Transferring an Earth-based adaptive optics technology to space telescopes

The present work is aimed at studying the application of a contactless voice-coil actuation technology, currently developed for some new-generation earth telescopes, to an hypothetical secondary mirror of a space telescope subjected to nanometric precision requirements. The technology involves a large number of electromagnetic actuators which are not in contact with the optical mirror. Each actuator is made by a fixed coil wound on a cold finger and a moving magnet, glued to the rear surface of the thin adaptive mirror. A stiff backplate provides the position reference. The gap between the mirror and the backplate is measured through co-located capacitive sensors. Since future space telescopes will operate at cryogenics temperatures, where the material damping is extremely small, the design of high-performance and stable active shape controllers without detrimental spillover effects is challenging. Due to the impossibility to exploit aerodynamic damping arising from the squeezed air film between the mirror and the backplate, as done on earth-based mirrors, there is the need of finding an alternative to damp out vibrations. For this purpose, the current-driven technology implemented in earth-based applications is replaced by a voltage-driven solution, which provides damping augmentation by means of eddy currents. This is crucial in achieving a sufficiently stable dynamic response. The related limitation in the control bandwidth is fully compatible with the promptness requirements of the control system for the application under study.