Modal trajectory generation for adaptive secondary mirrors in astronomical adaptive optics

The use of high speed adaptive secondary mirrors (AS) in cassegrain or gregorian telescopes shows high optical efficiency as well as the possibility to make Adaptive Optics (AO) available at all foci. After 4 years of extensive and successful use of a 336-actuator AS (MMT336) at the Multi Mirror Telescope (MMT), the need for faster control methods of the AS is arising. Recent wavefront sensors allow frame rates above 1kHz and the dynamically limiting part in the telescope's closed loop AO system is the AS. The development of two 672-actuator (LBT672) AS for the Large Binocular Telescope (LBT) underlines the need for accurate high speed control methods of AS systems with a large number of spatially distributed actuators in the near future. Currently, AS are controlled based on local position feedback for all actuators independently. Arising problems in this configuration are modedependent stiffness variations of the mirror shell, interacting actuators and the excitation of uncontrollable modal mirror modes in closed loop operation. Based on dynamic inversion of identified controllable modal eigenmodes of the deformable mirror shell we derive a feed-forward trajectory generator that excites only controllable modal mirror modes, compensates for the varying mirror stiffness, and the actuator interaction of the AS. Verified at the LBT672 prototype (P45), an experimental 45-actuator AS, we will show the benefits of modal feed-forward control including faster settling times and less overshoot for a setpoint change in closed loop operation.