The National Aeronautics and Space Administration’s (NASA) first dedicated exoplanetary spectroscopy mission, the Colorado Ultraviolet Transit Experiment (CUTE), is used to search for signatures of atmospheric escape, the process by which constituent gases depart a planetary atmosphere. Through transit spectroscopy, the signs of escape driven by the high level of ultraviolet (UV) radiation from their parent stars are detectable around close-in planets. CUTE is a 6U CubeSat developed and operated by the Laboratory for Atmospheric and Space Physics (LASP) of the University of Colorado in Boulder, Colorado, United States; it looks for these signs of escape by surveying close-in extrasolar planets in the near-UV (2479 to 3306 Å) with 208×84 mm Cassegrain telescope-fed, UV-enhanced charged coupled device. Funded through a NASA ROSES proposal in 2017 and forced to deal with a worldwide pandemic during the heart of its fabrication and test program, CUTE has demonstrated the capability of small satellites to launch on schedule and perform challenging astronomical measurements. We will highlight the CUTE mission’s science objectives, implementation, and tribulations on its road to delivering a successful science program while discussing lessons learned pertaining to the development of CubeSat programs and the application of those lessons for a CUTE-style follow-on mission in the future.

Electroforming replication technology at the Marshall Space Flight Center has a long heritage of producing high-quality, full-shell X-ray mirrors for various applications. Nickel alloys are electroformed onto a super-polished mandrel in the electroforming process and then separated to form the replicated full-shell optic. Various parameters in the electroplating configuration could result in the non-uniformity of the shell’s thickness. Thickness non-uniformities primarily occur due to the non-uniform electric field distribution in the electroforming tank during deposition. Using COMSOL Multiphysics simulations, we studied the electric field distributions during the deposition process. Using these studies, we optimized the electric field distribution and strength inside the tank using customized shields and insulating gaskets on the mandrel. These efforts reduced the thickness non-uniformity from over 20% to under 5%. Improving the thickness uniformity of the shell aids in better mounting and aligning shells in the optics module. Optimization of the electroforming process, in some cases, improved the optical performance of the shells. Using finite element modeling, we estimated the effect of electroforming stress on the figure errors of the replicated optics. We observed that the electroforming stress predominantly affects the figure toward the ends of the optics. We presented COMSOL optimization of the electroforming process and the experimental results validating these simulations. We also discuss modeling experimental results of the replication figure errors due to electroforming stresses.

The mirror repositioning system is one critical system in large-size deployable space telescopes that aids in correcting errors in mirror orientation once deployed. Stewart mechanism is employed for reorienting the mirror due to its potential for use in high-precision applications, and a high-range and high-accuracy Stewart platform for positioning the mirror was designed using dual-resolution actuators. System characterization is crucial for understanding, optimizing, and evaluating the performance of a system. It provides insight into a system’s behavior, strengths, weaknesses, and limitations, aiding in troubleshooting, design decisions, and quality assurance. Overall, it forms the foundation for ensuring the functionality, efficiency, and reliability of a system throughout its lifecycle. We discuss the techniques adopted for characterizing the mirror repositioning system and the methods employed for error reduction in the system.

Space observatories utilizing micro pore optics (MPOs) have been used and are planned for several future X-ray astronomy space missions. The optical systems are designed to facilitate the focusing of incoming photons onto the focal plane of telescopes. Unfortunately, as well as having a small solid angle “open” to the sky, MPOs also have the unintentional effect of focusing high-energy particles from the space radiation environment. This causes additional radiation damage to mission-critical imaging sensors with solar energetic particles being particularly focusable. Typically, processes such as sectoral analysis are used to estimate the predicted dose to components, which is a ray tracing approach, and does not include focusing effects. We investigated focused dose estimation techniques for MPOs using Monte Carlo (MC) simulations. The focused dose contribution was compared with the unfocused contribution for the Solar wind Magnetosphere Ionosphere Link Explorer mission. The unfocused dose estimates were calculated using a traditional sectoral shielding analysis. The Monte Carlo-focused dose simulations enabled dose mapping over the image sensor to be analyzed. This revealed a relatively uniform dose across the device with some focusing artifacts present. The simulations also showed that the total ionizing dose and total non-ionizing dose decreased with depth into the sensor from the entrance window. This is key when considering that charge is often stored at varying depths in imaging devices across different technologies, for example, in front or back illuminated devices.

