Open-Loop Woofer-Tweeter Control on the LAO Multi-Conjugate Adaptive Optics Testbed

Abstract

Advances in micro deformable mirror (DM) technologies such as MEMs, have stimulated interest in the characteristics of systems that include a high stroke mirror in series with a high actuator count mirror. This arrangement is referred to as a woofer-tweeter system. In certain situations it may be desirable or necessary to operate the woofer DM in open-loop. We present a simple method for controlling a woofer DM in open loop provided the device behaves in an approximately linear fashion. We have tested a mirror that we believe meets our criterion, the ALPAO DM52 mirror. Using our open-loop method we fit several test Kolmogorov wavefronts with the mirror and have achieved an accuracy of approximately 25 nm rms surface deviation over the whole clear aperture, and 20 nm rms over 90% of the aperture. We have also flattened the mirror in open loop to approximately 11 nm rms residual.