Optical performance of the ASTRO-F telescope at cryogenic temperatures

The telescope onboard Japanese infrared astronomical satellite, ASTRO-F, forms an F/6 Ritchey-Chretien system with a primary mirror of 670 mm in diameter, the total weight of which is about 42 kg. The primary and secondary mirrors are made of a sandwich-type SiC material, consisting of light porous core and dense CVD coat of SiC. The whole system will be cooled down to 5.8 K with a combined use of super-fluid liquid helium and mechanical coolers on orbit. In order to estimate optical performance of the flight-model telescope at operating cryogenic temperatures, the primary mirror alone was first cooled and tested, and then the whole telescope assembly was tested at cryogenic temperatures. In both cases, the changes in the surface figure were measured from outside the cryostat by an interferometer for the temperature range of 10 K to 300 K. As a result, non-negligible degradation in wave-front errors of the primary mirror and the telescope assembly was detected at low temperatures. The deformation of the primary mirror was found to be mainly due to the thermal contraction of support structures and heat anchors, and degradation by the SiC mirror itself was much smaller. The observed wave-front error of the telescope assembly at 13 K, which was found to originate mainly from the distortion of the primary mirror, marginally meets the requirement to achieve the diffraction-limited performance at 5 microns. This paper summarizes the optical performances thus achieved at cryogenic temperatures for the ASTRO-F telescope.