The National Aeronautics and Space Administration’s (NASA) Great Observatories Maturation Program (GOMAP) will advance the science definition, technology, and workforce needed for the Habitable Worlds Observatory (HWO) with the goal of a phase A start by the end of the current decade. GOMAP offers long-term cost and schedule savings compared with the “technology readiness level (TRL) 6 by preliminary design review” paradigm historically adopted by large NASA missions. Many of the key technologies in the development queue for HWO require the combined activities of (1) facility and process development for validation of technologies at the scale required for HWO and (2) deployment in the “real-world” environment of mission integration and test prior to on-orbit operations. We present a concept for the SmallSat Technology Accelerated Maturation Platform (STAMP), an integrated facility, laboratory, and instrument prototype development program that could be supported through the GOMAP framework and applied to any of NASA’s future Great Observatories (FGOs). This brief describes the recommendation for the first entrant into this program, “SmallSat Technology Accelerated Maturation Platform-1 (STAMP-1),” an ESPA Grande-class mission advancing key technologies to enable the ultraviolet capabilities of HWO. STAMP-1 would advance new broadband optical coatings, high-sensitivity ultraviolet detector systems, and multi-object target selection technology to TRL 6 with a flight demonstration. STAMP-1 advances HWO technology on an accelerated timescale, building on current research opportunities in space and earth sciences (ROSES) strategic astrophysics technology (SAT) + astrophysics research and analysis (APRA) programs, reducing cost and schedule risk for HWO while conducting a compelling program of preparatory science and workforce development with direct benefits for HWO mission implementation in the 2030s.

The primary goal of the CubeSpec mission is to show the feasibility of high-resolution optical astronomical spectroscopy from a small and cheap space platform. In its showcase mission, this 12U CubeSat space telescope will observe a time series of spectra of multiple β-Cephei pulsators. The inner structure of these stars can be derived from the spectral line profile variations. The optical payload, composed of an off-axis Cassegrain telescope and compact optical bench with a spectrograph, occupies half of the available volume and measures ∼10×20×30 cm3. During the observations, the star should be imaged on the spectrograph entrance slit of 20×50 μm (2.6″×6.5″) for at least 80% of the time to reach the sensitivity required. This leads to strict pointing requirements for the mission that cannot be met with the attitude determination and control system (ADCS) architecture of CubeSats. Therefore, we implemented a second control loop, based on a high-precision pointing platform (HPPP) that relies on its own sensor and actuator to measure the pointing error to a few arcsec accuracy and apply the proper compensation. In contrast to the ADCS, the HPPP does not change the spacecraft attitude but changes the optical path within the payload to project the image of the star inside the spectrograph slit. This paper describes the mechanical design of the fine-steering mirror (FSM), one of the key components of the HPPP. The design fits within the tight volume constraints of CubeSats and CubeSpec in particular and satisfies all mechanical challenges defined by the system dynamics and launch conditions. The three degrees of freedom design has a peak-to-peak tip/tilt range larger than 7 mrad along two tip/tilt axes and a first structural mode at 810 Hz while meeting the dimensional constraints. Especially the combination of a large actuated mirror with the small dimensions of the mechanism is a step forward with respect to commercially available FSM mechanisms, which typically do not match the CubeSat volume constraints. The concept can be easily scaled to various applications with different sets of requirements at a reasonable cost using commercial-of-the-shelf components.

