The World Space Observatory for Ultraviolet (WSO-UV) is an orbital optical telescope with a 1.7-m diameter primary mirror currently under development. The WSO-UV is aimed to operate in the 115- to 310-nm UV spectral range. Its two major science instruments are UV spectrographs and UV imaging field cameras with filter wheels. The WSO-UV project is currently in the implementation phase, with a tentative launch date in 2023. As designed, the telescope field of view in the focal plane is not fully occupied by instruments. Recently, two additional instruments devoted to exoplanets have been proposed for WSO-UV, which are the focus of this paper. UVSPEX, a UV-spectrograph for exoplanets, aims to determine atomic hydrogen and oxygen abundance in the exospheres of terrestrial exoplanets. The spectral range is 115 to 130 nm, which enables simultaneous measurement of hydrogen and oxygen emission intensities during an exoplanet transit. A study of exosphere transit photometric curves can help differentiate among different types of rocky planets. The exospheric temperature of an Earth-like planet is much higher than that of a Venus-like planet because of the low mixing ratio of the dominant coolant (CO₂) in the upper atmosphere of the former, which causes a large difference in transit depth at the oxygen emission line. Thus, whether the terrestrial exoplanet is Earth-like, Venus-like, or other can be determined. A Stellar Coronagraph for Exoplanet Direct Imaging (SCEDI) is aimed to directly detect the starlight reflected from exoplanets orbiting their parent stars or from the stellar vicinity including circumstellar disks, dust, and clumps. SCEDI will create an achromatic (optimized to 420- to 700-nm wavelength range), high-contrast stellocentric coronagraphic image of a circumstellar vicinity. The two instruments, such as UVSPEX and SCEDI, share common power and control modules. The present communication outlines the science goals of both proposed instruments and explains some of their engineering features.

We investigated the definition for circular off-axis aspheric mirrors (COAMs). The clear opening of the COAM obtained by projecting a circular aperture on the aspheric surface does not form a plane but a curved surface. To solve this problem, we propose to use a plane to cut the aspheric surface to obtain a COAM. The COAM has three characteristics: (i) an elliptical opening; (ii) the major axis of the opening lies on the meridional plane containing the original optical axis of the aspheric surface and the minor axis lies on the sagittal plane perpendicular to the meridian plane; and (iii) it is not deepest at the center of the ellipse. The third characteristic suggests two methods for defining the COAM using a plane: (i) with the center of the elliptical opening as a reference and (ii) with the deepest point of the mirror as a reference. All the formulas required to obtain the desired COAM using the two methods are presented.

W  /  B₄C multilayer (ML) mirrors with varying periodicities   (  d  )    =  1.6 to 5.4 nm are tested for rapid thermal and temporal stability, which are required for space-based x-ray telescopes for astronomy. The aging effects on the structural parameters over a period of 2 years are assessed through hard x-ray reflectivity (HXR) measurements. Multiwavelength performance of ML mirrors is studied over thermal cycling from −40  °  C to +50  °  C for 1, 3, and 10 days, which simulate the expected temperature variation in the low-earth orbit. The structural parameters of all samples remained nearly constant over the first 2 years. It is observed that the short-period MLs develop a contamination layer over time. Rapid thermal cycling results indicate no change in HXR for all ML mirrors. However, at soft x-rays, there is a reduction in reflectivity after thermal cycling. The variations in optical performance at hard and soft x-ray energies after thermal cycling are due to variation in interface roughness at different spatial frequencies.

