Performance of MEMS-based visible-light adaptive optics at Lick Observatory: closed- and open-loop control

Lower-limb exoskeletons are widely researched to improve walking performance and mobility. Low-level sensor-less exoskeleton motor control is attractive for consumer applications due to reduced device complexity and cost, but complex and variable transmission system configurations make the development of effective open-loop motor controllers that are responsive to user input challenging. The objective of this study was to develop and validate an open-loop motor control framework resulting in similar or greater performance vs. closed-loop torque control. We used generalized linear regression to develop two open-loop controllers by modeling motor current during exoskeleton-assisted walking; a "complex" model used desired torque and estimated ankle angular velocity as inputs, while a "simple" model used desired torque alone. Five participants walked at 1.0-1.3 m/s on a treadmill with closed-loop and both open-loop controllers providing ankle exoskeleton assistance. Both open-loop current controllers had similar root-mean-squared torque tracking error (p=0.23) compared to the closed-loop torque-feedback controller. Both open-loop controllers had improved relative average torque production (p<0.001 complex, p=0.022 simple), lower power consumption (p<0.001 for both), and reduced operating noise (p=0.002 complex, p<0.001 simple) over the closed-loop controller. New control models developed for a different ankle exoskeleton configuration showed similar improvements (lower torque error, greater average and peak torque production, lower power consumption) over closed-loop control during over-ground walking. These results demonstrate that our framework can produce open-loop motor controllers that match closed-loop control performance during exoskeleton operation.

at a 20 kHz frame-rate. 2,000 consecutive wavefronts of each of the two turret pointing angles measured were scaled to a tactical high energy laser (HEL) laser weapon scenario and their wavefront error statistics analyzed. The two wavefront sequences were decomposed using proper orthogonal decomposition (POD) modes and their spatial frequency content evaluated. From this, the minimum number of deformable mirror (DM) actuators needed to compensate these disturbances was estimated to be approximately 18 actuators across the aperture diameter. Finally, a the performance of a simple integrator-type adaptive optics (AO) control system against the two scaled wavefront sequences was investigated. It was found that the forward-looking aero-optics aberrations were about three-times less in magnitude than the rear-looking aero-optics aberrations. Consequently, the open-loop Strehl ratio of the forward-looking case was considerably greater than the open-loop Strehl ratio of the rear-looking case. Nevertheless, in for both the forwardlooking and rear-looking cases, in order to achieve compensation better than open-loop, it was found necessary to have AO sample rates greater than 10 kHz with one frame of latency. These results suggest that signicant AO compensation improvements will be achieved mainly by reducing the latency as much as possible, perhaps with adaptive/predictive AO controllers.

Most large telescopes are now implementing sodium laser guide star (LGS) adaptive optics (AO) systems. Most of these systems plan to use the Shack-Hartmann approach for wavefront sensing. In these systems, the laser spots that are imaged in the Shack-Hartmann subapertures suffer spot elongation due to the 10 km extent of the sodium layer. The spot elongation extends radially from the projection point, and increases linearly with the distance the subaperture is separated from the laser. For 8-meter class telescopes with laser projection behind the secondary mirror, the spot elongation is ~1 arc sec at the edge of the pupil, and does not significantly affect the performance of the AO system. However, for the coming generation of extremely large telescopes, sodium LGS spot elongation will significantly degrade the quality of wavefront measurement. Attention should now be given to the development of technologies that can reduce or eliminate the spot elongation problem. The laser spot elongation can be greatly reduced by projecting the sodium laser in a series of short (1-3 μsec) pulses. The Lawrence Livermore National Laboratory (LLNL) has been funded to develop a pulsed fiber laser. In parallel, a new kind of wavefront sensor detector must be developed to properly sense the pulsed laser return. In this paper, we present our project that will develop a novel CCD which is optimized for sensing the return from a pulsed sodium LGS. Our CCD design uses custom pixel morphology that aligns the pixels of each subaperture with the radial extension of the LGS spot. This pixel geometry will allow each subaperture to follow the yellow-orange rabbit (i.e. the 589 nm laser pulse) as it traverses the sodium layer, providing optimal sampling of a limited number of detected photons. This CCD will attain photon-noise limited performance at high frame rates, using MOSFET amplifiers that exist today (2-3 electrons noise). However, we seek even lower noise amplifiers, and as part of our project, we are testing a new generation of JFET amplifiers that may attain sub-electron noise performance. The test CCD will be a standard geometry, 160x160 pixel image area with split frame transfer and a total of 20 readout ports. This test CCD will easily surpass the performance of the CCDs presently in use in astronomical AO systems, and should provide a significant performance improvement in the AO systems of the 8-10 meter telescopes. This project is a collaboration between the Keck Observatory, MIT Lincoln Laboratory, SciMeasure Analytical Systems, Gemini Observatory, Lick Observatory, the University of California, and the Rockwell Scientific Company. Our efforts are being coordinated with the developments at LLNL so that the pulsed laser and novel geometry CCD can be mated together in 3 years when both are fully developed.

