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The MMT will be converted to a single 6.5 m primary mirror in 1997. The new telescope will be equipped with adaptive optics for high-resolution infrared astronomy at wavelengths between 1.6 microns and 5 microns. Atmospheric distortions will be corrected using an adaptive secondary mirror which directly feeds a high-resolution infrared detector. The use of an adaptive secondary maximizes emissivity, which provides high throughput to the imaging detector and efficient wavefront sensing by eliminating many optical elements normally needed in Coudé systems.

The advantages of using an adaptive secondary mirror for astronomical high resolution observations are very attractive. The drastic reduction of optical surfaces along the optical train of the adaptive system will allow a very compact system with high transmission and low emissivity. Also such a system has the potentiality of a very high actuator density.

The thin faceplate adaptive secondary mirrors are controlled by means of electromagnetic force actuators, which seem to be a suitable choice in terms of stroke, accuracy and cost [Del Vecchio, Gallieni et al.; Biasi, Gallieni et al. in this Conference]. Those actuators need a local control loop to achieve a linear position response. The position sensor is definitely a crucial component of this control loop: both high sensitivity and high speed have to be achieved at same time.

The new 6.5 m single mirror Multiple Mirror Telescope (MMT) will be equipped with adaptive optics capabilities. In particular, high resolution infrared astronomy will use a new adaptive secondary on a regular basis. Before we design the adaptive secondary in detail, we fabricated a smaller prototype mirror using similar actuators and actuator geometry. The prototype's purpose is to debug the components and allow comparisons with theoretical predictions. We are studying system interactions and looking for unexpected performance characteristics. Manufacturing techniques and reliability are also being tested, to reduce risks when building the adaptive secondary.