According to the demand for high-performance silver-based telescope mirrors, attempts are being made to develop surface coatings that protect the mirrors from corrosion. Aluminum nitride (AlN) is utilized for various optical coatings, and its high optical transparency and mechanical robustness make it potentially well suited as a protective coating of silver-based mirrors. However, according to our best knowledge, AlN with controlled oxygen content has never been used to protect silver mirrors. Various protective coatings based on AlN are prepared by RF magnetron sputtering. Specific amounts of oxygen are deliberately introduced to obtain aluminum oxynitride (AlON) protective layers with refractive indices within a certain range (i.e., high ∼2.1, medium ∼1.8, and low ∼1.6 at 400 nm). The designed protective layers are applied to two types of silver-based mirror test structures, and their performance was assessed in terms of optical reflectivity and structural integrity of the test structures that underwent environmental testing in a controlled atmosphere at 80°C with ∼80  %   relative humidity. Comprehensive analysis on the samples before and after the environmental testing indicates that AlON-based 41- and 56-nm protective layers with medium refractive index outperform similar samples using AlON with higher or lower refractive index. We suggest that the benefits of the best AlON barrier with specific refractive index are likely associated with the optical, chemical, and structural characteristics produced with sputtering conditions that yield a nitrogen/oxygen ratio of approximately one.

The integration of the Large Synoptic Survey Telescope (LSST) technical writing process, project planning, tools, and the software SOLIDWORKS^® Composer, as well as the review process and storage, is described. LSST creates highly effective visual work instructions with SOLIDWORKS^® Composer directly from existing 3-D CAD models, with content that can be updated automatically if the design and development change. We are able to create quickly any kind of interactive images and animations for the technical documentation, which helps clearly present even the most complex data without the ambiguity that may occur with traditional 2-D static images. By creating clear and easily understandable visual technical documentation, LSST minimizes both technical and personnel risks by ensuring workers fully understand all aspects of the required task. By illustrating what tools to use and where to use them, LSST protects the health and safety of their workers and equipment while streamlining the construction process. These documents also facilitate educating employees on best practices.

Modern observatories and instruments require optics fabricated at larger sizes with more stringent performance requirements. The Daniel K. Inouye Solar Telescope (DKIST) will be the world’s largest solar telescope at 4.0-m aperture delivering a 300 W beam and a 5 arc min field. Spatial variation of retardance is a limitation to calibration of the full field. Three polarimeters operate seven cameras simultaneously in narrow bandpasses from 380 to 1800 nm. The DKIST polarization calibration optics must be 120 mm in diameter at Gregorian focus to pass the beam and operate under high heat load, UV flux, and environmental variability. Similar constraints apply to the three retarders for modulation within the instrument suite with large beams near focal planes at F/18 to F/62. We assess how design factors can produce more spatial and spectral errors simulating elliptical retardance caused by polishing errors. We measure over 5-deg net circular retardance and spectral oscillations over ±2  deg for optics specified as strictly linear retarders. Spatial variations on scales >10  mm contain 90% of the variation. Different designs can be a factor of 2 more sensitive to polishing errors with dissimilar spatial distributions even when using identical retardance bias values and materials. The calibration of the on axis beam is not impacted once circular retardance is included. The calibration of the full field is limited by spatial retardance variation unless techniques account for this variation. We show calibration retarder variation at amplitudes of 1-deg retardance for field angles greater than roughly 1 arc min for both quartz and MgF₂ retarders at visible wavelengths with significant variation between the three DKIST calibration retarders. We present polishing error maps to inform calibration techniques attempting to deliver absolute accuracy of system calibration below effective cross talk levels of 1 deg retardance.

We design a distributed situational observer using formation flying in a displaced orbit. The main focus of our investigation is the relative dynamics and control of displaced orbits obtained by low-thrust propulsion. The spatial dynamics in Newtonian form are used to derive the numerical relative motions, and their natural frequencies discovered by eigenvalue decomposition separate from each other at a critical height that differentiates the structural stability, bifurcation, and instability. Using the Jordan decomposition, six fundamental motions are achieved, including the stationary multiequilibria, the periodic oscillations that correspond to the natural frequencies, and the maximum leaving or approaching velocity caused by the different geometric and algebraic multiplicities. Off-axis equilibrium is obtained by a proposed open-loop control, and the motions nearby are proven to be equivalent to the numerical relative motions. The reduced dynamics in Hamiltonian form are used to derive the analytical solutions for linearized relative motions. Bounded relative trajectories with arbitrary initial values are achieved by two extraclosed-loop controls. Using the off-axis equilibrium and resonance of natural frequencies, the applications of a fixed relative baseline vector for interferometric SAR or Fresnel zone lens missions and repeating relative ground tracks for a phased array antenna mission are addressed in terms of the trajectory design.

