mercury cadmium telluride (HgCdTe) 15  μm cutoff wavelength detector arrays for future space missions. To reach the 15-μm cutoff goal, we took an intermediate step by developing four ∼13-μm cutoff wavelength arrays to identify any unforeseen effects related to increasing the cutoff wavelength from the extensively characterized 10-μm cutoff wavelength detector arrays developed for the NEOCam mission. The characterization of the ∼13-μm cutoff wavelength HgCdTe arrays at the University of Rochester allowed us to determine the key dark current mechanisms that limit the performance of these HgCdTe detector arrays at different temperatures and biases when the cutoff wavelength is increased. We present initial dark current and well depth measurements of a 15-μm cutoff array that shows dark current values two orders of magnitude smaller at large reverse bias than would be expected from our previous best structures.

Tree rings are one of the sensor effects that may affect precise measurements in the Large Synoptic Survey Telescope (LSST). The effect is caused by silicon wafer manufacturing process, resulting in formation of circular patterns due to variations of the silicon dopant concentration. We have analyzed flat-field images taken at Brookhaven National Laboratory and SLAC for all production sensors used to build the LSST camera in order to measure the amplitudes and periods of the tree ring patterns as a function of the radius, illumination wavelength, and sensor back bias voltage. With nominal back bias voltage settings, the tree ring amplitudes and periods for both ITL and e2v sensors are considered to have small impact on the galaxy shear measurement in LSST.

Interpixel capacitance (IPC) between adjacent pixels in hybridized arrays gives rise to an electrostatic cross talk. This cross talk causes MTF degradation and blurring of images or spectra collected using these devices. As pixel size is driven down from the 18-μm pixel pitch of the H2RG read out circuits to the 10- or 15-μm H4RGs IPC is driven up resulting in greater cross talk, all else being equal. Mounting evidence indicates that IPC varies as a function of depletion state of the photo-active diodes. For single pixel events, increasing the event intensity corresponds to a decreasing fractional coupling. If left uncorrected, IPC can give rise to systematic errors in precision astrometric and photometric measurements, in particular when dealing with confused point sources or spatially extended structures for shape measurements as demonstrated through comparison of registered sources from ESO HAWK-I and HST ACS WFC datasets. Furthermore, these errors will be the most significant when operating near the sensitivity limit of these devices. Deconvolution-based correction methods are invalidated by this same signal dependence. Instead, a numerical method of successive approximation can be used to correct coupling due to a well-characterized IPC. Examination of single pixel reset data above flat fields could be used to characterize IPC’s functional relationship for neighboring pixels. This higher quality characterization can result in more accurate correction.

We present the status of on-going detector development efforts for our joint NASA/Centre National d’Études Spatiales balloon-borne UV multiobject spectrograph, the Faint Intergalactic Redshifted Emission Balloon (FIREBall-2; FB-2). FB-2 demonstrates a UV detector technology, the delta-doped electron-multiplying CCD (EMCCD), in a low-risk suborbital environment, to prove the performance of EMCCDs for future space missions and technology readiness level advancement. EMCCDs can be used in photon-counting mode to achieve extremely low readout noise (<1 electron). Our testing has focused on reducing clock-induced-charge (CIC) through wave shaping and well-depth optimization with a Nüvü V2 CCCP controller, measuring CIC at 0.001  e⁻  /  pixel  /  frame. This optimization also includes methods for reducing dark current, via cooling, and substrate voltage levels. We discuss the challenges of removing cosmic rays, which are also amplified by these detectors, as well as a data reduction pipeline designed for our noise measurement objectives. FB-2 flew in 2018, providing the first time an EMCCD, was used for UV observations in the stratosphere. FB-2 is currently being built up to fly again in 2020, and improvements are being made to the EMCCD to continue optimizing its performance for better noise control.

