Design and structural/optical analysis of a kinematic mount for the testing of silicon carbide mirrors at cryogenic temperatures

Abstract

A kinematic mount has been designed to support two Silicon Carbide-based spherical mirrors during cryogenic testing at the Goddard Space Flight Center. The mirrors are flight representative test mirrors for the NIRSpec Instrument of the James Webb Space Telescope (JWST), provided by Galileo Avionica of Florence, Italy. One is cold-pressed Silicon Carbide (SiC) and one is Carbon reinforced Silicon Carbide (C/SiC); both are coated in a SiC-based chemical vapor deposit. Each is lightweighted and has an integral mount on the rear surface. The integral mount is used as an interface to the kinematic mount, which is designed to support the mirrors during cryogenic testing while minimizing distortions induced by CTE mismatch among the materials. Additionally, an alternative simply supported mount is used to hold the mirrors around the outer edge of the optical surface. This eliminates the bending of the integral mount under the weight of the mirror and evaluates the effectiveness of the kinematic mount. The mirrors were analyzed for optical performance during testing from room temperature to 20K using Finite Element Analysis (FEA) with MSC/NASTRAN 2001. Predicted surface figure error (SFE) based on the removal of bias, tilt, and power was calculated using an in-house Matlab script for spherical mirrors. SFE was verified using the SigFit optical post-processing program to provide Zernike polynomial input for analysis with the Zemax optical software. The results show that the kinematic mount induces minimal figure error on the optical surface.