Toward an era of x-ray astronomy, next-generation x-ray optics are indispensable. To meet a demand for telescopes lighter than the foil optics but with a better angular resolution <1  arcmin, we are developing micropore x-ray optics based on micromaching technologies. Using sidewalls of micropores through a thin silicon wafer, this type can be the lightest x-ray telescope ever achieved. Two Japanese missions, ORBIS and GEO-X, will carry this telescope. ORBIS is a small x-ray astronomy mission to monitor supermassive blackholes, while GEO-X is a small exploration mission of the Earth’s magnetosphere. Both missions need an ultralightweight (<1  kg) telescope with moderately good angular resolution (<10  arcmin) at an extremely short focal length (<30  cm). We plan to demonstrate this type of telescope in these two missions around 2020.

The Ngari (Ali) observatory is located in Ngari, Tibet, a region known as “the roof of the roof of the world.” The observatory benefits from abundant photometric nights, low perceptible water vapor, high transmittance, and good seeing. Due to these advantages, it promises to be one of the best locations in the world at which to make infrared and submillimeter observations. However, no data on the sky background radiation at this location are available, impacting the planning of future facilities at the observatory. To remedy this deficiency, a near-infrared sky brightness monitor (NISBM) has been designed to obtain data in the J, H, and Ks bands. This monitor is based on an InGaAs photoelectric diode and uses chopper modulation and digital lock-in amplifier processing, which considerably enhance its signal-to-noise ratio, detectivity, and data acquisition speed. An independent device has been designed for each band (J, H, and Ks) and calibrated in the laboratory. The NISBM was installed at the Ngari observatory in July 2017 and has obtained the first NIR sky brightness data for that location.

Aluminum mirrors offer great potential for satisfying the increasing demand in high-performance optical components for visible and ultraviolet applications. Ion beam figuring is an established finishing technology and in particular a promising technique for direct aluminum figure error correction. For the machining of strongly curved or arbitrarily shaped surfaces as well as the correction of low-to-mid spatial frequency figure errors, the usage of a high-performance ion beam source with low tool width is mandatory. For that reason, two different concepts of ion beam generation with high ion current density and narrow beam width are discussed. (1) A concave ion beam extraction grid system is used for apertureless constriction of ion beams in the low millimeter range. An oxygen ion beam with a full-width at half-maximum (FWHM) of 4.0 mm with an ion current density of 29.8  mA  /  cm² was achieved. (2) For even smaller ion beams, a conic aperture design with a submillimeter-sized exit opening was tested. A nitrogen ion beam with an FWHM down to 0.62 mm with an ion current density of 4.6  mA  /  cm² was obtained. In situ ion current density mapping is performed by scanning Faraday probe measurements. Special interest is set on the data evaluation for submillimeter ion beam analysis.

The Transiting Exoplanet Survey Satellite (TESS) is an MIT-led, NASA-funded Explorer-class planet finder launched in April 2018. TESS will carry out a 2-year all-sky survey with the primary goal of detecting small transiting exoplanets around bright and nearby stars. The TESS instrument consists of four wide-field cameras in a stacked configuration, providing a combined field of view of 24  deg  ×   96  deg that spans approximately from the ecliptic plane to the ecliptic pole. In order to achieve the desired photometric precision necessary for the mission, TESS uses the instrument cameras as star trackers during fine-pointing mode to enhance attitude accuracy and stabilization for science operations. We present our approach in quantifying the expected performance of the fine-pointing system and assessing the impact of pointing performance on the overall photometric precision of the mission. First, we describe the operational details of the fine-pointing system with the science instrument being used for star-tracking. Next, we present the testing framework used to quantify the attitude determination performance of the system and the expected attitude knowledge accuracy results, both in coarse-fine pointing hand-off and in nominal fine-pointing conditions. By combining simulations of the instrument and the spacecraft bus, we quantify the closed-loop fine-pointing stability performance of the system in nominal science operations as well as in the case of camera unavailability due to Earth/Moon interference. Finally, we assess the impact of platform pointing stability on the photometric precision of the system using detailed system modeling and discuss the applicability of mitigation techniques to reduce the effect of jitter on TESS science data.

