Dynamic analysis of the active control system for the CGST

Abstract

The Chinese Giant Solar Telescope (CGST) is the China's next generation solar telescope with an aperture of 8 m in diameter. The unique feature of the CGST is its ring primary, which facilitates the polarization detection and thermal control. The CGST is now in its design and development phase. A mosaic mirror with 24 trapezoidal segments is a candidate for its primary mirror, which highly relies on a segment active control system to achieve competitive optical quality of a monolithic mirror. The CGST is designed to operate in open-air observation mode, its active control system thus faces new challenges. As the CGST has an aperture larger than that of current solar telescopes, and as the magnitude of wind load in open air is greater than that of a stellar telescope with a similar aperture yet under the protection of a dome and/or an enclosure. Furthermore, as a mosaic mirror, high precision real-time tip sensing is required to serve the feedback of its control system. The accuracy depends on integration time (or working bandwidth) when an optical metrology is adopted, which should match the bandwidth of the segment control system. In this paper, a dynamic analysis of the segment control system of the CGST is presented. We demonstrate how the dynamic interaction between the segment control system and the telescope structure impacts the telescope's optical performance under wind disturbances. The dynamic analysis helps to understand the bandwidth limit for the segment control system, and further to clarify technical requirements for tip sensing implementation, telescope structure design and wind shielding design.