Initial Control Results for the Thirty Meter Telescope

Abstract

The next generation of large ground-based optical telescopes are likely to involve a highly segmented primary mirror that must be controlled in the presence of wind and other disturbances, resulting in a new set of challenges for control. The current design concept for the Thirty Meter Telescope (TMT) has 738 segments in the primary mirror, with the out-of-plane degrees of freedom actively controlled. The secondary mirror also requires at least 5 degree of freedom control. We discuss control issues for extremely large segmented-mirror telescopes and present preliminary simulation results for the current TMT design. The most significant departure from existing telescope control systems is that wind buffeting due to turbulence inside the telescope enclosure drives the desired control bandwidth higher, and hence limitations resulting from control-structure-interaction must be understood. The bandwidth of the main telescope elevation drive is limited by interaction with structural modes. In order to achieve a significant reduction in image motion, a fast tip/tilt control of the secondary requires momentum compensation to mitigate interaction with structural dynamics. Control of the primary mirror segments is limited only by interaction with the segment support resonant frequencies, and not with the “global” modes of the telescope structure.