We developed a table-top multipass spectrograph with a 610-MHz resolution (corresponding to a resolving power of 450,000) and a throughput of 10%. The spectrograph was calibrated with a 4-GHz optical frequency comb (OFC) that did not require filtering cavities, which would hinder long-term operation. The OFC is centered at a wavelength of 1  μm, which makes it suitable for the investigation of M dwarf stars and the compact size of the OFC-calibrated spectrograph makes it suitable for use in small to mid-scale observatories.

The soft gamma-ray concentrator is a telescope mission concept utilizing a suitable arrangement of bent multilayer structures of alternating low- and high-density materials. This lens is able to channel gamma-ray photons via total external reflection and concentrate the incident radiation to a point. The channeling technique offers the potential for concentrating gamma rays with focal lengths <10  m and energies >100  keV, beyond the reach of current grazing-incidence hard x-ray mirrors. For the performance estimation of such an instrument, we have developed a flexible set of computer modeling tools to compute the optical properties of multilayer structures, predict the channeling efficiency for a given multilayer configuration, and aid in the optimization of potential gamma-ray concentrator-based telescope designs. This modeling includes the multilayer optical properties calculated by the IMD software, the ray tracing using an IDL code, and the focal plane detector simulation by MEGAlib. We illustrate the potential of this approach by presenting simulated astronomical observations from a balloon-borne platform. The final result, including simulated effective area, instrument sensitivity, and polarization performance, shows that the gamma-ray concentrator will provide greatly increased sensitivity for next-generation soft gamma-ray missions with modest cost and complexity.

The Advanced Mirror Technology Development (AMTD) project was a 6-year effort to mature technologies required to enable 4-m-or-larger monolithic or segmented ultraviolet/optical/infrared space-telescope primary-mirror assemblies for general astrophysics and exoplanet missions. AMTD used a science-driven systems-engineering approach. Starting with science requirements, engineering specifications were derived for the primary mirror aperture diameter, areal density, surface error, and stability. The most impactful specification may be 10 pm per 10-min wavefront stability. Advances were made to six key technologies: (1) fabricating large-aperture low-areal-density high-stiffness mirror substrates; (2) designing support systems; (3) correcting mid/high-spatial frequency figure error; (4) mitigating segment edge diffraction; (5) phasing segment-to-segment gaps; and (6) validating integrated models. AMTD successfully demonstrated a process to make substrates as large as 1.5 m and as thick as 40 cm by stacking multiple core elements and low-temperature fusing them together. To help predict on-orbit performance and assist in architecture trade studies, integrated models were created for two mirror assemblies (1.5-m Ultra-Low-Expansion (ULE^®) mirror fabricated by AMTD partner Harris Corp. and 1.2-m Zerodur^® mirror owned by Schott North American). X-ray-computed tomography was used to construct the 1.5-m ULE^® mirror’s “as-built” model. These models were validated by testing full- and subscale components in relevant thermovacuum environments.

A generic fiber positioning strategy and a fabrication path are presented for microlens-fiber-coupled integral field units (IFUs). It is assumed that microlens-produced microimages are carried to the spectrograph input through a step-index, multimode fiber, but our results apply to micropupil reimaging applications as well. Considered are the performance trades between the filling percentage of the fiber core with the microimage versus throughput and observing efficiency. A merit function is defined as the product of the transmission efficiency and the étendue loss. For a hexagonal packing of spatial elements, the merit function has been found to be maximized to 94% of an ideal fiber IFU merit value (which has zero transmission loss and does not increase the étendue) with a microlens-fiber alignment (centering) tolerance of 1-μm RMS. The maximum acceptable relative tilt between the fiber and the microlens face has been analyzed through optical modeling and found to be 0.3 deg RMS for input f-ratio slower than f  /  3.5, but it is much more relaxed for faster beams. From the acceptable tilt, we have deduced a minimum thickness of the fiber holder to be 3 mm for 5  μm clearance in hole diameter relative to the fiber outer diameter. Several options of fabricating fiber holders have been compared to identify cost-effective solutions that deliver the desired fiber positioning accuracy. Femto-second laser-drilling methods from commercial vendors deliver holes arrayed on plates with a relative position accuracy of ±1.5-μm RMS, similar diameter accuracy, and with an aspect ratio of 1:10 (diameter:thickness). One commercial vendor combines femtosecond laser-drilling with photolithographic etching to produce plates with thickness of 5 mm, but with similar (±1-μm RMS) positioning accuracy and conical entry ports. Both of these techniques are found to be moderately expensive. A purely photolithographic technique performed at Wisconsin Center for Advanced Microelectronics (a facility at the University of Wisconsin, Madison), in tandem with deep reactive ion etching, has been used to produce a repeatable recipe with 100% yield. Photolithography is more precise (0.5-μm RMS) in terms of hole positioning and similar diameter accuracy (1-μm RMS) but the plate can only have a thickness of 250  μm.