Electron-multiplying charge-coupled devices (EMCCDs) allow for subelectron effective read noise and thus for imaging at extremely low flux levels. In the ultraviolet, quantum yield creates an additional source of stochastic gain variation, which can be difficult to quantify using existing techniques. We propose a method for measuring the quantum yield gain of these devices, independent of existing methods, using images that are part of the existing test regimen for new EMCCDs. With this method, we were able to recover the quantum yield used to create simulated images within an accuracy of ∼5  %   and the method provided consistent results with test images after only minor modifications. However, the measured quantum yield remains anomalously low, consistent with other measurements on Teledyne-e2v devices. We hypothesize that this discrepancy is due to lateral transfer of secondary electrons between pixels at the surface explained by the band structure and crystal geometry of typical silicon wafers used in array detector manufacture.

For fabrication of high-performance mirror devices, technical aluminum alloys Al6061 or Al905 are widely used. The surface error topography after manufacturing by single-point diamond turning is applicable in the infrared spectral range. For increasing demands on the optical surface quality in the shortwave visible and ultraviolet spectral range, further improvement of the surface roughness is required. Hence, a promising alternative process to attain the required surface quality is evaluated. Within the ion beam planarization technique, a photoresist layer is deposited by conventional spin coating or spray coating technologies exhibiting an ultrasmooth surface. When removing the resist by reactive ion beam etch (RIBE) processing using nitrogen process gas, the ultrasmooth surface topography of the resist is transferred into the substrate. We optimized the photoresist thermal pretreatment to realize roughness preservation and a steady-state material removal rate during RIBE machining. The optimum preparation steps are explored based on roughness evaluation, chemical modification, and etch resistance of the negative photoresist. Reactive ion beam etching-based planarization is conducted on single-point diamond turned RSA Al905 and RSA Al6061 samples made of rapidly solidified aluminum (RSA) in a two-step process. The optimum process and the roughness evaluation are explored by topographic analysis applying a combination of white light interferometry and atomic force microscopy measurements.

Thin aperture light collector (TALC) is the next generation of telescopes for space exploration. TALC consists of deployable annular segmented mirrors supported on a central mast with the help of cables. The dynamic stability of the telescope is of immense importance in order to make sure that the telescope is pointing in the right direction during the observation period. We present a control strategy for the dynamic stabilization of the segmented TALC structure using active rods. The active rods consist of collocated pairs of piezoelectric stack actuators and sensors. Decentralized integral force feedback is proposed to enhance the dynamic stability of the TALC. The effectiveness of the strategy is demonstrated on a 1/10th scaled mock-up model of the TALC. For numerical investigation, finite element analysis of the TALC is carried out and a reduced order model is extracted using the Craig–Bampton method. This reduced order model is then used for the design and numerical validation of the controller. Experiments are conducted on the mock-up model of the TALC to evaluate the performance of the proposed strategy. It is found that the proposed strategy is quite effective for dynamic stabilization of TALC. It is found to reduce both steady state and transient responses of the TALC.

HaloSat is the first mission funded by NASA’s Astrophysics Division to use the CubeSat platform. Using three co-aligned silicon drift detectors, the HaloSat observatory measures soft (0.4 to 7 keV) x-ray emission from sources of diffuse emission such as the hot, gaseous halo of the Milky Way. We describe the design and construction of the science payload on HaloSat and the reasoning behind many of the choices. As a direct result of the design choices and adherence to best practices during construction, the HaloSat science payload continues to perform well after more than one year on-orbit.

Imaging spectropolarimetry is an important observational tool in solar physics because it provides fast-cadence spectral scans with high spectral resolution, large field-of-view, and inherent suitability for post facto image restoration. Fabry–Pérot etalons are the key optical elements of these instruments. Their optical quality critically defines the instrument’s performance. The two etalons of the GREGOR Fabry–Pérot interferometer were used for more than 10 years, raising questions about the potential deterioration of etalon coatings. We present an assessment of the etalons’ optical quality, describe the inspection method based on Zernike polynomials, discuss the field dependence of the finesse and its consequences for instrument design, and investigate the impact of the measurement technique to achieve plate parallelism. We find that extended exposure to sunlight affects the etalon coatings, i.e., lowering the peak transmission and leaving an imprint of the pupil of the GREGOR solar telescope on the etalon that is directly exposed to sunlight. The finesse of both etalons, however, remains high, so the impact on imaging spectropolarimetry is negligible.

