Adjustable bipod flexures for mounting mirrors in a space telescope

Abstract

A new mirror mounting technique applicable to the primary mirror in a space telescope is presented. This mounting technique replaces conventional bipod flexures with flexures having mechanical shims so that adjustments can be made to counter the effects of gravitational distortion of the mirror surface while being tested in the horizontal position. Astigmatic aberration due to the gravitational changes is effectively reduced by adjusting the shim thickness, and the relation between the astigmatism and the shim thickness is investigated. We tested the mirror interferometrically at the center of curvature using a null lens. Then we repeated the test after rotating the mirror about its optical axis by 180° in the horizontal setup, and searched for the minimum system error. With the proposed flexure mount, the gravitational stress at the adhesive coupling between the mirror and the mount is reduced by half that of a conventional bipod flexure for better mechanical safety under launch loads. Analytical results using finite element methods are compared with experimental results from the optical interferometer. Vibration tests verified the mechanical safety and optical stability, and qualified their use in space applications.

Bipod flexure for 1-m primary mirror system

“…The apex of the triangle formed by a bipod flexure should point to the mass center of the mirror or equivalently the shear center of the mirror such that gravityinduced surface distortion can be effectively reduced. 8 Each bipod flexure has a symmetric combination of two tangential blades and a radial blade as shown in Fig. 1(b), and they are named after the role of each part.…”

“…This is also proven in the 800-mm mirror system. 8 In such cases, the fundamental frequency F 1 can be calculated using C 41 or C 63 in Eq. (19), and can be expressed as…”

“…6,7 We reported an adjustable bipod flexure for mounting the 800-mm primary mirror in a space telescope. 8 We used FEA (finite element analysis) as an initial design and optimization tool for the flexure, and verified the design with experimental results obtained from vibration tests and optical interferometric testing. Initial design of the bipod flexures required exhaustive iterations with modeling and FEA tools to explore multidimensional space.…”

Analysis and experimental investigation for collimator reflective mirror surface deformation adjustment

“…There is no doubt that the location and the direction of a mounting flexure are determined using kinematic principles and the ideal support location is in the plane through the mirror centroid and the lateral force should be vertical to the optical axis (Vukobratovich and Richard 1988;Yoder 2008;Chu et al 2011;Kihm 2012). In mounting large mirrors, the common mount types are the V-type, radial, mercury tube, strap, and push-pull mounts for the horizontal optical axis.…”

Alignment and assembly process for primary mirror subsystem of a spaceborne telescope

“…The design reduced the error caused by the gravity effect to less than root mean square 10 nm. 7 Lin et al presented an optomechanical design and analysis method for a mirror mount integrated with a Cassegrain telescope. The deformation of the primary mirror caused by the gravity load from various mirror mounting positions can be predicted using finite element analysis (FEA) before primary mirror assembly.…”