The adaptive secondary mirror for the 6.5 conversion of the Multiple Mirror Telescope

Abstract

We present the laboratory results obtained with a non-conventional secondary mirror that will be used as corrector for the adaptive optics system of the 6.5m conversion of MMT. The design of the unit consists of a 2mm-thick 642mmdiameter convex deformable mirror whose figure is controlled by 336 electromagnetic force actuators, a thick shell used as position reference and an aluminum shell as actuator support and cooling. The actuator forces are controlled using feed-forward and de-centralized closed loop compensation thanks to the 40kHz feedback signals from the 336 capacitive position sensors. The digital real-time control and the unit monitoring is obtained using custom-made onboard electronics based on 168 DSPs. This paper will report on extensive static and dynamical measurements aimed at characterizing the mirror in terms of electric and optical response.