Ground-based direct imaging surveys such as the Gemini Planet Imager Exoplanet Survey (GPIES) rely on adaptive optics (AO) systems to image and characterize exoplanets that are up to a million times fainter than their host stars. One factor that can reduce AO performance is turbulence induced by temperature differences in the instrument’s immediate surroundings (e.g., “dome seeing” or “mirror seeing”). In this analysis, we use science observations, AO telemetry, and environmental data from September 2014 to February 2017 of the GPIES campaign to quantify the effects of mirror seeing on the performance of the Gemini Planet Imager (GPI) instrument. We show that GPI performance is optimal when the primary mirror (M1) is in equilibrium with the outside air temperature. We then examine the characteristics of mirror seeing by calculating the power spectral densities (PSDs) of spatial and temporal Fourier modes. Inside the inertial range of the PSDs, we find that the spatial PSD amplitude increases when M1 is out of equilibrium and that the integrated turbulence may exhibit deviations from Kolmogorov atmospheric turbulence models and from the one-layer frozen flow model. We conclude with an assessment of the current temperature control and ventilation strategy at Gemini South.

A key challenge for starshades is formation flying. To successfully image exoplanets, the telescope boresight and starshade must be aligned to ∼1  m at separations of tens of thousands of kilometers. This challenge has two parts: first, the relative position of the starshade with respect to the telescope must be sensed; second, sensor measurements must be combined with a control law to keep the two spacecraft aligned in the presence of gravitational and other disturbances. In this work, we present an optical sensing approach using a pupil imaging camera in a 2.4-m telescope that can measure the relative spacecraft bearing to a few centimeters in 1 s, much faster than any relevant dynamical disturbances. A companion paper will describe how this sensor can be combined with a control law to keep the two spacecraft aligned with minimal interruptions to science observations.

A direct current (DC) magnetron sputtering system has been developed at NASA Marshall Space Flight Center for depositing broadband x-ray optic multilayer coatings. The system has the capability to house up to four, 2-in.-diameter direct-cooled DC magnetron cathodes. The 2-in.-diameter cathodes facilitate low-cost, early stage research and development of various types of optical coatings. Despite the small target diameter, good coating uniformity on 4-in.-diameter substrates is demonstrated. Sources of both systematic and random error in the deposition of depth-graded multilayer coatings were identified and compensated for, resulting in the ability to deposit coating that required ∼7.5  h of total deposition time. Final verification of the system was concluded with the deposition of an NuSTAR W/Si depth-graded multilayer coating design (flight recipe 10).

We describe the design considerations, installation, and technical challenges of coupling a cyclical spatial heterodyne spectrometer (SHS) with the Cassegrain telescope at the Lick Observatory on Mt. Hamilton, California. The SHS instrument (named Khayyam after the mathematician) is mounted to a fixed focal plane shared by the 0.6-m Cassegrain Coudé Auxiliary Telescope (CAT) and has the field-of-view of ∼4  arc min, on the sky, spectral resolving power (R) of 48,000 and a tunable wavelength bandpass range over Δλ_B  ∼  150  Å. This instrument–telescope pairing is optimal for temporal observations of extended astronomical targets, e.g., cometary coma, when significant observing time is available since it provides high-resolution spectra from small input apertures. Khayyam’s approach contrasts with traditional high spectral resolution spectrometers that need to be coupled to large aperture telescopes to compensate for their low throughput. Based on our reference lamp results, we were able to formulate the undesirable impact of the spider pattern on the SHS data that prohibited us from acquiring spectra from our sky targets. However, more analysis is needed to investigate if we can create a framework to systematically eliminate the diffracted spider pattern shadow from the fringe pattern without compromising the integrity and quality of the data.