Current high-contrast "extreme" adaptive optics (ExAO) systems are partially limited by deformable mirror technology. Mirror requirements specify thousands of actuators, all of which must be functional within the clear aperture, and which give nanometer flatness yet micron stroke when operated in closed loop.¹ Micro-electrical mechanical-systems (MEMS) deformable mirrors have been shown to meet ExAO actuator yield, wavefront error, and cost considerations. This study presents the performance of Boston Micromachines' 1024-actuator continuous-facesheet MEMS deformable mirrors under tests for actuator stability, position repeatability, and practical operating stroke. To explore whether MEMS actuators are susceptible to temporal variation, a series of long-term stability experiments were conducted. Each actuator was held fixed and the motion over 40 minutes was measured. The median displacement of all the actuators tested was 0.08 nm surface, inclusive of system error. MEMS devices are also appealing for adaptive optics architectures based on open-loop correction. In experiments of actuator position repeatability, 100% of the tested actuators returned repeatedly to their starting point with a precision of < 1 nm surface. Finally, MEMS devices were tested for maximum stroke achieved under application of spatially varying one-dimensional sinusoids. Given a specified amplitude in voltage, the measured stroke was 1 &#956;m surface at the low spatial frequencies, decreasing to 0.2 &#956;m surface for the highest spatial frequency. Stroke varied somewhat linearly as inverse spatial frequency, with a flattening in the relation at the high spatial frequency end.

As already noticed in other telescopes, the presence of large telescope spiders and of a segmented deformable mirror in an Adaptive Optics system leads to pupil fragmentation and may create phase discontinuities. On the ELT telescope, a typical effect is the differential piston, where all disconnected areas of the pupil create their own piston, unseen locally but drastically degrading the final image quality. The poor sensitivity of the Pyramid WFS to differential piston will lead to these modes been badly seen and therefore badly controlled by the adaptive optics (AO) loop. In close loop operation, differential pistons between segments will start to appear and settle around integer values of the average sensing wavelength. These additional differential pistons are artificially injected by the adaptive optics control loop but do not have any real physical origin, contrary to the Low Wind Effect. In an attempt to reduce the impact of unwanted differential pistons that are injected by the AO loop, we propose a novel approach based on the pair-wise coupling of the actuators sitting on the edges of the deformable mirror segments. In this paper, we present the correction principle, its performance in nominal seeing condition, and its robustness relative to changing seeing conditions, wind speed and natural guide star magnitude. We show that the edge actuator coupling is a simple and robust solution and that the additional quadratic error relative to the reference case (i.e. no spiders) is of only 40 nm RMS, well within the requirements for HARMONI.

