Using 50-mm electrostatic membrane deformable mirror in astronomical adaptive optics

Membrane micro-machined deformable mirrors (MMDM) feature low cost, low power consumption, small size and absence of hysteresis. Interested in using such a device for the adaptive optics system at the SOAR 4.1-m telescope, we evaluated the performance of a 79-channel 50-mm (pupil size 35mm) MMDM from OKO Technologies. The measured influence functions match the solutions of the Poisson equation with a fixed boundary. The maximum achievable modulation of the local radius of curvature of the mirror is +-25 m. Simulations of the AO system with this MMDM show that the curvature saturation becomes apparent even at a median seeing of 0.67". A new control algorithm that re-distributes the signals of saturated electrodes to adjacent ones significantly improves compensation under conditions of partial saturation. Limited curvature range and pupil diameter restrict the suitability of MMDM technology to moderate order AO systems at telescopes with diameter of up to 3m.