Minimizing the wavefront error degradation for primary mirror segments with failed hexapod actuators

Abstract

The performance of a segmented space telescope depends in part upon the ability to maintain the alignment and phasing of its primary mirror segments. Failures of segment control actuators pose a threat to mission success, but their effects can be mitigated by using the remaining segment actuators to optimize the pose of each affected segment. This paper considers the effect of actuator failures on the final wavefront error of a segmented space telescope whose primary mirror consists of 18 hexagonal segments, each controlled by a 3-6 hexapod. Optimization algorithms that minimize the wavefront error for single- and multiple-failure cases are developed, and simulation results are presented. When one actuator fails, the affected segment can still attain a pose with zero wavefront error by exploiting the rotational symmetry of the primary. When two actuators fail, the resulting wavefront error depends upon which hexapod legs fail and at what lengths; cases where both legs of a bipod fail are an order of magnitude worse than other cases. Finally, Monte Carlo simulations of many failures randomly distributed across an initially well-phased segmented primary show that more than 10% of the actuators must fail before the root-mean-square wavefront error degrades significantly.