Design and prototype tests of an adaptive secondary mirror for the new 6.5-m single-mirror MMT

In 1996, the Multiple Mirror Telescope will be replaced with a 6.5 m single primary mirror. Development is currently underway on a sodium laser guide star adaptive optical system for the new telescope. One unique feature will be an adaptive secondary mirror, consisting of 320 individually controlled voice coil actuators on the back side of a thin, 64 cm diameter mirror. This paper describes initial tests on a 15 cm diameter, thin, flat prototype mirror with a single actuator. The thin mirror is held near a thick substrate which also serves as an immovable reference surface. A novel voice coil actuator connects the two glass parts. A custom capacitance sensor surrounding the voice coil actuator measures the absolute distance between the mirror back side and the reference surface. This prototype provides detailed performance measurements, including temporal and spatial actuator response functions. Assembly, alignment, and calibration techniques for the 64 cm mirror will be debugged. The data will help optimize the design and performance of the adaptive secondary.