Final prototype design for the adaptive secondary mirror of the 6.5-m MMT

The upgraded 6.5 m MMT in Arizona will use an adaptive secondary to optimize performance in the near infrared spectral region. The secondary mirror is a 2 mm thick, 640 mm diameter Zerodur shell suspended only by a flexible center hub. Three hundred voice coil actuators installed in an aluminum reference surface deform the shell according to commands from a wavefront sensor. Capacitor position sensors surrounding each actuator provide feedback in an inner servo loop, much faster than the exterior wavefront sensor control bandwidth. A 60 actuator prototype, nearly identical to the final adaptive secondary size, has been built and is currently being tested.