The Habitable Worlds Observatory (HWO) will seek to detect and characterize potentially Earth-like planets around other stars. To ensure that the mission achieves the Astro2020 Decadal’s recommended goal of 25 exoEarth candidates (EECs), we must take into account the probabilistic nature of exoplanet detections and provide a “science margin” to budget for astrophysical uncertainties with a reasonable level of confidence. We explore the probabilistic distributions of yields to be expected from a blind exoEarth survey conducted by such a mission. We identify and estimate the impact of all major known sources of astrophysical uncertainty on the EEC yield. As expected, η⊕ uncertainties dominate the uncertainty in EEC yield, but we show that sampling uncertainties inherent to a blind survey are another important source of uncertainty that should be budgeted for during mission design. We adopt the Large UV/Optical/IR Surveyor Design B (LUVOIR-B) as a baseline and modify the telescope diameter to estimate the science margin provided by a larger telescope. We then depart from the LUVOIR-B baseline design and identify six possible design changes that, when compiled, provide large gains in EEC yield and more than an order of magnitude reduction in exposure times for the highest priority targets. We conclude that a combination of telescope diameter increase and design improvements could provide robust exoplanet science margins for HWO.

According to the working principle of the telescope, we know that the telescope requires stray light from the system to reach the order of 10−10 of the output laser power. In this article, given the roughness of the M1 mirror of 3 Å and the roughness of the M2∼M4 mirror of 1.8 Å, through separate analysis of the four mirror surfaces, we found that M4 has the greatest impact on the backward stray light of the telescope, and as the angle of M4 incident light increases, the level of stray light in the system decreases; after adjusting the M4 incidence angle and considering only the roughness, the stray light level of the telescope system reaches 10−11 of the power of the outgoing laser, which meets the expected requirements. Subsequently, we calculated the impact of particle pollution on the stray light of the system, and based on our analysis results, we determined that the cleanliness level of the telescope testing and storage environment was better than 100. Then, we conducted surface defect calculations and obtained the surface defect requirements for M1 to M4, and it is concluded that as the scattering angle decreases, the main contribution of bidirectional reflectance distribution function (BRDF) changes from geometric optics to diffraction effects. Finally, we conducted actual measurements on the surface quality of the ultra-smooth mirror sample, and the measured BRDF value was substituted into the simulation analysis, resulting in a telescope stray light of 8.29×10−11, meeting the expected requirements.

Future space telescope coronagraph instruments hinge on the integration of high-performance masks and precise wavefront sensing and control techniques to create dark holes essential for exoplanet detection. Recent advancements in wavefront control algorithms might exhibit differing performances depending on the coronagraph used. This research investigates three model-free and model-based algorithms in conjunction with either a vector vortex coronagraph or a scalar vortex coronagraph under identical laboratory conditions: pairwise probing with electric field conjugation, the self-coherent camera with electric field conjugation, and implicit electric field conjugation. We present experimental results in narrowband and broadband light from the In-Air Coronagraph Testbed at the Jet Propulsion Laboratory. We find that model-free dark hole digging methods achieve broadband contrasts comparable to model-based methods, and we highlight the calibration costs of model-free methods compared with model-based approaches. This study also reports the first time that electric field conjugation with the self-coherent camera has been applied for simultaneous multi-subband correction with a field stop. This study compares the advantages and disadvantages of each of these wavefront sensing and control algorithms with respect to their potential for future space telescopes.

We present the integration of a new calibration system into the Faint Intergalactic-medium Redshifted Emission Balloon-2 (FIREBall-2), which added in-flight calibration capability for the recent September 2023 flight. This system is composed of a calibration source box containing zinc and deuterium lamp sources, focusing optics, electronics, sensors, and a fiber-fed calibration cap with an optical shutter mounted on the spectrograph tank. We discuss how the calibration cap is optimized to be evenly illuminated through non-sequential modeling for the near-UV (191 to 221 nm) for spectrograph slit mask position calibration, electron multiplying charged-coupled device (EMCCD) gain amplification verification, and wavelength calibration. Then, we present the pre-flight performance testing results of the calibration system and their implications for in-flight measurements. FIREBall-2 flew in 2023, but did not collect calibration data due to early termination of the flight.

