Adaptive beam-combining mirror for the MMT

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Abstract
We present a novel, high performance, and economical design for tip-tilt mirrors. The two- dimensional tilt of each mirror is sensed through capacitive displacement sensors sensitive to approximately 1 nm rms out to 10 KHz. An analogue PID circuit utilizes this positional feedback to lock the mirrors to the commanded tilt via voice coil drivers. These mirrors achieve a 10 - 90% risetime of 3.0 ms and have a critically damped response at 100 - 150 Hz update rates. We have incorporated six of these tip-tilt mirrors to adaptively combine all six beams from the MMT. During first light of the instrument in April 1995, individual beams improved from 1.1 inch to 0.6 inch FWHM in 60 s exposures corrected with 60 Hz update rates.

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