The Large Binocular Telescope primary mirror support control system description and current performance results

The Large Binocular Telescope (LBT) is built around two lightweight borosilicate honeycomb mirrors which, at 8.4 meters in diameter, are the largest operational examples of this technology. Since the mirrors are relatively stiff, the LBT mirror support system relies on passive position control and active force control. Passive position control is performed by six extendable hardpoints organized as a truncated hexapod, which may be positioned as required by the active optics control loop. The hardpoints rely on their axial stiffness to maintain the mirror position against residual external disturbances. The active force control system minimizes the force exerted by the hardpoints on the glass. Additionally, the axial component of the nominally uniform active support forces can be perturbed to distort the mirror as required by the active optics control loop. Because of the relatively large CTE of borosilicate glass, the differential temperature of the mirror is critical. Thus, the force control system must support a 16 metric ton mirror using less than 100 Watts of electrical power. The authors present a description of the primary mirror support system as implemented at the LBT. Initial stability problems made the mirrors nearly unusable in freezing temperatures. The authors explain the reason for this instability and describe the solutions implemented. Data demonstrating the current performance of the primary mirror support system are also presented.