The Suborbital Imaging Spectrograph for Transition region Irradiance from Nearby Exoplanet host stars (SISTINE) is a rocket-borne ultraviolet (UV) imaging spectrograph designed to probe the radiation environment of nearby stars. SISTINE operates over a bandpass of 98 to 127 and 130 to 158 nm, capturing a broad suite of emission lines tracing the full 104−105 K formation temperature range critical for reconstructing the full UV radiation field incident on planets orbiting solar-type stars. SISTINE serves as a platform for key technology developments for future ultraviolet observatories. SISTINE operates at moderate resolving power (R∼1500), while providing spectral imaging over an angular extent of ∼6′, with ∼2″ resolution at the slit center. The instrument is composed of an f/14 Cassegrain telescope that feeds a 2.1× magnifying spectrograph, utilizing a blazed holographically ruled diffraction grating and a powered fold mirror. Spectra are captured on a large format microchannel plate (MCP) detector consisting of two 113×42 mm segments each read out by a cross-delay line anode. Several novel technologies are employed in SISTINE to advance their technical maturity in support of future NASA UV/optical astronomy missions. These include enhanced aluminum lithium fluoride coatings (eLiF), atomic layer deposition (ALD) protective optical coatings, and ALD-processed large-format MCPs. SISTINE was launched a total of three times with two of the three launches successfully observing targets Procyon A and α Centauri A and B.

We summarize the current best polychromatic (∼10% to 20% bandwidth) contrast performance demonstrated in the laboratory by different starlight suppression approaches and systems designed to directly characterize exoplanets around nearby stars. We present results obtained by internal coronagraph and external starshade experimental testbeds using entrance apertures equivalent to off-axis or on-axis telescopes, either monolithic or segmented. For a given angular separation and spectral bandwidth, the performance of each starlight suppression system is characterized by the values of “raw” contrast (before image processing), off-axis (exoplanet) core throughput, and post-calibration contrast (the final 1-sigma detection limit of off-axis point sources, after image processing). Together, the first two parameters set the minimum exposure time required for observations of exoplanets at a given signal-to-noise, i.e., assuming perfect subtraction of background residuals down to the photon noise limit. In practice, residual starlight speckle fluctuations during the exposure will not be perfectly estimated nor subtracted, resulting in a finite post-calibrated contrast and exoplanet detection limit whatever the exposure time. To place the current laboratory results in the perspective of the future Habitable Worlds Observatory (HWO) mission, we simulate visible observations of a fiducial Earth/Sun twin system at 12 pc, assuming a 6 m (inscribed diameter) collecting aperture and a realistic end-to-end optical throughput. The exposure times required for broadband exo-Earth detection (20% bandwidth around λ=0.55 μm) and visible spectroscopic observations (R=70) are then computed assuming various levels of starlight suppression performance, including the values currently demonstrated in the laboratory. Using spectroscopic exposure time as a simple metric, our results point to key starlight suppression system design performance improvements and trades to be conducted in support of HWO’s exoplanet science capabilities. These trades may be explored via numerical studies, lab experiments, and high-contrast space-based observations and demonstrations.