Astronomers everywhere were saddened by the death of Astronomer Emeritus J. H. Moore on March 15, 1949. Following a highly distinguished career, Dr. Moore had retired from the staff of the Lick Observatory on July 1, 1948. At the time of his death he was residing in Oakland. Though he had retired from his position as astronomer, this retirement did not extend to his work. He was completing work on several problems, the investigation of which started while he was at the Lick Observatory. The University as a whole and the members of the Lick Observatory staff in particular have many reasons to be grateful to Dr. Moore, not only for his valued services and counsel throughout his long career at the Observatory, but especially for the way in which, as director, he kept it operating successfully during the difficult war years. Dr. George Herbig was appointed junior astronomer as of July 1, 1948, but he was given leave of absence for one year to permit him to accept a National Research Council fellowship. He spent the year partly at the Mount Wilson Observatory but mostly at the Yerkes and McDonald Observatories. In July 1949 Dr. Herbig returned to the Lick Observatory. Dr. Olin J. Eggen was in residence at the Lick Observatory from July 1, 1948, until June 30, 1949, as research associate on a contract with the Office of Naval Research. As of July 1, 1949, Dr. Eggen was appointed junior astronomer in the Lick Observatory. Professor Joel Stebbins, who in 1948 retired as director of the Washburn Observatory, University of Wisconsin, came to the Lick Observatory as Alexander F. Morrison Research Associate early in November 1948, serving in this capacity for two months. He was appointed research associate under contract with the Of-

We describe a project to enable remote observing on the Nickel 1-meter Telescope at Lick Observatory. The purpose was to increase the subscription rate and create more economical means for graduate- and undergraduate students to observe with this telescope. The Nickel Telescope resides in a 125 year old dome on Mount Hamilton. Remote observers may work from any of the University of California (UC) remote observing facilities that have been created to support remote work at both Keck Observatory and Lick Observatory. The project included hardware and software upgrades to enable computer control of all equipment that must be operated by the astronomer; a remote observing architecture that is closely modeled on UCO/Lick's work to implement remote observing between UC campuses and Keck Observatory; new policies to ensure safety of Observatory staff and equipment, while ensuring that the telescope subsystems would be suitably configured for remote use; and new software to enforce the safety-related policies. The results increased the subscription rate from a few nights per month to nearly full subscription, and has spurred the installation of remote observing sites at more UC campuses. Thanks to the increased automation and computer control, local observing has also benefitted and is more efficient. Remote observing is now being implemented for the Shane 3- meter telescope.

Modern bio-imaging techniques such as light-sheet, multiphoton and PALM/STORM are now aiming to image more complex biological samples at larger depth and therefore face larger-amplitude and more complex aberrations. We provide an analysis of key requirements driving optimal implementation of adaptive optics (AO) in microscopy, with a focus on wavefront modulators. We show that some specifications of wavefront modulators such as linearity, hysteresis or actuators performance &amp; layout can end up to better AO performance in microscopy systems, when specifically optimized for such use. We then provide design details and characterization results of a newly developed deformable mirror, and report on experimental images obtained from AO-enhanced microscopes based on the device, for several modalities such as light-sheet, multiphoton or super-resolution single molecule localization systems. Finally, we provide recommendations on how to define the right set of AO components, algorithms and overall method depending on modality, instrument and sample constraints.

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.

The forthcoming Extremely Large Telescopes, and the new generation of Extreme Adaptive Optics systems, carry on a boost in the number of actuators that makes the real-time correction of the atmospheric aberration computationally challenging. It is necessary to study new algorithms for performing Adaptive Optics at the required speed. Among the last generation algorithms that are being studied, the Fourier Transform Reconstructor (FTR) appears as a promising candidate. Its feasibility to be used for Single-Conjugate Adaptive Optics has been extensively proved by Poyneer et al.^[1] As part of the activities supported by the ELT Design Study (European Community's Framework Programme 6) we have studied the performance of this algorithm applied to the case of the European ELT, in two different cases: single-conjugate and ground-layer adaptive optics and we are studying different approaches to apply it to the more complex multi-conjugate case. The algorithm has been tested on ESO's OCTOPUS software, which simulates the atmosphere, the deformable mirror, the sensor and the closed-loop control. The performance has been compared with other algorithms as well as their response in the presence of noise and with various atmospheric conditions. The good results on performance and robustness, and the possibility of parallelizing the algorithm (shown by Rodr&#237;guez-Ramos and Marichal-Hern&#225;ndez) make it an excellent alternative to the typically used Matrix-Vector Multiply algorithm.

