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6 July 2018 LSST M1M3 figure actuator final design, fabrication, and test
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Abstract
The Large Synoptic Survey Telescope1 (LSST) is an altitude-azimuth mounted three mirror telescope and camera. The primary (M1) and tertiary (M3) mirrors are integrated into a single, monolithic borosilicate substrate 8.42 m diameter. The annular secondary (M2) mirror is located above the M1M3 mirror and the camera is nested inside the M2. The M1M3 mirror is supported on a mirror cell by two independent systems: one system is for Active Mode and the other for Static Mode.

During observing, or Active Mode2, the M1M3 mirror is supported by an array of 156 support and figure control actuators consisting of 268 pneumatic cylinders that react to gravity and inertial loads and provide figure error correction. Load cells on the actuators measure forces that are communicated to the M1M3 control system. However, the figure actuators do not define the mirror position. This is defined with six axially stiff linear actuators called hardpoints3 arranged in a hexapod pattern to restrain rigid body motion of the mirror in a kinematic fashion. By adjusting the length of each hardpoint, the mirror can be adjusted in all six degrees of freedom with respect to the cell. Displacement sensors and load cells on the hardpoints communicate displacements and forces to the control system, which processes the telemetry and issues force corrections to the figure actuators to zero out any loads and moments on the hardpoints.

In Static Mode, the M1M3 mirror is no longer supported by figure actuators and the position sensing of the hard point hexapod is inactive. A second support system consisting of 288 wire rope isolators called Static Supports come into play. The static supports mechanically capture the mirror whether in Active or Static Mode and in the event the mirror experiences motion beyond the active motion range in any direction. The static supports also safely support the mirror during seismic events for all elevation angles. In active mode, the static supports do not contact the mirror and thus, do not affect the mirror positioning or figure.

This paper focuses on the detailed design, development, testing, integration, and current status of the M1M3 pneumatic figure actuators.
© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Gary P. Muller, Edward A. Hileman, Felipe Daruich, Michael Warner, Oliver M. Wiecha, Constanza Araujo, N. G. Mills, Brian E. Johnson, Edward Stover, Michael T. Booth, Ron C. Harris, Douglas R. Neill, William J. Gressler, and Andrew W. Serio "LSST M1M3 figure actuator final design, fabrication, and test", Proc. SPIE 10700, Ground-based and Airborne Telescopes VII, 107003V (6 July 2018); https://doi.org/10.1117/12.2313598
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