Development of a Fast Steering Secondary Mirror Prototype for the Giant Magellan Telescope

The Giant Magellan Telescope (GMT) will be a 25m class telescope currently in the design and development phase. The GMT will be a Gregorian telescope and equipped with a fast-steering secondary mirror (FSM). This secondary mirror is 3.2 m in diameter and built as seven 1.1 m diameter circular segments conjugated 1: 1 to the seven 8.4m segments of the primary. The prototype of FSM (FSMP) development effort is led by the Korea Astronomy and Space Science Institute (KASI) with several collaborators in Korea, and the National Optical Astronomy Observatory (NOAO) in USA. The FSM has a tip-tilt feature to compensate image motions from the telescope structure jitters and the wind buffeting. For its dynamic performance, each of the FSM segments is designed in a lightweight mirror. Support system of the lightweight mirror consists of three axial actuators, one lateral support at the center, and a vacuum system. A parametric design study to optimize the FSM mirror configuration was performed. In this trade study, the optical image qualities and structure functions for the axial and lateral gravity print-through cases, thermal gradient effects, and dynamic performances will be discussed.