We propose a concept of multiplexing lobster-eye (MuLE) optics to achieve significant reductions in the number of focal plane imagers in lobster-eye (LE) wide-field x-ray monitors. In the MuLE configuration, an LE mirror is divided into several segments and the x-rays reflected on each of these segments are focused on a single image sensor in a multiplexed configuration. If each LE segment assumes a different rotation angle, the azimuthal rotation angle of a cross-like image reconstructed from a point source by the LE optics identifies the specific segment that focuses the x-rays on the imager. With a focal length of 30 cm and LE segments with areas of 10  ×  10  cm², ∼1  sr of the sky can be covered with 36 LE segments and only four imagers (with total areas of 10  ×  10  cm²). A ray tracing simulation was performed to evaluate the nine-segment MuLE configuration. The simulation showed that the flux (0.5 to 2 keV) associated with the 5σ detection limit was ∼2  ×  10^−  10  erg cm^−  2 s^−  1 (10 mCrab) for a transient with a duration of 100 s. The simulation also showed that the direction of the transient for flux in the range of 14 to 17 mCrab at 0.6 keV was determined correctly with a 99.7% confidence limit. We conclude that the MuLE configuration can become an effective on-board device for small satellites for future x-ray wide-field transient monitoring.

As the precision frontier of large-area survey astrophysics advances toward the one millimagnitude level, flux calibration of astronomical instrumentation remains an ongoing challenge. We describe initial testing of silicon solar cells (SCs) as large-aperture precise calibration photodiodes. We present measurements of dark current, linearity, frequency response, spatial response uniformity, and noise characteristics of the Sunpower C60 SCs, an interdigitated back-contact 125  ×  125  mm² monocrystalline SC. We find that these devices may hold promise as large-area flux calibration sensors and that further analyses over a broader range of operating conditions are necessary.

The High-Resolution Energetic X-ray Imager (HREXI) cadmium zinc telluride (CZT) detector development program at Harvard is aimed at developing tiled arrays of finely pixelated CZT detectors for use in wide-field coded aperture 3 to 200 keV x-ray telescopes. A pixel size of ∼600  μm has already been achieved in the ProtoEXIST2 (P2) detector plane with CZT readout by the NuSTAR application-specific integrated circuits. This paves the way for even smaller than 300-μm pixels in the next-generation HREXI detectors. We describe a new HREXI calibration facility (HCF) that enables a high-resolution subpixel-level (100  μm) two-dimensional (2D) scan of a 256-cm² tiled array of 2  ×  2  cm² CZT detectors illuminated by a bright x-ray AmpTek Mini-X tube source at timescales of around a day. HCF is a significant improvement from the previous apparatus used for scanning these detectors, which took ∼3 weeks to complete a one-dimensional (1D) scan of a similar detector plane. Moreover, HCF has the capability to scan a large tiled array of CZT detectors (32  ×  32  cm²) at 100-μm resolution in the 10- to 50-keV energy range, which was not possible previously. We describe the design, construction, and implementation of HCF for the calibration of the P2 detector plane.

