The deformable mirror demonstration mission (DeMi) CubeSat: optomechanical design validation and laboratory calibration

The Deformable Mirror Demonstration Mission (DeMi) is a 6U CubeSat that will characterize the on-orbit performance of a Microelectromechanical Systems (MEMS) deformable mirror (DM) with both an image plane wavefront sensor and a Shack-Hartmann wavefront sensor (SHWFS). Coronagraphs on future space telescopes will require precise wavefront control to detect and characterize Earth-like exoplanets. High-actuator count MEMS deformable mirrors can provide wavefront control with low size, weight, and power. The DeMi payload will characterize the on-orbit performance of a 140 actuator MEMS Deformable Mirror (DM) with 5.5 μm maximum stroke, with a goal of measuring individual actuator wavefront displacement contributions to a precision of 12 nm. The payload will be able to measure low order aberrations to λ/10 accuracy and λ/50 precision, and will correct static and dynamic wavefront phase errors to less than 100 nm RMS. We present an overview of the payload design, the assembly, integration, and test process, and report on the development and validation of an optical diffraction model of the payload. Launch is planned for late 2019.

The next generation of extremely large telescopes (ELTs) that will be built on a scale of 30–50 meters in diameter necessarily require an increased ability to correct wavefront errors. Currently, wavefront correction is achieved through the use of a deformable mirror (DM). The degree and amount of correction depends on amechanical displacement of segments of themirror (stroke), and the number of actuators in the mirror surface (order). An effective ELTwill require a high-stroke (10microns) and high-order (100X100 actuators) mirror, a costly prospect using current technology.1 Lowering the cost while improving the performance of DMs is possible using microelectromechanical systems (MEMS) technology. However, current surface machining processes used to fabricate MEMS deformable mirrors, such as the Sandia ultra-planar multilevel MEMS technology (SUMMiT) and MEMSCAP’s PolyMUMPS process, limit a DM’s stroke due to the use of thin-film sacrificial layers (which are dissolved away after the etching process to free up micromechanical components in the actuators).2, 3 Commercially available surface micromachined MEMS mirrors are limited in stroke to approximately 5.5 microns, about half of the stroke required by ELTs. A high aspect ratio fabrication process capable of depositing thick layers will allow the DM’s to provide both high-stroke and highorder corrections, thus bypassing the need for a complex dual woofer (high stroke, low order)/tweeter (high order, low stroke) DM configuration.4 In our work, we tested different actuator designs with a bonded faceplate constructed using the LIGA (German acronym for lithography, electroplating, and molding) process, enabling multilayer fabrication of MEMS devices.5 Various types of high-stroke gold actuators consisting of folded springs with Figure 1. Diagram showing the final release of the monolithic fabricated structure created using our technique. The gap for the Xbeam electrostatic actuators is 20 microns, enabling a mirror stroke of 10 microns. WMS-15: specific glass-ceramic substrate. Not to scale.

For satisfying the boarder application requirement of adaptive optics (AO) and solving the problem of large volume and high cost of conventional deformable mirrors (DM), micro DM based on micro-electro-mechanical system (MEMS) technology is developed and measured. The developed DM has 140 hexagonal parallel plate capacitor electrostatic actuators. The actuators are arranged as a square array and the pitch is 400 μm. A DM prototype is fabricated by MEMS surface micromachining process and packaged by a ceramic pin grid array (CPGA). A miniaturization multi-channel high voltage driver for the DM is developed too. The measurement results show that the prototype has a surface PV value of 411 nm, RMS value of 78 nm, reflectivity of about 80% in 600 nm to 900 nm wavelength, stroke of 1.8 μm, actuator coupling of 15%, working bandwidth of 13 kHz and step response time of 23 μs. Thus the DM has the advantages of small volume, low cost and fast response. Besides the measurement of single element, the whole DM is controlled open loop to fit Zernike aberration and its fitting capability is demonstrated. Above results indicate that the DM prototype can satisfy initially the requirement of AO system.

