LSST Telescope Primary/Tertiary Mirror Hardpoints

The Large Synoptic Survey Telescope (LSST) utilizes an 8.4-meter cast borosilicate primary/tertiary mirror (M1M3). This mirror system has stringent vibration and stiffness requirements because the LSST optical system does not include a fast steering mirror and the mission requires a short slew and settling time. The position stability of the M1M3 relative to the mirror cell is controlled by six displacement controlled actuators (subsequently referred to as "hardpoints") that form a large hexapod. This design is based largely on previous hardpoints implemented for borosilicate mirror positioning. Traditionally, all dynamic forces applied to these mirrors are reacted through their hardpoints. Consequently, the characteristics of these hardpoints critically affect the ability of the telescope to meet the stringent dynamic requirements without overstressing the mirror. The hardpoints must have a high stiffness of 120 N/um in the axial direction, while protecting the mirror by limiting the loads in all six degrees of freedom. The non-axial direction loads are limited by flexures. The axial loads are limited by a pneumatic breakaway mechanism. Since the hardpoints react the dynamic mirror loads, the axial breakaway force may limit the telescope's slewing accelerations. The travel of the breakaway mechanism must accommodate the transfer of the mirror from its active supports to its static supports. The hardpoint positioning mechanism must have sufficient travel and resolution to properly position the mirror relative to the mirror cell. Fulfilling these functions also requires numerous sensors, including a precision axial load cell which is paramount in determining the figure control actuator forces.