Fast-steering Secondary Mirror Research Articles

Tip-Tilt Tests by Using Eddy Current Measuring Sensors

Fast-steering Secondary Mirror (FSM) will be installed to the Giant Magellan Telescope (GMT) as a first generation secondary mirror. The FSM implements a tip-tilt mechanism which compensates image degradations caused by wind turbulence and structure jitter. A prototype segment of the FSM has been developed, which comprised of a dummy mirror, a mirror cell, axial and lateral supports, vacuum devices, etc. Several test-bed frames have been developed to hang the segment and to test tip-tilt functionalities. They have been designed, manufactured, assembled, and verified successfully step by step. Tip-tilt measurements were performed mainly by displacement measuring sensors which detect eddy currents precisely. The results proved that the prototype segment worked successfully, by complying the requirements of the FSM for the tip-tilt actuations.

Random Vibration Analysis of the Tip-tilt System in the GMT Fast Steering Secondary Mirror

A random vibration analysis was accomplished on the tip-tilt system of the fast steering secondary mirror (FSM) for the Giant Magellan Telescope (GMT). As the FSM was to be mounted on the top end of the secondary truss and disturbed by the winds, dynamic effects of the FSM disturbances on the tip-tilt correction performance was studied. The coupled dynamic responses of the FSM segments were evaluated with a suggested tip-tilt correction modeling. Dynamic equations for the tip-tilt system were derived from the force and moment equilibrium on the segment mirror and the geometric compatibility conditions with four design parameters. Statically stationary responses for the tip-tilt actuations to correct the wind-induced disturbances were studied with two design parameters based on the spectral density function of the star image errors in the frequency domain. Frequency response functions and root mean square values of the dynamic responses and the residual star image errors were numerically calculated for the off-axis and on-axis segments of the FSM. A prototype of on-axis segment of the FSM was developed for tip-tilt actuation tests to confirm the ratio of tip-tilt force to tip-tilt angle calculated from the suggested dynamic equations of the tip-tilt system. Tip-tilt actuation tests were executed at 4, 8 and 12 Hz by measuring displacements of piezoelectric actuators and reaction forces acting on the axial supports. The derived ratios of rms tip-tilt force to rms tip-tilt angle from tests showed a good correlation with the numerical results. The suggested process of random vibration analysis on the tip-tilt system to correct the wind-induced disturbances of the FSM segments would be useful to advance the FSM design and upgrade the capability to achieve the least residual star image errors by understanding the details of dynamics.

대형 망원경용 FSM(Fast Steering Secondary Mirror)을 위한 팁틸트 액츄에이터의 성능평가

For ground-based telescope application, the performance assessment of tip-tilt actuator is important because the optical quality of telescope depends upon the windshake compensation ability of the fast steering secondary mirror. But it is difficult to measure the performance characteristics of the actuators due to the large size mirror and test facilities including the vacuum support and structural frame. In this paper, the full-scale tip-tilt test bed for the large size secondary mirror with diameter of 1m is built and the several tests are performed including the range, resolution and frequency response function. From the measurement results, it is shown that the tip-tilt actuator can successfully compensate the windshake with frequency of maximum 12 Hz and be a candidate for the Giant Magellan Telescope.