We present a revision to the astrometric calibration of the Gemini Planet Imager (GPI), an instrument designed to achieve the high contrast at small angular separations necessary to image substellar and planetary-mass companions around nearby, young stars. We identified several issues with the GPI data reduction pipeline (DRP) that significantly affected the determination of the angle of north in reduced GPI images. As well as introducing a small error in position angle measurements for targets observed at small zenith distances, this error led to a significant error in the previous astrometric calibration that has affected all subsequent astrometric measurements. We present a detailed description of these issues and how they were corrected. We reduced GPI observations of calibration binaries taken periodically since the instrument was commissioned in 2014 using an updated version of the DRP. These measurements were compared to observations obtained with the NIRC2 instrument on Keck II, an instrument with an excellent astrometric calibration, allowing us to derive an updated plate scale and north offset angle for GPI. This revised astrometric calibration should be used to calibrate all measurements obtained with GPI for the purposes of precision astrometry.

Many future space missions including Large UV/Optical/Infrared Surveyor and Laser Interferometer Space Antenna require metering structures of exceptional geometric stability in order to meet their science goals. Thermal expansion and contraction of these structures is typically a primary concern. The application of a known high-stability bonding method, hydroxide catalysis bonding, to a new structural material, ALLVAR Alloy 30, with the uniquely useful property among metals of a negative coefficient of thermal expansion is studied. We report on measurements of the shear breaking strength of Allvar mated with fused silica (glass), grade 5 Ti-6Al-4V (Ti64), and aluminum 6061-T6 (aluminum). Measured shear breaking strengths ranged from 3 to 16 MPa with a mean of ∼7.5  MPa. Initial series of tests indicated no significant correlation between bonding strength and surface roughness over the range of surface roughness tested.

We present the analysis of James Webb Space Telescope near-infrared H2RG detectors with a 5-μm cutoff, which shows that, at temperatures <60  K, there is no measurable dark current. Instead, the observed signal in dark exposures is almost entirely due to multiplexer glow that arises as each pixel is selected. We are able to separate the per-sample glow from the time-dependent dark current by comparing the observed signal in both continuous and sparsely sampled dark exposures. Such explicit tests are required to break the degeneracy between dark current and uniform amplifier glow. We show that the glow is lower within the regions of the detector that are missing the epoxy back fill (voids). We also find that the glow from each pixel extends out to a radius of several pixels. Because of the higher sampling frequency of subarray observations, the per-sample glow leads to a higher apparent dark current in subarray exposures. Finally, we show that the magnitude of the glow is affected by the pixel source follower current, the pixel clocking rate, and the number of outputs running in parallel. Our measurement of an insignificant dark current shows that the detector noise is no longer limited by the quality of the mercury cadmium telluride layer but instead by the multiplexer and readout electronics.

We report on the results of an experiment to determine the effects of radiation damage caused by high-energy protons on an x-ray hybrid CMOS detector. This detector was utilized in a previous proton radiation experiment, which delivered a total dose to a selected region of ∼3  krad (Si). With updated hardware and experimental procedures, we further irradiated the detector with 7-MeV protons, delivering an additional 1.5 krad (Si) (2.78  ×  10⁹  protons  /  cm² 10 MeV equivalent) with increased uniformity to an overlapping region. The effects of this radiation on several important detector characteristics were analyzed after delivering doses of 0.5 and 1.0 krad. After 16 h of annealing at room temperature, detector performance was found to be unchanged in both cases.

