Control Loop Analysis and Characterization for an Adaptive Optics Experiment on the Starfire 3.5m Telescope

Au experiment with a nasmyth adaptive optics system mounted on the SOR 3.5m telescope employs an adaptive optical imager. A 148-subaperture hexagonal lenslet array provides a Hartmann wavefront sensor for the 163 actuator mirror to which it is mated. Our analysis of the control loop of the adaptive optical system is edified, and the actual implementation is explained. Performance tradeoffs between various subaperture geometries, based on reconstructor noise gain and stability considerations, are presented. A fast algorithm for measuring Hartmann spot positions is combined with a computationally efficient reconstructor running on a pair of i860XP processors to achieve 1000 reconstructions per second. Moreover, the reconstructor can perform much of its processing on the fly while the frame is being transferred from the pupil plane camera, resulting in significantly reduced servo phase lag. We describe the control loop design, with particular emphasis on the creative use of hardware data paths.