We present an algorithm, effective over a broad range of planet populations and instruments, for optimizing integration times of an exoplanet direct imaging observation schedule, to maximize the number of unique exoplanet detections under realistic mission constraints. Our planning process uses “completeness” as a reward metric and the nonlinear combination of optimal integration time per target and constant overhead time per target as a cost metric constrained by a total mission time. We validate our planned target list and integration times for a specific telescope by running a Monte Carlo of full mission simulations using EXOSIMS, a code base for simulating telescope survey missions. These simulations encapsulate dynamic details such as time-varying local zodiacal light for each star, planet keep-out regions, exoplanet positions, and strict enforcement of observatory use over time. We test our methods on the Wide-Field Infrared Survey Telescope (WFIRST) coronagraphic instrument (CGI). We find that planet, Sun, and solar panel keep-out regions limit some target per-annum visibility to <28  %   and that the mean local zodiacal light flux for optimally scheduled observations is 22.79 mag arcsec^−  2. Both these values are more pessimistic than previous approximations and impact the simulated mission yield. We find that the WFIRST CGI detects 5.48  ±  0.17 and 16.26  ±  0.51 exoplanets, on average, when observing two different planet populations based on Kepler Q1-Q6 data and the full Kepler data release, respectively. Optimizing our planned observations using completeness derived from the more pessimistic planet population (in terms of overall planet occurrence rates) results in a more robust yield than optimization based on the more optimistic planet population. We also find optimization based on the more pessimistic population results in more small planet detections than optimization with the more optimistic population.

There is increasing demand for space telescopes with ultralarge apertures in the order of 100 to 1000 m for space exploration. It is difficult to expand a space telescope beyond 100 m because of the extensive structural controls that are needed to satisfy optical interference. Large apertures can be achieved using classical formation flying satellites, but it is difficult to achieve suitable relative position and altitude control. Therefore, a novel composite formation flying strategy for distributed space telescopes that provides an ultralarge aperture and ultrahigh accuracy is presented. The proposed strategy combines coarse and fine formations (FFs) provided by support and payload modules (PMs), respectively. The support and PMs are actively connected by noncontact voice coil actuators (VCAs) in each formation satellite. A model and a VCA collision avoidance control approach are also provided for the proposed satellites. The coarse formation is controlled by small thrusters and gives the relative position accuracy in the order of centimeters. The FF is controlled by the VCAs and gives the relative position accuracy in the order of micrometers. The performance is evaluated using simulation studies.

An innovative model of a static spectropolarimeter able to cover the entire Stokes vector is discussed. The optical layout is based on a birefringent modulator formed by two antiparallel prisms stuck together with the help of an intermediary part of the same material. This optical model has the advantage of being extremely compact. It avoids any movable parts or rotating components. By its architecture, the device induces a complete modulation on the vertical direction of any incoming polarized light, facilitating the determination of the entire Stokes vector through a single measurement. Because the modulation is also wavelength-dependent, spectral dependencies of the polarization states can be derived. The behavior of the model was first investigated in noise-free conditions. The existence of a unique solution was proven in the absence of noise and for any Stokes vector configuration. Under noisy conditions, the uncertainty on the Stokes parameters and the efficiency of the modulation scheme were evaluated as a function of the analyzer’s angle and for two different configurations of the modulator. The simulations show that an almost ideal efficiency is reachable, qualifying the concept for the high-precision measurement of the polarization.

Evidence of “fringing” due to optical etaloning was observed in narrowband hyperspectral image cubes of Jupiter collected prior to 2018 at the Apache Point Observatory 3.5-m telescope with the New Mexico State University Acousto-optic Imaging Camera. The etaloning resulted from the use of a back-illuminated, high quantum efficiency CCD. Otherwise successful flat-field correction was ineffective in removing fringes at some wavelengths associated with Jupiter’s absorption regions. We describe an etaloning correction method based on a mathematical interference model that assumes a single detection layer. A two-dimensional thickness function for the sensor layer was derived and found to have an overall “dish-shaped” variation along with some finely spaced surface polishing marks. Synthetic fringe frames corresponding to the flat-field and science images were created using the thickness function. Optimized contrast values were found for the synthetic frames and defringed images of Jupiter were generated by separately correcting flat-field and science images using the synthetic fringe frames before applying the final flat-field division. Quantitative analyses of defringed flat-field images showed fringe contrast reductions by factors of 2 to 8 on average and a disk-averaged spectrum of defringed Jupiter data compared favorably with an established spectrum. This defringing approach is applicable to other detectors that can be modeled with a single detection layer and where a sequence of spectral images with adequate wavelength resolution is available.