Coherent laser radar (CLR) multilateration for mapping a large dish relative to antenna design is discussed. Multilateration is more accurate than typical mapping methods used for analysis of antenna design, such as photogrammetry and tracking interferometers. Two CLR designs are considered: using four independent CLRs, and using a single CLR in which the laser is divided into four channels. The accuracy/cost trade-off shows that the four independent CLRs are about 3  ×   more accurate than the single four-channel CLR, while the cost is 3  ×   more. For a laser beam on a 50-m dish, the estimated CLR measurement uncertainty is between 20 and 31  μm, while the average root mean square, including environmental considerations for four independent CLRs, is 28 and 83  μm for the single CLR with four channels. The antenna can be measured at any elevation angle and dynamic measurements are possible. CLR is immune to sunlight, so measurements can be collected during daytime or nighttime. CLR operating wavelength (1500 nm) is sufficiently different from millimeter frequencies, so simultaneous measurements may be possible.

We investigate the conceptual design and in-orbit assembly mission planning problem of a large space telescope (LST). The segmented mirror design has been proposed, and the robotic assembly concept considering the manipulator work space coverage is developed. To reduce the in-orbit assembly period and protect the fragile mirror structure, the assembly paths of the robots are optimized by several new algorithms. First, a mapping between the assembly path and the assembled piece number is established to rapidly generate the candidate solution to the optimization problem. Second, the two-level hybrid optimization framework that combines the ant-colony-inspired algorithm and the genetic algorithm is proposed. The hybrid optimization method is designed to be able to converge rapidly to a solution that is close to the global optimal point. The proposed models and algorithms are verified by simulation, and the results show that the methods developed can dramatically increase the in-orbit assembly mission efficiency of an LST.

The recent burgeoning interest in massive multiobject spectroscopy has pushed the development of massive optical fiber positioning systems. These systems rely on precise fiber placement to detect the light spectra of many stars and galaxies. One successful approach is the use of robotic fiber positioners, which allow one to automate and scale up observations. However, due to the need for high precision and accuracy, each positioner must be calibrated and verified to comply with the requirements. The calibration measurements are nontrivial, and the large number of the robotic positioners up to thousands can lead to a prohibitively long time for calibration. We describe and validate an optical calibration setup and procedure for robotic fiber positioning systems. Based on the measurements results, we have developed models describing the behavior of the positioners and we introduce new performance metrics that allow one to verify the stringent positioner specifications and furthermore help to identify and analyze design and manufacturing flaws.

Camera lenses are increasingly used in wide-field astronomical surveys due to their high performance, wide field-of-view (FOV) unreachable from traditional telescope optics, and modest cost. The machining and assembly tolerances for commercially available optical systems cause a slight misalignment (tilt) between the lens and CCD, resulting in point spread function (PSF) degradation. We have built an automated alignment system (Robotilters) to solve this challenge, optimizing four degrees of freedom—two tilt axes, a separation axis (the distance between the CCD and lens), and the lens focus (the built-in focus of the lens by turning the lens focusing ring, which moves the optical elements relative to one another) in a compact and low-cost package. The Robotilters remove tilt and optimize focus at the sub-10-μm level, are completely automated, take ≈2  h to run, and remain stable for multiple years once aligned. The Robotilters were built for the Evryscope telescope (a 780-MPix 22-camera array with an 8150-sq. deg FOV and continuous 2-min cadence) designed to detect short-timescale events across extremely large sky areas simultaneously. Variance in quality across the image field, especially the corners and edges compared to the center, is a significant challenge in wide-field astronomical surveys like the Evryscope. The individual star PSFs (which typically extend only a few pixels) are highly susceptible to slight increases in optical aberrations in this situation. The Robotilter solution resulted in a limiting magnitude improvement of 0.5 mag in the center of the image and 1.0 mag in the corners for typical Evryscope cameras, with less distorted and smaller PSFs (half the extent in the corners and edges in many cases). We describe the Robotilter mechanical and software design, camera alignment results, long-term stability, and image improvement. The potential for general use in wide-field astronomical surveys is also explored.

