Development of lightweight SiC mirrors for the space infrared telescope for cosmology and astrophysics (SPICA) mission

SPICA (Space Infrared Telescope for Cosmology and Astrophysics) is a Japanese astronomical infrared satellite project with a 3.5-m telescope. The target year for launch is 2017. The telescope is cooled down to 4.5 K in space by a combination of newly-developed mechanical coolers with an efficient radiative cooling system at the L2 point. The SPICA telescope has requirements for its total weight to be lighter than 700 kg and for the imaging performance to be diffraction-limited at 5 pm at 4.5 K. Material for the SPICA telescope mirrors is silicon carbide (SiC). Among various types of SiC, primary candidates comprise normally-sintered SiC, reaction-sintered SiC, and carbon-fiber-reinforced SiC; the latter two have been being developed in Japan. This paper reports the current design and status of the SPICA telescope along with our recent activities on the cryogenic optical testing of SiC and C/SiC composite mirrors, including the development of an innovative support mechanism for cryogenic mirrors, which are based on lessons learned from a SiC 70 cm telescope onboard the previous Japanese infrared astronomical mission AKARI.