The generation of agile satellites has better flexible three-axis maneuverability, which makes dynamic imaging possible. A mission planning algorithm is proposed to deal with the area target observation problems of agile satellites, which takes the drift angle constraint of the observation instrument into account. First, an area target observation path planning algorithm is presented based on the dynamic imaging mode and the grid discretization approach. Second, the constraint of drift angle is introduced into the mission planning to account for the effect of image motion on time delay integration charge coupled device imaging. Third, an improved ant colony algorithm based on sensational and consciousness strategy is developed to solve the travelling salesman problem with constraints. Finally, the computer simulation results show that the proposed algorithm can effectively solve the area target mission planning problem.

We address the issue of the observability and performance of three autonomous navigation schemes with different combinations of measurements for the terminal cruise phase of Mars exploration. First, the system dynamic model and navigation measurement models are established to facilitate subsequent design and analysis. Second, the detailed degree of observability analysis of three navigation schemes is conduced based on the observability analysis theory of the piece-wise constant system. Third, both extended Kalman filter and unscented Kalman filter are adopted to assess the performance of different navigation approaches. Finally, the performance of three navigation schemes with different measurements is comparatively studied by theoretical analysis and computer simulation. Meanwhile, the optimal integrated navigation scheme (Mars/Sun/one x-ray pulsar measurement) is suggested based on comprehensive consideration of scheme performance and engineering constraints.

Chromospheric magnetic fields are of paramount importance in understanding the dynamics of energetic events in the solar atmosphere. At the Kodaikanal Solar Observatory, several polarimeters were developed in the past to study the active region magnetic fields. A polarimeter has been developed and installed at Kodaikanal Tower-Tunnel Telescope to study the active regions at the chromospheric level, in Ca ii 8542 Å spectral line. Design aspects of the instrument and polarimetry strategy are discussed. Telescope instrumental polarization has been revisited, and possible ways to reduce it have been proposed. The telescope polarization model developed in Zemax to examine the analytical instrumental polarization model is discussed. The polarimeter control unit, and the software developed to operate the polarimeter are briefly described. Polarimetric calibration of the instrument, observations, corrections for instrumental polarization, and the sample Stokes profiles are presented. Polarimetric accuracy and sensitivity are estimated to be better than 3  ×  10^−  2 and 3  ×  ^{10  −  3}, respectively.

We present an introduction to the heasim multimission observation and skyback background, high-energy pseudo Monte Carlo astrophysical simulation tools. Heasim may be used to accurately and efficiently construct flexible image transport system (FITS) event files for simple or composite sources with a wide range of standard and user-defined spatial, spectral, and temporal characteristics. Skyback is designed to enable users to assess the impact of background discrete and diffuse emission on prospective observations, and skyback output may be directly input into heasim. We present a brief overview of heasim and skyback input, algorithms, usage, and output. We also introduce the sxsbranch tool that computes Hitomi soft X-ray spectrometer resolution grade branching ratios, emphasizing its application to simulations. We include several examples of particular relevance to the Hitomi mission.

Digital baseband converters (DBBC) are among the core equipment in a very long baseline interferometry (VLBI) system, which undertakes missions of data acquisition, analog-to-digital conversion, and frequency selection. In order to enhance the measurement resolution of the Chinese VLBI Network (CVN), a new generation of DBBC named CDAS2 (the second-generation Chinese VLBI data acquisition system) has been developed. CDAS2 uses polyphase filter banks (PFB) and fast Fourier transform to convert signals from intermediate frequency to baseband. A high-speed sampling analog-to-digital converter is adopted to guarantee CDAS2’s broadband observation ability. In order to facilitate the broadband observation ability of CDAS2, a high-speed data transmission system is designed to transmit data from CDAS2 to a data recording server. The data transmission system includes a Mark5B formatter and a 10G network transmission system. We describe the detailed design of PFB and the data transmission system. We installed two CDAS2s in two radio astronomical telescopes of Shanghai Astronomical Observatory to carry out observation experiments for the radio source of 4C39.25. After processing with DiFX, we got interference fringes successfully. Our experiment demonstrates the validity and performance of CDAS2.