We study a mid-order wavefront sensor (MOWFS) to address fine cophasing errors in exoplanet imaging with future large segmented aperture space telescopes. Observing Earth analogs around Sun-like stars requires contrasts down to 10−10 in visible light. One promising solution consists of producing a high-contrast dark zone in the image of an observed star. In a space observatory, this dark region will be altered by several effects, and among them, the small misalignments of the telescope mirror segments due to fine thermo-mechanical drifts. To correct for these errors in real time, we investigate a wavefront control loop based on a MOWFS with a Zernike sensor. Such a MOWFS was installed on the high-contrast imager for complex aperture telescopes (HiCAT) testbed in Baltimore in June 2023. The bench uses a 37-segment Iris-AO deformable mirror to mimic telescope segmentation and some wavefront control strategies to produce a dark zone with such an aperture. In this contribution, we first use the MOWFS to characterize the Iris-AO segment discretization steps. For the central segment, we find a minimal step of 125 ±31 pm. This result will help us to assess the contribution of the Iris-AO DM on the contrast in HiCAT. We then determine the detection limits of the MOWFS, estimating wavefront error amplitudes of 119 and 102 pm for 10 s and 1 min exposure time with a SNR of 3. These values inform us about the measurement capabilities of our wavefront sensor on the testbed. These preliminary results will be useful to provide insights on metrology and stability for exo-Earth observations with the Habitable Worlds Observatory.

Adaptive optics (AO) applications in astronomy and vision science require deformable mirrors (DMs) with high-stroke, high-order packing density at a lower cost than the currently available technology. The required AO specifications are achievable with microelectromechanical systems (MEMS) devices fabricated with LIGA (lithographie galvanofomung abformung) high-aspect-ratio processing techniques. Different actuator designs and a bonded faceplate fabricated in a LIGA process, enabling multilayer fabrication of MEMS devices, are investigated. Various types of high-stroke gold actuators for AO consisting of folded springs with rectangular and circular membranes as well as x-beam actuators supported diagonally by fixed-guided springs are designed, simulated, and fabricated individually and as part of a continuous-face-sheet DM system. The actuators and DM displacement versus voltage are measured with an interferometer and the corresponding results are compared to finite element analysis simulations. Simulations and interferometer scans show the ability of the actuators to achieve displacements of greater than 1/3 of the initial gap. A stroke of ~9.4 µm is achieved, thus showing that this fabrication process holds promise in the manufacture of future MEMS DMs for the next generation of extremely large telescopes.

Since atmosphere turbulence has an effect on laser propagation, it causes wavefront error usually , changes intensity and coherence of laser, disturbs detection of lidar. The adaptive optical system has broad application in the field of laser transmission because it can adjust characters of optical system ,detect and correct the wavefront error at the same time. Adaptive optics technology uses deformable mirrors to perform dynamic phase modulation and endow optical system the ability to decrease the influence of dynamic wavefront errors. In this paper ,a correction method of the micro-miniature adaptive optical system based on Micro Electromechanical System (MEMS) technology is proposed by analyzing the working theory of the adaptive optical system. An experimental system including deformable mirror based on Micro Electromechanical System (MEMS) technology is designed to correct a factitious wavefront error.The influence function and voltage-deflection curve are researched, and the voltage control matrix is educed. By using the voltage control , the static wavefront aberration is corrected. Several important capabilities of deformable mirrors is tested. With the voltage control matrix, the corrected capability of the adaptive optical system is achieved successfully .The experimental results show that the adaptive optical system can preferably correct the wavefront error, that has small volume and steady capability, and greatly improve the capability of lidar detection.

