A tip/tilt mirror with large dynamic range for the ESO VLT Four Laser Guide Star Facility

N. Rijnveld, R. Henselmans, B. Nijland, Alson E. Hatheway
2011 Optomechanics 2011: Innovations and Solutions  
One of the critical elements in the Four Laser Guide Star Facility (4LGSF) for the ESO Very Large Telescope (VLT) is the Optical Tube Assembly (OTA), consisting of a stable 20x laser beam expander and an active tip/tilt mirror, the Field Selector Mechanism (FSM). This paper describes the design and performance testing of the FSM. The driving requirement for the FSM is its large stroke of ±6.1 mrad, in combination with less than 1.5 µrad RMS absolute accuracy. The FSM design consists of a
more » ... onsists of a Zerodur mirror, bonded to a membrane spring and strut combination to allow only tip and tilt. Two spindle drives actuate the mirror, using a stiffness based transmission to increase resolution. Absolute accuracy is achieved with two differential inductive sensor pairs. A prototype of the FSM is realized to optimize the control configuration and measure its performance. Friction in the spindle drive is overcome by creating a local velocity control loop between the spindle drives and the shaft encoders. Accuracy is achieved by using a cascaded low bandwidth control loop with feedback from the inductive sensors. The pointing jitter and settling time of the FSM are measured with an autocollimator. The system performance meets the strict requirements, and is ready to be implemented in the first OTA.
doi:10.1117/12.893682 fatcat:iafah65hxbha5erhalfe6jsyim