Analysis of Non-Uniform Gain for Control of a Deformable Mirror in an Adaptive-Optics System

Abstract

The Air Force Research Laboratory's (AFRL) Sodium Guidestar Adaptive Optics for Space Situational Awareness program (NGAS) sponsored research on spatially non-uniform gain for the servo-loop controller of an adaptive optics (AO) system. The edge subapertures of a Shack-Hartmann wavefront sensor have lower signal-to-noise ratios and are more susceptible to measurement errors than fully illuminated center subapertures. These measurement errors produce errant commands over the corresponding edge actuators and can induce instabilities over these regions in strong turbulence conditions. The Non-uniform Gain Experiment (NUGE) concentrated on the development and experimental analysis of spatially varying gain maps on the servo-loop controller of a deformable mirror. The goal was to improve AO system performance and mitigate instabilities that can occur over the edge actuators of a deformable mirror. A gain map with a ring of lower filter gains b over just the outer actuators was experimentally shown to increase the overall Strehl ratio of the AO system in all of the tested turbulence conditions. A Gaussian gain map was also shown to significantly reduce the overall residual phase variance over the edge actuators thereby reducing the formation of the instabilities. Experiments were conducted at the Starfire Optical Range (SOR), AFRL, Kirtland AFB.