The demonstrated performance and cost-effectiveness of complementary metal–oxide–semiconductor (CMOS) sensors make them a potentially attractive option for low-cost space-based X-ray observatories. We have previously reported on the performance of a commercially available backside-illuminated Sony IMX290LLR-C CMOS sensor and found it to offer X-ray spectral resolutions comparable to the charged coupled devices (CCDs) aboard Suzaku and Chandra and to have a sufficient radiation hardness for use in low Earth orbit. In this work, we report on the quantum efficiency (QE) of this sensor, an essential metric for modeling the sensitivity of an instrument as an X-ray detector. Using the Advanced Photon Source at Argonne National Laboratory, we measure the soft X-ray QE of this CMOS sensor to be 0.28±0.02 at a photon energy of 490.5 eV. This energy was chosen for its proximity to the astrophysically important O VII triplet emission lines (∼574 eV) studied by the HaloSat mission. Although not surpassing that of the back-illuminated CCDs aboard Suzaku and Chandra, this QE compares favorably to that of the front-illuminated CCDs aboard the same observatories and is competitive with that of the silicon drift detectors used aboard HaloSat, making it a strong candidate for use on future X-ray small satellite (SmallSat) missions.

CIS221-X is a prototype monolithic complementary metal-oxide-semiconductor (CMOS) image sensor, optimized for soft X-ray astronomy and developed for the proposed European Space Agency Transient High Energy Sky and Early Universe Surveyor (THESEUS) mission. One significant advantage of CMOS technology is its resistance to radiation damage. To assess this resistance, three backside-illuminated CIS221-X detectors have been irradiated with 10 MeV protons using the MC40 Cyclotron Facility at the University of Birmingham, United Kingdom. Each detector received 1/2, 1, and 2 THESEUS end-of-life proton fluences (6.65×108 p+/cm2). One had already been exposed to ionizing radiation [up to 59.04 krad total ionizing dose (TID)] during a previous radiation campaign. Using unirradiated readout electronics, the electro-optical performance of each device has been measured before and after proton irradiation. No significant change was observed in the readout noise and image lag. An increase in mean dark current was recorded, as was an increase in the number of hot pixels. The degradation of CIS221-X performance due to non-ionizing radiation effects is similar to that of comparable CMOS image sensors and has been attributed to an increase in the number of bulk silicon defects.

We present the results of a radiation test program for a 1-megapixel single-photon-counting and photon-number-resolving CMOS image sensor. The results include pre- and post-radiation values for dark current, voltage shift at the pixels’ output, read noise, quantum efficiency (QE), conversion gain, and photon counting ability. The Center for Detectors at the Rochester Institute of Technology exposed the sensor to a 50-krad(Si) dose of 60-MeV protons, equivalent to the dose absorbed over 10 11-year space missions at L2 with 1-cm aluminum shielding. The median dark current of the sensor increased from 0.00085 to 0.0085 e−/s/pix at 258 K and from 0.0075 to 0.075 e−/s/pix at 282 K. This is an increase of 2.0 fA/cm2/krad(Si) and 17.8 fA/cm2/krad(Si), respectively. Performance in other metrics remained constant: 0.34 e− median read noise, 85% peak QE at 490 nm, and photon number resolution. We report mostly total ionizing dose and displacement damage dose effects and compare the radiation tolerance of the device to the performance of state-of-the-art charge coupled devices and CMOS devices. The detector exhibits a comparable radiation tolerance to the expected tolerance of modern CMOS devices.

The fifth Sloan Digital Sky Survey Local Volume Mapper (LVM) is a wide-field integral field unit survey that uses an array of four 160 mm fixed telescopes with siderostats to minimize the number of moving parts. An individual telescope observes the science or calibration field independently and is synchronized with the science exposure. We developed the LVM Acquisition and Guiding Package (LVMAGP)-optimized telescope control software program for LVM observations, which can simultaneously control four focusers, three K-mirrors, one fiber selector, four mounts (siderostats), and seven guide cameras. This software is built on a hierarchical architecture and the SDSS framework and provides three key sequences: autofocus, field acquisition, and autoguide. We designed and fabricated a proto-model siderostat to test the telescope pointing model and LVMAGP software. The mirrors of the proto-model were designed as an isogrid open-back type, which reduced the weight by 46% and enabled reaching thermal equilibrium quickly. In addition, deflection due to bolting torque, self-gravity, and thermal deformation was simulated, and the maximum scatter of the pointing model induced by the tilt of optomechanics was predicted to be 4′.4, which can be compensated for by the field acquisition sequence. We performed a real sky test of LVMAGP with the proto-model siderostat and obtained field acquisition and autoguide accuracies of 0″.38 and 1″.5, respectively. It met all requirements except for the autoguide specification, which will be resolved by more precise alignment among the hardware components at Las Campanas Observatory.

