The lightweight cryogenic telescope on board the Japanese infrared astronomical satellite, ASTRO-F, which is scheduled to be launched early in 2006, forms an F/6 Ritchey-Chretien system with a primary mirror of 710 mm in diameter. The mirrors of the ASTRO-F telescope are made of sandwich-type silicon carbide (SiC) material, comprising a porous core and a chemical-vapor-deposited coat of SiC on the surface. To estimate the optical performance of the flight model telescope, the telescope assembly was tested at cryogenic temperatures, the total wavefront errors of which were measured by an interferometer from outside a liquid-helium chamber. As a result, the wavefront error obtained at 9 K shows that the imaging performance of the ASTRO-F telescope is diffraction limited at a wavelength of 6.2 microm, which is a little worse than our original goal of diffraction-limited performance at 5.0 microm.

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.

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In this paper, we describe our recent activities on wave-front measurement of space infrared telescopes. Optical performance of the 685-mm lightweight telescope on board the Japanese infrared astronomical satellite, ASTRO-F, has been evaluated at cryogenic temperatures. The mirrors of the ASTRO-F telescope are made of sandwich-type silicon carbide (SiC) material, comprising porous core and CVD coat of SiC on the surface. The total wavefront errors of the telescope were measured with an interferometer from outside a liquid-helium chamber; a 75-cm reflecting flat mirror was used for auto-collimating the light from the interferometer. The cryogenic deformation of the flat mirror was derived independently by shifting it in the chamber and its contribution to the wavefront error was removed. In addition to the ASTRO-F telescope, we are currently developing a 3.5-m telescope system for SPICA, the next Japanese infrared astronomical satellite project. Details of our methodology for the ASTRO-F telescope, together with our optical test plan for the SPICA telescope, are reported.