Modern control for the secondary mirror of a Giant Segmented Mirror Telescope

Abstract

To achieve diffraction-limited seeing with the next generation of giant telescopes will require a multi-tiered approach employing both active and adaptive optics. Before removing atmospheric effects via adaptive optics, the telescope structure must be actively controlled to remove structural dynamics effects from the optical wavefront. While low frequency thermal and gravitational effects may be removed by the primary mirror control system, a more difficult challenge is the higher frequency, wind-induced vibrations of the telescope structure. This paper will address the control system design for the rigid secondary mirror support structure using modern control methods. Multivariable control methods are motivated by the large number of coupled structural modes that contribute to the wavefront error at the secondary mirror. H₂ methods are applied to the secondary mirror control system in order to validate the design approach for achieving nominal performance at the system level. The approach investigated for this paper involves using wavefront information to remove wavefront error at the secondary mirror. This work will serve as a basis for demonstrating the feasibility of the overall control architecture for the GSMT point design study.