Several starshade concepts for imaging exo-Earths would operate at the second Earth–Sun Lagrange point (L2) and consist of a starshade flying in formation tens to hundreds of thousands of kilometers from a telescope. The starshade would need to maintain meter-level lateral alignment with the line of sight from telescope to target star. A companion paper describes an optical sensing scheme using a pupil imaging camera in the telescope that can sense the relative lateral position to a few centimeters. A full flight-traceable formation flying framework that leverages this sensor is presented. In particular, a two-dimensional “disk deadbanding” algorithm is introduced for lateral control. The framework also maximizes the drift time between thruster burns to reduce interruption to scientific observations. The main sources of uncertainty affecting the control performance are compared, and it is found that spacecraft mass uncertainty is a driving factor. The formation flying environment is also analyzed to identify conditions that lead to worst-case differential gravity and solar radiation pressure disturbances. Finally, for a representative observation scenario with the Wide Field Infrared Space Telescope, this control system is tested through Monte Carlo simulations. The results show robust meter-level control with essentially optimal drift time between thruster burns.

We propose an analytical nonlinear hysteresis model of the bimorph beam bending piezoelectric actuator for a deformable mirror system. An explicit solution is derived from the analytic method based on the nonlinear elasticity. As an application example, the nonlinear hysteresis behaviors of a bimorph beam bending piezoelectric actuator, which is sandwich compounds consisting of a lower and an upper piezoelectric ceramic (PZT8-50) layer and a middle layer made of the aluminum alloy, are examined with the model. The numerical calculations based on the analytical solution are presented. The numerical simulations have shown that the nonlinear hysteresis behaviors of the bending actuator are depended on the start values of the exciting electric field and the geometric parameters of the metallic layer. In addition, the numerical results have also shown that the thickness of metallic layer has little influence on the relative hysteresis, and the relationships between relative hysteresis Δd  /  ΔD and the driving electric field   |  E_start  |   are linear.

Segmented mirror space telescopes have many advantages in both observation capacity and engineering feasibility. However, the alignment procedures for them are particularly complicated. Meanwhile, global alignment is one of the most important steps, in which the misalignments of each segment should be determined and corrected before image stacking is performed. Therefore, segment-level wavefront sensing is needed in this process. At present, traditional iterative phase retrieval algorithm is used to recover the segmented-level wavefront phase. However, the efficiency of this algorithm is comparatively low, especially given that there is an array of segment-level wavefront maps that need to be recovered. In addition, the magnitudes of misalignments are comparatively large in this stage and the iterative phase retrieval algorithm can be trapped in a local minimum for large-scale wavefront sensing. An analytic approach is proposed to estimate the segment-level wavefront aberrations based on the analysis of the geometrical features of one defocused point spread function (PSF) image. Meanwhile, some aberration properties of the misaligned system are also utilized. Simulations and an experiment are performed to verify the effectiveness of the proposed approach. This work can not only improve the efficiency and robustness of the global alignment of segmented mirror space telescopes, but also provide an intuitive and in-depth understanding for the mechanism of aberration calculation using PSF image features.

Although adaptive optics (AO) systems have been developed to compensate for atmospheric turbulence, their application is mostly limited to large size (1.5 to 8 m) telescopes due to high cost and complexity. In particular, for small to medium aperture telescopes for which the need for AO correction is critical, AO systems have different requirements such as compactness and transportability. We report on the realization and the on-sky test of an extremely small adaptive optics module (XSAO) based on a multiactuator adaptive lens. Observations were performed on an 11-in. commercial telescope and a 1-m scientific telescope with minimal modifications. The results show a strong improvement of the acquired images for different types of observed objects, such as bright stars, double stars, planets, and deep sky objects.