As one of the backend modules aboard the China Space Station Telescope, the high-sensitivity terahertz detection module (HSTDM) needs to be rationally scheduled to conduct various observation tasks to fulfill and maximize its scientific goals. This is because HSTDM cannot operate simultaneously with other modules, and the observable time windows determined by constrained and changeable conditions are randomly distributed and limited; even worse, the total allocated time is estimated to account for less than 10% of the total in-orbit time. We develop a modified genetic algorithm (MGA) to better solve this problem. Compared with conventional genetics algorithm (CGA), the core uniqueness of this method are as follows: (1) reduce the search space of chromosomes by pre-calculating the observable time windows of observing objects; (2) accelerate the exploration and exploitation of chromosomes by a transformation process that reduces the chromosome length through recombination of non-zero valued genes, followed by increasing the initial population diversity through the proposed similarity avoidance based population generation method and then by adopting stochastic universal sampling and elitism selection combined parents selection method; and (3) design a compound fitness function that can simultaneously achieve three optimization criteria through evolution process. The effectiveness of the proposed method is validated on a simulated scenario, and performance comparisons with CGA suggest that MGA can generate more profitable solutions (as much as 46% improvement) in fewer (as much as 90% reduction) generations.

The advent of back-illuminated complementary metal–oxide–semiconductor (CMOS) sensors and their well-known advantages over charge-coupled devices make them an attractive technology for future X-ray missions. However, numerous challenges remain, including improving their depletion depth and identifying effective methods to calculate per-pixel gain conversion. We have tested a commercial Sony IMX290LLR CMOS sensor under X-ray light using an Fe55 radioactive source and collected X-ray photons for ∼15 consecutive days under stable conditions at regulated temperatures of 21°C and 26°C. At each temperature, the data set contained enough X-ray photons to produce one spectrum per pixel consisting only of single-pixel events. We determined the gain dispersion of its 2.1 million pixels using the peak fitting and the energy calibration via correlation (ECC) methods. We measured a gain dispersion of 0.4% at both temperatures and demonstrated the advantage of the ECC method in the case of spectra with low statistics. The energy resolution at 5.9 keV after the per-pixel gain correction is improved by ≳10 eV for single-pixel and all event spectra, with single-pixel event energy resolution reaching 123.6±0.2 eV, close to the Fano limit of silicon sensors at room temperature. Finally, our long data acquisition demonstrated the excellent stability of the detector over more than 30 days under a flux of 104 photons per second.

Time delay error is a significant error source in adaptive optics (AO) systems. It arises from the latency between sensing the wavefront and applying the correction. Predictive control algorithms reduce the time delay error, providing significant performance gains, especially for high-contrast imaging. However, the predictive controller’s performance depends on factors such as the wavefront sensor (WFS) type, the measurement noise level, the AO system’s geometry, and the atmospheric conditions. We study the limits of prediction under different imaging conditions through spatiotemporal Gaussian process models. The method provides a predictive reconstructor that is optimal in the least-squares sense, conditioned on the fixed times series of WFS data and our knowledge of the atmospheric conditions. We demonstrate that knowledge is power in predictive AO control. With a Shack–Hartmann sensor-based extreme AO instrument, perfect knowledge of the wind and atmospheric profile and exact frozen flow evolution lead to a reduction of the residual wavefront phase variance up to a factor of 3.5 compared with a non-predictive approach. If there is uncertainty in the profile or evolution models, the gain is more modest. Still, assuming that only effective wind speed is available (without direction) led to reductions in variance by a factor of ∼2.3. We also study the value of data for predictive filters by computing the experimental utility for different scenarios to answer questions such as how many past telemetry frames should the prediction filter consider and whether is it always most advantageous to use the most recent data. We show that within the scenarios considered, more data provide a consistent increase in prediction accuracy. Furthermore, we demonstrate that given a computational limitation on how many past frames, we can use an optimized selection of n past frames, which leads to a 10% to 15% additional improvement in root mean square over using the n latest consecutive frames of data.

