Progress in Developing a Low-Cost Large Deformable Mirror

Large (> 1m) deformable mirrors with hundreds or thousands of actuators are attractive for extremely large telescopes. Use of force actuators coupled to the mirror via suction cups, and electret microphones for position sensing, has the potential of substantially reducing costs. However, a mirror controlled with force actuators will have many structural resonances within the desired system bandwidth, shifting the emphasis somewhat of the control aspects. Local velocity and position loop for each actuator can add significant damping, but gives poor performance at high spatial frequencies. We therefore introduce a novel control strategy with many parallel "actuator families", each controlled by single-input-single-output controllers. This family approach provides performance close to that of global control, but without the accompanying robustness challenges. Using a complete simulation model of a representative large deformable mirror, we demonstrate feasibility of the approach. This paper describes the challenges of non-ideal actuators and sensors. The results presented give an understanding of the required actuator bandwidth and the effects of the sensors dynamics. The conclusion is that the introduction of actuator and sensor dynamics does not limit the control system of the deformable mirror.