We present an algorithm for the detection of candidate astronomical pulses. It is implemented in several steps. First, a spectrogram of a dispersed astronomical pulse is linearized in observing frequency followed by application of the Radon transform. The result of the transformation is displayed as a two-dimensional function. Next, the function is smoothed using a spatial low-pass filter. Finally, the maximum of the function above 90-deg angle is compared to the maximum of the standard deviation of the noise below 90-deg angle and a decision in favor of an astronomical pulse present or absent is made. Once pulse is detected, its dispersion measure (DM) is estimated by means of a basic equation relating the slope of the linearized dispersed pulse and the DM value. Performance of the algorithm is analyzed by applying it to a set of simulated fast radio bursts, experimental data of Masui pulse, and of seven rotating radio transients. The detection algorithm demonstrates results comparable to those by the conventional pulse detection algorithm.

Space instruments such as solar arrays, radiators, or optics can be strongly impacted by molecular contaminants outgassed from spacecraft materials. For optics, transmittance and reflectance performances could indeed be modified by the deposit of contaminants. We report the transmittance measurements and predictions in the ultraviolet–visible–near-infrared range of contaminated optics from the outgassing of a mixture of two common materials used in space industry: EC2216 material (epoxy compound) and RTVS691 material (silicone compound). The Swanepoel model, commonly used in many fields, was employed for the first time in such conditions to easily and quickly predict transmittance. Transmittance was fully recovered at 20°C; a decontamination plan could be based on heating at this temperature at least during a duration depending on the silicone/epoxy contaminants layer thickness.

The James Webb Space Telescope (JWST) cryogenic testing required measurement systems that both obtain a very high degree of accuracy and can function in that environment. Close-range photogrammetry was identified as meeting those criteria. Extensive modeling prior to installation verified that the design would meet the desired accuracy goals. Extensive validation work was done to ensure that the actual as-built system met accuracy and repeatability requirements. The simulated image data predicted the uncertainty in measurement to be within specification and this prediction was borne out experimentally. Uncertainty at all levels of a measurement volume that comprised 8  ×  8  m was verified experimentally to be <0.1  mm with a repeatability of <0.03  mm at the primary mirror (PM) level, achieving measurement accuracies on par with a laser tracker or radar system. During a 3-month Optical Telescope Element and Integrated Science Instrument Module thermal vacuum test performed in chamber A at Johnson Space Center, the photogrammetric system provided key data to allow for verification of actuator ranges needed for on-orbit alignment of JWST, alignment of the PM and secondary mirror (SM), and positional information needed for thermal model verification.

Future large space telescopes will be equipped with adaptive optics (AO) to overcome wavefront aberrations and achieve high contrast for imaging faint astronomical objects, such as Earth-like exoplanets and debris disks. In contrast to AO that is widely used in ground telescopes, space-based AO systems will use focal plane wavefront sensing to measure the wavefront aberrations. Focal plane wavefront sensing is a class of techniques that reconstructs the light field based on multiple focal plane images distorted by deformable mirror (DM) probing perturbations. We report an efficient focal plane wavefront sensing approach for space-based AO that optimizes the DM probing perturbation and thus also the integration time for each image. Simulation of the AO system equipped with a vortex coronagraph has demonstrated that our approach enables efficient information acquisition and significantly reduces the time needed for achieving high contrast in space.

The free-vibration modes of an annular mirror (FVMAM), derived from the thin plate theory and reflecting the intrinsic characteristics of the physical phenomenon of resonance, have been applied to compensate the aberrations of the active optics system. As an application example, the compensations of some low-order aberrations of the 2.5-m Wide-Field Survey Telescope with the FVMAM have been presented. In addition, a quantitative comparative study of the aberration corrections between the FVMAM and the annular Zernike polynomials has been carried out. The results have shown that the FVMAM are more effective to correct the aberrations.