We tested the optical performance at cryogenic temperatures of an $800\mathrm{mm}$ diameter lightweight mirror, consisting of carbon-fiber reinforced silicon carbide and with a mass of $11.2\mathrm{kg}$ . 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 $18\mathrm{K}$ 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 $110\mathrm{nm}$ 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.