Optimization of a lightweight mirror with reduced sensitivity to the mount location.

Abstract

Due to optical performance requirements, the primary mirror assembly must be unaffected by environmental influences. These environmental influences include gravity, axial assembly error, flatness error of mounting interface, and thermal change, which can degrade the mirror surface's accuracy. The flexure mounts can be used to isolate the load transfers to the mirror in case of a flatness error and thermal change. The mirror surface's accuracy will degenerate significantly when the flexure mounts have deviations from the optimum axial mount location due to mirror fabrication and a testing error at the center of gravity. These two error terms introduce an accuracy of mount locations on the order of millimeters. In this paper, we describe a method to reduce the sensitivity of a lightweight mirror to the mount location. First, we introduce a design criterion that determines the sensitivity. Then, the topology and parametric optimization are used to specify selective reinforcement of the mirror structure in which the design criterion is taken as the objective function. With our method, the lightweight ratio of a 2 m mirror has been improved from 86.8% to 88.5%, and its sensitivity to the mount location has been reduced from ${1}\;{\rm nm/} \pm {1}\;{\rm mm}$1nm/±1mm to $ {0.6}\;{\rm nm/} \pm {1}\;{\rm mm} $0.6nm/±1mm.