Microscopic surface structure of C/SiC composite mirrors for space cryogenic telescopes

TL;DR

This study analyzes the microscopic surface structure of C/SiC composite mirrors used in space cryogenic telescopes, demonstrating that a SiSiC slurry coating significantly reduces surface roughness and maintains stability at cryogenic temperatures.

Contribution

It provides detailed microscopic analysis of C/SiC mirrors and shows that SiSiC slurry coating improves surface quality and stability for space telescope applications.

Findings

Surface roughness reduced from 20-100 nm to 2 nm with coating

Surface structure remains stable down to 95 K

Coated mirrors are promising for cryogenic space telescopes

Abstract

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.