High average power operation of the Mercury Laser induces dynamic aberrations to the laser beam wavefront. Analysis of recent data indicates that up to 4 waves of low order aberration (mainly focus error or power, with spatial resolution < 0.5 cm{sup -1}) could be expected at each pass. Because of the magnitude of the wavefront error, the logical position is to place a deformable mirror (DM) at the M11 position, where the DM will correct the beam between passes 1 & 2 and 3 & 4. Currently, there are only two established commercial vendors offering complete adaptive optic (AO) systems that can accommodate the Mercury beam size (45 x 75 mm) which are compatible with high damage threshold coatings. Xinetics (MA, USA) offers a complete AO system along with a Shack-Hartmann wavefront sensor. The Xinetics DM is based on lead magnesium niobate (PMN) technology. A number of US aerospace firms as well as NIF use Xinetics PMN technology for their DMs. Phasics (Paris, France) offers a complete AO solution with its proprietary SID-4, a four-way shearing interferometric wavefront sensor capable of high resolution (over 100 x 100 sampling points). The Phasics system includes a bimorph deformable mirror from Night-n-Opt (Moscow, Russia) that uses lead zirconate titanate (PZT) technology. Various high power laser laboratories around the world such as LULI (France), HELEN (UK), and GEKKO (Japan) are using the PZT-based bimorph DM in their system. While both DM technologies are equivalent and have been deployed in high-energy laser systems, the PZT based bimorph DM offers two distinct features that makes it more attractive for high average power laser systems. The bimorph DM uses two layers of PZT actuators with the outer layer acting as power correctors, capable of correcting up to 20 waves of power. The Xinetics DM offers a maximum stroke of 4 waves. In addition, Night-N-Opt has also designed a water-cooled DM with a silicon based substrate (as opposed to a glass substrate) specifically for high average power laser systems--an option that is currently not available for PMN based DMs.

We present a variant of the model introduced by Vogel and Yang [J. Opt. Soc. Am. A23, 1074 (2006)] for point-actuated deformable mirrors (DMs) with continuous facesheets, and we describe a robust efficient regularized- output least-squares computational scheme to estimate the parameters in the model, given noisy discrete observations of the DM response to known actuation. We demonstrate the effectiveness of this approach with experimental data obtained from a pair of DMs--a piezo-actuated prototype DM built by CILAS for the Thirty Meter Telescope Project and an electrostatically actuated commercial micro-electro-mechanical systems (MEMS) DM produced by Boston Micromachines Corporation.

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

NASA’s Laser Communication Relay Demonstration (LCRD) aims to demonstrate a geosynchronous satellite laser communications (lasercom) relay between two independent ground terminals. We report on the design of two adaptive optics (AO) techniques for LCRD Ground Station #2 (GS-2). GS-2 leverages the ground terminal developed for NASA’s Lunar Laser Communications Demonstration (LLCD). Equipping GS-2’s 40cm diameter receive telescope with AO to mitigate atmospheric turbulence effects will enable the use of single mode, optically preamplified receivers for high data-rate near-Earth relay applications. In this work a direct wavefront sensing AO approach using a Shack-Hartmann sensor and a continuous facesheet micro-electro-mechanical system (MEMS) deformable mirror (DM) was compared with an indirect sensing, hill-climbing or multidither approach using a segmented MEMS DM. Design concepts and recent experimental progress for the two approaches are presented.

Future space telescopes may use large diameter (20-meter plus), ∞exible, lightweight membrane-like optical quality mirrors to overcome launch weight and packaging limitations. In-plane actuators, such as piezoelectric elements embedded within the mirror’s structure, could be used to actively shape these deformable mirrors. Thus, the active deformable mirrors could form commanded conjugate surfaces necessary to remove atmospheric distortions in an incoming wavefront, as well as countering any manufacturing imperfections or space-atmospheric disturbances. A stepping stone to realizing these large scale structures is the creation of laboratory test articles and investigating their behavior. To this end, a flve-inch in-plane actuated, tensioned, deformable mirror was constructed and evaluated for optical level responses. The research presented herein is a comparison of experimental data with advanced flnite element modelling techniques using MSC.Nastran software, and the implementation of control algorithms developed from the results of the flnite element modelling used to achieve optical precision quasi-static shaping of the experimental mirror’s surface.