The goal of deformable mirrors (DMs) is to correct aberrated optical wavefronts in spaceborne electro-optical (EO) payloads. It is used as part of an active/adaptive optics system. A continuous-surface, metal-based DM is highly reliable and less complex to assemble, has better stability of the active surface, is less expensive, and can be manufactured quickly. In addition, metal DM with actuation away from the active surface makes the overall configuration scalable. Continuing our previous work on deformable metal mirrors, this work presents the design, validation, and qualification of an aluminum DM using 25 piezoelectric actuators, which include an actuator in the center of the mirror, to improve the spherical aberration correction accuracy. The optomechanical design and analysis of the deformable mirror assembly (DMA) are also presented for performance and survival loads. Later, a qualification model (QM) was built with vacuum-compatible closed-loop piezoelectric actuators. The correction accuracy was demonstrated at the QM by correcting aberrations in the mirror itself. The QM was successfully tested in the space environment in the ThermoVac for operating temperature limits of 20°C±5°C and demonstrated survivability for storage temperature limits of 20°C±40°C. Likewise, the survivability of QM for launch environments such as sinusoidal and random vibration loads is demonstrated. The successful completion of all these tests has improved the maturity of this technology to the technology readiness level of 7 and is now ready to be configured for the appropriate spaceborne EO payload.

Several proposed future X-ray missions will require thin (≤0.5 mm thick) mirrors with precise surface figures to maintain high angular resolution (≤0.5 arcsec). To study methods of meeting these requirements, adjustable X-ray optics have been fabricated with thin-film piezoelectric actuators to perform figure correction. The fabrication and actuator performance for an adjustable X-ray mirror that forms a conical approximation to a Wolter-I telescope are reported. The individual responses of actuator cells were measured and shown to induce a figure change of 870 nm peak-to-valley on average. These measured responses were compared with predicted responses generated using a finite-element analysis algorithm. On average, the measured and predicted cell responses agreed to within 60 nm root mean square. A set of representative mirror distortions and the measured cell responses were used to simulate figure corrections and calculate the half-power diameter (HPD, single reflection at 1 keV) achieved. These simulations showed an improvement in 4.5 to 9 arcsec mirrors to 0.5 to 1.5 arcsec HPD. The disagreements between the predicted and measured cells’ performance in actuation and figure correction were attributed to a high spatial frequency metrology error and differences in mirror bonding considerations between the finite-element analysis model and the as-built mirror mount.

The Indian Institute of Astrophysics is developing a Multi-Conjugate Adaptive Optics system for the Kodaikanal Tower Telescope. In this context, we measured the daytime turbulence strength profile at the Kodaikanal Observatory. The first method based on wavefront sensor images, called solar differential image motion monitor+, was used to estimate the higher altitude turbulence up to a height of 5 to 6 km. The second method used balloon-borne temperature sensors to measure the near-Earth turbulence up to 350 m. We also carried out simulations to validate the performance of our system. We report the first-ever daytime turbulence strength profile measurements at the observatory. We identified the presence of a strong turbulence layer ∼3 km above the observatory. The measured near-Earth turbulence matches the trend that is expected from the model for a daytime component of turbulence and gives an integrated r0 of ∼4 cm at 500 nm. This is consistent with earlier seeing measurements. This shows that a low-cost setup with a small telescope and a simple array of temperature sensors can be used for estimating the turbulence strength profile at the site.