The SOAR Telescope project has embarked on the development of a very high quality 4.2-meter diameter optical telescope to be sited on Cerro Pachon in Chile. The telescope will feature an image quality of 0.18 arc seconds, a moderate field of 11 arc minutes, a very large instrument payload capacity for as many as 9 hot instruments, and an Active Optical System optimized for the optical to near IR wavelengths. The active optical system features a 10 cm thick ULETM primary mirror supported by 120 electro- mechanical actuators for a highly correctable surface. the 0.6 meter diameter secondary is articulated by a hexapod for real time optical alignment. The 0.6-meter class tertiary will provide fast beam steering to compensate for atmospheric turbulence at 50 hertz and a turret for directing the light to either of two nasmyth or three-bent cassegrain ports. Both the secondary and tertiary are light- weighted by machining to achieve cost-effective low weight mirrors. This paper discusses the unique features of this development effort including many commercial products and software programs that enable its technical feasibility and high cost efficiency.
The influence function matrix of an adaptive optical system defines the mapping between commands to the deformable mirror actuators and the outputs of the wavefront sensor. The condition number of that matrix can be used to quantify the controllability and observability of the system. This condition number is a function of both deformable mirror parameters and wavefront sensor parameters. Its dependence on the grating period and the subaperture size for a shearing interferometer wavefront sensor is examined through simulation studies and useful design guidelines are developed.
Design of a wavefront sensing system for an adaptive optical system requires knowledge of the spatial content of the aberrations to be sensed. Rules of thumb using quantities such as the number of actuators across the diameter of a deformable mirror are often used, but these may not work well for system with irregular actuator configurations or significant inter-actuator coupling. A novel approach using Fourier transform analysis of a matrix decomposition of the deformable mirror influence function matrix is presented. This approach allows the calculation of the spatial content of the aberrations corrected by a deformable mirror with an arbitrary actuator configuration and the design of a wavefront sensor to support that deformable mirror.
KEYWORDS: Wavefronts, Actuators, Wavefront sensors, Sensors, Space sensors, Condition numbers, Control systems design, Deformable mirrors, Matrices, Control systems
A lightweight segmented adaptive optical telescope for spaceborne applications is described and details of a hardware demonstration program presented. This program demonstrates, at a 4-meter aperture, a configuration and technologies for large deployable imaging systems. Real-time sensing and control is achieved using a suite of sensors to continuously measure wavefront error and segment phasing. The resulting state vector is operated on by the control algorithms and the resultant optimization commands applied to precision actuators to correct the system wavefront. The demonstrated technologies are discussed, along with details of the space qualifiable hardware configuration. These technologies include: a shearing interferometer wavefront sensor, autonomous hierarchical control sequences, lightweight graphite composite structures, and large lightweight optics.
During 1988 - 1993, Itek Optical Systems developed software for a demonstration program of several critical technologies for adaptive optical systems. The software implements a variety of data acquisition and control functions for an adaptive telescope system with hundreds of degrees of freedom. The completed software contains approximately 250,000 lines of real- time Ada code and was developed using a tailored DoD-STD-2167A methodology. Testing of the optical system was successfully conducted in December 1993. The system-level requirements for this software, the design of the software, the development methodology, and some lessons learned are discussed.
The influence function of an active optical system defines the relationship between commands to deformable mirror actuators and the changes in wavefront error that result from those commands. Accurate knowledge of the influence function is critical for stable closed-loop operation of the system. A recent program at Itek Optical Systems used a 2.6-meter-diameter, ULE deformable mirror with 144 lead-magnesium-niobate electrostrictive actuators. The influence functions of this mirror were measured under operational conditions over a period of fourteen months and no significant changes in the influence functions were found over that time. This is a very encouraging result for future applications of active optical systems where stable, closed-loop operation is required for many months between remeasurement of influence functions.
Active optical systems are complex systems that may be expected to operate in hostile environments such as space. The ability of such a system either to tolerate failures of components or to reconfigure to accommodate failed components could significantly increase the useful lifetime of the system. Active optical systems often contain hundreds of actuators and sensor channels but have an inherent redundancy, i.e., more actuators or sensor channels than the minimum needed to achieve the required performance. A failure detection and isolation system can be used to find and accommodate failures. One type of failure is the failure of an actuator. The effect of actuator failure on the ability of a deformable mirror to correct aberrations is analyzed using a finite-element model of the deformable mirror, and a general analytical procedure for determining the effect of actuator failures on system performance is given. The application of model-based failure detection, isolation and identification algorithms to active optical systems is outlined.
A correctability model of a large deformable mirror in a center-of-curvature test configuration is developed using finite-element techniques. The mirror is a one-meter diameter, solid ULE facesheet with electrostrictive actuators. The correctability model allows simulation of the performance of the active optical system, as measured by the wavefront error (optical path difference) over the aperture, in response either to commands to the actuators or to noise/disturbance processes. The performance of the closed-loop active optical system, assuming interferometric measurement of the wavefront error, is analyzed as a function of the number of actuators, the breadth of the actuator influence functions, and the spatial content of the disturbance aberration. The measures of performance used are: the correctability ratio, the required actuator stroke and the forces applied to the mirror.
A large deformable mirror with 144 actuators was tested in a center-of-curvature configuration. The influence function of an actuator, i.e., the change in wavefront error produced by a unit command to that actuator, was measured interferometrically and compared to predictions made by a finite-element model of the mirror. Good agreement between the measurement and the prediction in both magnitude and shape was found, serving to validate the finite-element model. The measurement process, the finite-element model, error sources, and the dependence of the influence function on the electrostrictive actuator response are discussed.
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