A design methodology was developed that optimizes the performance of active mirrors. Patterns of parameterized ellipsoidal actuators are overlaid onto the mirror, and then numerically optimized to improve performance of the mirror in optical modes that are typically difficult to correct, while also improving performance in other optical modes.
The actuation patterns that have been established in this invention are novel. They have been demonstrated to perform better than mirrors with traditional actuation patterns, and more than twice the number of actuators. The actuation system consists of a pattern of electrodes printed on a continuous layer of piezoelectric material that is bonded to an optical-quality substrate. The electrodes provide almost full coverage of the piezoelectric layer in order to maximize the amount of active material available for actuation. Their shape is optimized to maximize the correctability and stroke of the mirror for a chosen number of independent actuators and for a dominant imperfection mode.
The starting point for the design of the electrodes is the observation that the correction of a figure error that has at least two planes of mirror symmetry is optimally done with twin actuators that have the same optimized shape, but are rotated with respect to each other. Additional sets of optimized twin actuators are defined within the intersection of the twin actuators, and an arbitrarily fine actuation pattern can be generated. It is shown that this approach leads to actuator systems that provide higher correctability than simple, geometrically based actuators with more than twice as many actuators. Several actuator patterns to correct third-order astigmatism aberrations have been assessed, including the results from an experimental demonstration of a 41-actuator mirror.
The mirrors created through this method are lightweight, relatively inexpensive, and provide a sufficiently large shape correction capability to allow the use of nominally identical mirror segments in large segmented apertures. Accurate shape control should also allow active compensation for thermal effects and long-term material effects such as creep and aging.