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

Author: Rijnveld, N. · Henselmans, R. · Nijland, B.A.H.
Type:article
Date:2011
Source:Optomechanics 2011: Innovations and Solutions, 22-25 August 2011, San Diego, CA, USA, 8125
series:
Proceedings of SPIE - The International Society for Optical Engineering
Identifier: 445726
ISBN: 9780819487353
Article number: No.: 812503
Keywords: Electronics · Adaptive Optics · Field Selector · Laser Guide Star · Mechatronics · OTA · Tip/tilt mirror · VLT · Field Selector · Laser Guide Star · OTA · Tip/tilt mirror · VLT · Inductive sensors · Jitter · Optical telescopes · Sensors · Stars · Mirrors · High Tech Systems & Materials · Industrial Innovation · Mechatronics, Mechanics & Materials · OM - Opto-Mechatronics · TS - Technical Sciences

Abstract

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 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. © 2011 SPIE.