We describe the tools and the underlying methods and principles for generating the spectral response functions for the four imaging instruments that were flown on the Hitomi x-ray astronomy satellite [Soft X-ray Spectrometer, or SXS; Soft X-ray Imager, or SXI, and two Hard X-ray Imagers, or HXI]. In essence, the spectral response function is a temporally and spatially averaged effective area and line-spread-function. For model-fitting x-ray spectra from an instrument, the spectral response function encapsulates the end-to-end physics of the entire system from telescope to detector, and also includes satellite attitude drift, exposure corrections, and in the case of the HXIs, drift in the telescope/detector alignment system. Accuracy in the construction of the spectral response functions is, therefore, critical to maximize the science return from the data.

Vibrations are a key source of image degradation in ground-based instrumentation, especially for high-contrast imaging instruments. Vibrations reduce the quality of the correction provided by the adaptive optics system, blurring the science image, and reducing the sensitivity of most science modules. We studied vibrations using the Subaru coronagraphic extreme adaptive optics instrument at the Subaru Telescope as it is the most vibration-sensitive system installed on the telescope. We observed vibrations for all targets, usually at low frequency, below 10 Hz. Using accelerometers on the telescope, we confirmed that these vibrations were introduced by the telescope itself, and not the instrument. It was determined that they were related to the pitch of the encoders of the telescope drive system, both in altitude and azimuth, with frequencies evolving proportionally to the rotational speed of the telescope. Another strong vibration was found in the altitude axis of the telescope, around the time of transit of the target, when the altitude rotational speed is below 0.12  arc sec  /  s. These vibrations are amplified by the 10-Hz control loop of the telescope, especially in a region between 4 and 6 Hz. We demonstrate an accurate characterization of the frequencies of the telescope vibrations using only the coordinates—right ascension and declination—of the target and provide a means by which we can predict them for any telescope pointing. This will be a powerful tool that can be used by more advanced wavefront control algorithms, especially predictive control that uses information about the disturbance to calculate the best correction.

We present a multiconjugate adaptive optics (MCAO) system simulator bench, Herzberg NFIRAOS Optical Simulator (HeNOS). HeNOS is developed to validate the performance of the MCAO system for the Thirty Meter Telescope, as well as to demonstrate techniques critical for future AO developments. We focus on describing the derivations of parameters that scale the 30-m telescope AO system down to a bench experiment and explain how these parameters are practically implemented on an optical bench. While referring to other papers for details of AO technique developments using HeNOS, we introduce the functionality of HeNOS, in particular, three different single-conjugate AO modes that HeNOS currently offers: a laser guide star AO with a Shack–Hartmann wavefront sensor, a natural guide star AO with a pyramid wavefront sensor, and a laser guide star AO with a sodium spot elongation on the Shack–Hartmann corrected by a truth wavefront sensing on a natural guide star. Laser tomography AO and ultimate MCAO are being prepared to be implemented in the near future.

Modern ground-based telescopes like the planned extremely large telescope (ELT) depend heavily on adaptive optics (AO) systems to correct for atmospheric turbulence. Even though AO correction is used, the quality of astronomical images still is degraded due to the time delay stemming from the wavefront sensor integration time and temporal response of the deformable mirror(s) (DM). This results in a blur, which can be mathematically described by a convolution of the true image with the point spread function (PSF). We present an algorithm for single-conjugate adaptive optics PSF reconstruction adapted to the needs of ELTs in a storage efficient way. In particular, the classical PSF reconstruction algorithm is changed in several points to give a more accurate estimate for the post-AO PSF. Bilinear splines are used as basis functions to minimize the computational effort. Results obtained in an end-to-end simulation tool show qualitatively good reconstruction of the PSF compared with the PSF calculated directly from the simulated incoming wavefront. Furthermore, the used algorithm has a reasonable runtime and memory consumption.

