Development of GMT fast steering secondary mirror assembly

Abstract

The Giant Magellan Telescope (GMT) is one of Extremely large telescopes, which is 25m in diameter featured with two Gregorian secondary mirrors, an adaptive secondary mirror (ASM) and a fast-steering secondary mirror (FSM). The FSM is 3.2 m in diameter and built as seven 1.1 m diameter circular segments conjugated 1:1 to the seven 8.4 m segments of the primary. The guiding philosophy in the design of the FSM segment mirror is to minimize development and fabrication risks ensuring a set of secondary mirrors are available on schedule for telescope commissioning and early operations in a seeing limited mode. Each FSM segment contains a tip-tilt capability for fine co-alignment of the telescope sub-apertures and fast guiding to attenuate telescope wind shake and mount control jitter, thus optimizing the seeing limited performance of the telescope. The final design of the FSM mirror and support system configuration was optimized using finite element analyses and optical performance analyses. The optical surface deformations, image qualities, and structure functions for the gravity print-through cases, thermal gradient effects, and dynamic performances were evaluated. The results indicated that the GMT FSM mirror and its support system will favorably meet the optical performance goals for residual surface error and the FSM surface figure accuracy requirement defined by encircled energy (EE80) in the focal plane. The mirror cell assembly analysis indicated an excellent dynamic stiffness which will support the goal of tip-tilt operation.

Overview and status of the Giant Magellan Telescope project

“…The functional requirements of the two secondary mirror systems and baseline designs have been described in prior reviews. 22,23 We have considered two approaches to the interface between the secondary mirrors and the telescope. The first is a singlestage interface consisting of one large stroke hexapod per segment that provides both positioning and tip-tilt control.…”

Experimental study of breakaway system for the fast-steering secondary mirror prototype of GMT

“…The requirement of the preload of the spring is one to three times of gravity force of mirror [7] . The mass of the mirror is nearly 100 kg and there are three axial support in the FSM [3], [4], [8] . The preload range for single BAS is from 333N to 999N.…”

Development status of the fast-steering secondary mirror of GMT

“…Rigid mirror of each FSM segment is mounted in an individual FSM cell as shown in Figure 2. Three tip/tilt actuators provide pointing corrections at frequencies above the control bandwidth of the Telescope Mount [1] [2] . The main purpose of the FSM tip/tilt control is to mitigate image motion due to wind disturbances.…”