This study presents a survey on quantum control theory and applications from a control systems perspective. Some of the basic concepts and main developments (including open-loop control and closed-loop control) in quantum control theory are reviewed. In the area of open-loop quantum control, the paper surveys the notion of controllability for quantum systems and presents several control design strategies including optimal control, Lyapunov-based methodologies, variable structure control and quantum incoherent control. In the area of closed-loop quantum control, this study reviews closed-loop learning control and several important issues related to quantum feedback control including quantum filtering, feedback stabilisation, linear-quadratic-Gaussian control and robust quantum control.

Visualization of the retinal structure is crucial for understanding the pathophysiology of ophthalmic diseases, as well as for monitoring their course and treatment effects. Until recently, evaluation of the retina at the cellular level was only possible using histological methods, because the available retinal imaging technology had insufficient resolution due to aberrations caused by the optics of the eye. Adaptive optics (AO) technology improved the resolution of optical systems to 2 µm by correcting optical wave-front aberrations, thereby revolutionizing methods for studying eye structures in vivo. Within 25 years of its first application in ophthalmology, AO has been integrated into almost all existing retinal imaging devices, such as the fundus camera (FC), scanning laser ophthalmoscopy (SLO), and optical coherence tomography (OCT). Numerous studies have evaluated individual retinal structures, such as photoreceptors, blood vessels, nerve fibers, ganglion cells, lamina cribrosa, and trabeculum. AO technology has been applied in imaging structures in healthy eyes and in various ocular diseases. This article aims to review the roles of AO imaging in the diagnosis, management, and monitoring of age-related macular degeneration (AMD), diabetic retinopathy (DR), glaucoma, hypertensive retinopathy (HR), central serous chorioretinopathy (CSCR), and inherited retinal diseases (IRDs).

Aberration correction based on wavefront sensorless adaptive optics in membrane diffractive optical telescope

Stabilogram-diffusion analysis was used to examine how prolonged periods in microgravity affect the open-loop and closed-loop postural control mechanisms. It was hypothesized that following spaceflight: (1) the effective stochastic activity of the open-loop postural control schemes in astronauts is increased; (2) the effective stochastic activity and uncorrelated behavior, respectively, of the closed-loop postural control mechanisms in astronauts are increased; and (3) astronauts utilized open-loop postural controls schemes for shorter time intervals and smaller displacements. Four crew members and two alternates from the 14-day Spacelab Life Sciences 2 Mission were included in the study. Each subject was tested under eyes-open, quiet-standing conditions on multiple preflight and postflight days. The subjects' center-of-pressure trajectories were measured with a force platform and analyzed according to stabilogram-diffusion analysis. It was found that the effective stochastic activity of the open-loop postural control schemes in three of the four crew members was increased following spaceflight. This result is interpreted as an indication that there may be in-flight adaptations to higher-level descending postural control pathways, e.g., a postflight increase in the tonic activation of postural muscles. This change may also be the consequence of a compensatory (e.g., "stiffening") postural control strategy that is adopted by astronauts to account for general feeling of postflight unsteadiness. The crew members, as a group, did not exhibit any consistent preflight/postflight differences in the steady-state behavior of their closed-loop postural control mechanisms or in the functional interaction of their open-loop and closed-loop postural control mechanisms. These results are interpreted as indications that although there may be in-flight adaptations to the vestibular system and/or proprioceptive system, input from the visual system can compensate for such changes during undisturbed stance.

It is well known that using two or more deformable mirrors (DMs) can improve the compensation ability of an adaptive optics (AO) system. However, to keep the stability of an AO system, the correlation between the multiple DMs must be suppressed during the correction. In this paper, we proposed a slope-based decoupling algorithm to simultaneous control the multiple DMs. In order to examine the validity and practicality of this algorithm, a typical woofer-tweeter (W-T) AO system was set up. For the W-T system, a theory model was simulated and the results indicated in theory that the algorithm we presented can selectively make woofer and tweeter correct different spatial frequency aberration and suppress the cross coupling between the dual DMs. At the same time, the experimental results for the W-T AO system were consistent with the results of the simulation, which demonstrated in practice that this algorithm is practical for the AO system with dual DMs.