Structural vibrations usually feature tip–tilt modes and affect the closed-loop performance of the tip–tilt mirror control system in astronomical telescopes. Long integration time of the image sensor always restricts the closed-loop control bandwidth, resulting in difficulties in rejecting structural vibrations. An error-based plug-in controller to mitigate multimodal structural vibrations is proposed. Only position error is used in this proposed controller, which can be plugged into existing feedback loops to create high gain at the frequencies of multimodal disturbances, and thereby the closed-loop performance can be optimized by designing an appropriate Q-filter. When the centered frequencies of energetic disturbances are determined, a band-pass filter is suggested to mitigate multimodal vibrations. This improved band-pass filter can avoid big amplifications at other frequencies. Due to not being restricted by an accurate model, the plug-in controller does not lead to a compromise between vibration rejection and control stability. Both simulation and experiment validate the effectiveness of the proposed method in the tip–tilt mirror control system under the condition of vibrations.

The generation of Extremely Large Telescopes (ELTs) with mirror diameters up to 40 m has thick secondary mirror support structures (also known as spider legs), which cause difficulties in the wavefront reconstruction process. These spider legs create areas where the information of the phase is disconnected on the wavefront sensor detector, leading to pupil fragmentation and a loss of data on selected subapertures. The effects on wavefront reconstruction are differential pistons between segmented areas, leading to poor wavefront reconstruction. The resulting errors make the majority of existing control algorithms unfeasible for telescope systems having spider legs incorporated. A solution, named the split approach, is presented, which suggests to separate reconstruction of segment piston modes from the rest of the wavefront. Further, two methods are introduced for the direct reconstruction of the segment pistons. Due to the separate handling of the piston offsets on the segments, the split approach makes any of the existing phase reconstruction algorithms developed for nonsegmented pupils suitable for wavefront control in the presence of telescope spiders. We present end-to-end simulation results showing accurate, stable, and extremely fast wavefront reconstruction for the first light instrument mid-infrared ELT imager and spectograph of the ELT that is currently under construction.

All coronagraphic instruments for exoplanet high-contrast imaging need wavefront correction systems to reject optical aberrations and create sufficiently dark holes. Since the most efficient wavefront correction algorithms (controllers and estimators) are usually model-based, the modeling accuracy of the system influences the ultimate wavefront correction performance. Currently, wavefront correction systems are typically approximated as linear systems using Fourier optics. However, the Fourier optics model is usually biased due to inaccuracies in the layout measurements, the imperfect diagnoses of inherent optical aberrations, and a lack of knowledge of the deformable mirrors (actuator gains and influence functions). Moreover, the telescope optical system varies over time because of instrument instabilities and environmental effects. We present an expectation–maximization (E-M) approach for identifying and real-time adapting the linear telescope model from data. By iterating between the E-step (a Kalman filter and a Rauch smoother) and the M-step (analytical or gradient-based optimization), the algorithm is able to recover the system even if the model depends on the electric fields, which are unmeasurable hidden variables. Simulations and experiments in Princeton’s High Contrast Imaging Lab demonstrate that this algorithm improves the model accuracy and increases the efficiency and speed of the wavefront correction.

Phase diversity (PD) technique is an effective method for wavefront sensing and image restoration in adaptive optics (AO). Classical PD with Tikhonov regularization can achieve proper wavefront estimation but constantly results in overly smooth images. Nonlocal centralized sparse representation (NCSR) based on nonlocal self-similarity and the sparsity model is combined with PD to obtain high-resolution images. The proposed method contains two steps: the first step is obtaining wavefront from ordinary PD with Tikhonov regularization, and the second step is deblurring the image with NCSR other than Tikhonov regularization. Numerical simulations show that the peak signal-to-noise ratios and structural similarity index metrics of deblurred images by the proposed method are higher than those by the traditional method. This work also studies the influence of weak noise. Initially, the proposed method is applied to a liquid crystal AO system, where the highest spatial resolutions that can be clearly distinguished are 1.59  ×   diffraction limitation with AO on, 1.41  ×   diffraction limitation with traditional PD, and 1.26  ×   diffraction limitation with the proposed method. The proposed approach can be widely used for AO postprocessing in ground-based telescopes, fluorescence microscopes, and other applications.

This erratum corrects an error in an equation.