Active lateral support system for a thin 1-meter meniscus mirror with virtual fixed points

Abstract

This publication presents an active lateral support system for the thin 1-m meniscus mirror of a Ritchey-Chrétien telescope system. The goal is to keep the mirror at its position under varying external influences, such as gravity and temperature, therefore maintaining the alignment and optical image quality. To achieve this, the lateral support consists of eight actuators, based on stepper motors with gearboxes, with local force measurement and three laser triangulation sensors to measure the mirror’s lateral position. A cascaded control structure with local force feedback in the inner loop and position feedback in the outer loop is proposed. In the outer loop, three single-input single-output PI-controllers are implemented to maintain mirror position in the three lateral degrees of freedom. The developed system is able to position the mirror with root mean square errors of 0.27 and 0.18 μm in translational directions and 5.1 μrad in the rotational direction over the operational altitude range with a slewing speed of 0.2 deg / s.