Cryogenic optical testing of an 800 mm lightweight C/SiC composite mirror mounted on a C/SiC optical bench.

We tested the optical performance at cryogenic temperatures of an 800 mm diameter lightweight mirror, consisting of carbon-fiber reinforced silicon carbide and with a mass of 11.2 kg. The ceramic composite of the mirror was HB-Cesic, developed by ECM, Germany, and Mitsubishi Electric Corporation, Japan. The test was carried out while the mirror was mounted, via Invar stress relief supports, on a lightweight optical bench also made of HB-Cesic. During the test, both the mirror and the optical bench were cooled to 18 K in a liquid-helium chamber. The test consisted of measuring the mirror's change of surface figure with an interferometer installed outside the cryo-chamber. The cryogenic deformation of the mirror was 110 nm RMS with no significant residual deformation after cooling, which is very promising for the applicability of the HB-Cesic composite to large lightweight cryogenic space optics.