  We report on the microscopic surface structure of carbon-fiber-reinforced
silicon carbide (C/SiC) composite mirrors that have been improved for the Space
Infrared Telescope for Cosmology and Astrophysics (SPICA) and other cooled
telescopes. The C/SiC composite consists of carbon fiber, silicon carbide, and
residual silicon. Specific microscopic structures are found on the surface of
the bare C/SiC mirrors after polishing. These structures are considered to be
caused by the different hardness of those materials. The roughness obtained for
the bare mirrors is 20 nm rms for flat surfaces and 100 nm rms for curved
surfaces. It was confirmed that a SiSiC slurry coating is effective in reducing
the roughness to 2 nm rms. The scattering properties of the mirrors were
measured at room temperature and also at 95 K. No significant change was found
in the scattering properties through cooling, which suggests that the
microscopic surface structure is stable with changes in temperature down to
cryogenic values. The C/SiC mirror with the SiSiC slurry coating is a promising
candidate for the SPICA telescope.
