Design and realization of ancillary control loops for the MMT adaptive optics system

The adaptive optics system of the Multiple Mirror Telescope is going to realize a high speed (1 kHz bandwidth) and high order (336 actuators) wavefront correction. However, to achieve the required 0.08 arcsec pointing stability the focal point of the Shack-Hartman wavefront sensor must be kept aligned to the Cassegrain focus better than 10 μm in spite of the non-common path tip/tilt error due to mechanical and thermal deformation of the telescope structure. The wave-front sensor must also be rotated with high precision to keep it aligned with the deformable secondary mirror in spite of the parallactic angle correction of the telescope. Our approach is to use a feed-forward loop to eliminate the adverse effect of deformation. A fast, deterministic field bus is applied to interconnect the actuators, sensors and computers. The bandwidth (500 kbs) and latency (less than 1 ms) of the DeviceNet serial bus is adequate to support our distributed control system. The field bus architecture simplifies and standardizes the control software as well as improves the reliability of the electronics by reducing the wiring.