This thesis presents a concept for ultra-lightweight deformable mirrors based on a thin substrate of optical surface quality coated with continuous active piezopolymer layers that provide modes of actuation and shape correction. This concept eliminates any kind of stiff backing structure for the mirror surface and exploits micro-fabrication technologies to provide a tight integration of the active materials into the mirror structure, to avoid actuator print-through effects. Proof-of-concept, 10-cm-diameter mirrors with a low areal density of about 0.5 kg/m² have been designed, built and tested to measure their shape-correction performance and verify the models used for design. The low cost manufacturing scheme uses replication techniques, and strives for minimizing residual stresses that deviate the optical figure from the master mandrel. It does not require precision tolerancing, is lightweight, and is therefore potentially scalable to larger diameters for use in large, modular space telescopes. Other potential applications for such a laminate could include ground-based mirrors for solar energy collection, adaptive optics for atmospheric turbulence, laser communications, and other shape control applications. The immediate application for these mirrors is for the Autonomous Assembly and Reconfiguration of a Space Telescope (AAReST) mission, which is a university mission under development by Caltech, the University of Surrey, and JPL. The design concept, fabrication methodology, material behaviors and measurements, mirror modeling, mounting and control electronics design, shape control experiments, predictive performance analysis, and remaining challenges are presented herein. The experiments have validated numerical models of the mirror, and the mirror models have been used within a model of the telescope in order to predict the optical performance. A demonstration of this mirror concept, along with other new telescope technologies, is planned to take place during the AAReST mission.

Adaptive optics for the next generation of extremely large telescopes (30 - 50 meter diameter primary mirrors) requires high-stroke (10 microns), high-order (100x100) deformable mirrors at lower-cost than current technology. Lowering the cost while improving the performance of deformable mirrors is possible using Micro-Electro-Mechanical Systems (MEMS) technology. In this paper the fabrication and testing of an array of high-stroke gold MEMS X-beam actuators attached to a continuous gold facesheet will be described. Both the actuator and the facesheet were fabricated monolithically in gold plated onto a thermally matched ceramic-glass substrate (WMS-15) using a high-aspect ratio fabrication process. Continuous facesheets that are deformed due to stress gradients have been annealed at high temperature and for an extended amount of time. The facesheet was flattened to the point where features such as etch holes and support post topography were easily distinguishable. Initial root-mean-square (RMS) topography at center of facesheet attached to a 16x16 X-beam actuator array with 1mm pitch was measured to be ~13.8&#956;m. After annealing, the surface topography was measured to be ~1.0&#956;m.

Future large aperture space telescopes may use lightweight correctable active mirrors. The Naval Postgraduate School's Segmented Mirror Telescope (SMT) test bed uses 1-meter silicon carbide (SiC) active mirror segments to form a sixsegment deployable 3-meter telescope. The active segments suffer from residual surface errors after a correction is applied. A deformable mirror is added at the SMT pupil plane to improve this residual error. The large active SMT segment represents the woofer, and a small continuous micro-electro-mechanical system (MEMS) deformable mirror represents the tweeter. A global influence matrix and closed loop constrained least squares controller command the active segment and additional deformable mirror as a single device. An interferometer measures the surface error and provides feedback to the controller. Simulation and experimental results demonstrate a significant improvement in wavefront error compared to a 2-step sequential woofer-tweeter constrained least squares control approach.

The purpose of this study was to evaluate the performance of a bimorph deformable mirror from AOptix, inserted into an adaptive optics system designed for in-vivo retinal imaging at high resolution. We wanted to determine its suitability as a wavefront corrector for vision science and ophthalmological instrumentation. We presented results obtained in a closed-loop system, and compared them with previous open-loop performance measurements. Our goal was to obtain precise wavefront reconstruction with rapid convergence of the control algorithm. The quality of the reconstruction was expressed in terms of root-mean-squared wavefront residual error (RMS), and number of frames required to perform compensation. Our instrument used a Hartmann-Shack sensor for the wavefront measurements. We also determined the precision and ability of the deformable mirror to compensate the most common types of aberrations present in the human eye (defocus, cylinder, astigmatism and coma), and the quality of its correction, in terms of maximum amplitude of the corrected wavefront. In addition to wavefront correction, we had also used the closed-loop system to generate an arbitrary aberration pattern by entering the desired Hartmann-Shack centroid locations as input to the AO controller. These centroid locations were computed in Matlab for a user-defined aberration pattern, allowing us to test the ability of the DM to generate and compensate for various aberrations. We conclude that this device, in combination with another DM based on Micro-Electro Mechanical Systems (MEMS) technology, may provide better compensation of the higher-order ocular wavefront aberrations of the human eye