Reflective optical system made entirely of ultra low thermal expansion ceramics: a possibility of genuine athermal cryogenic IR instrument

Reflective optical system free from chromatic aberration is essential for astronomical instruments, which usually require wider wavelength coverage. However, it cannot always be the optimum choice compared with refractive optical system in terms of cost-effectiveness because mirrors require high surface accuracy and also because non-co-axial systems force tough alignment work. This dilemma could be overcome by a monolithic reflective optical system made entirely of cordierite CO-720, a ceramic material by Kyocera, which is the first material that offers both high-precision 3D-shaping and surface polishing for optical quality. This material also possesses a very low coefficient of thermal expansion (CTE) enabling a genuine athermal system useful for various astronomical applications. This athermality could make a significant breakthrough especially for cryogenic infrared instruments since optical systems made of cordierite are expected to keep as-built performance throughout the cooling process, providing extremely high wavefront accuracy that has never been possible at cryogenic temperature with conventional optical systems made of glasses or metals. In this paper, we report the first cryogenic optical testing of a small cordierite-made imaging optical system that was simply assembled with mechanical accuracy at room temperature. We confirmed that the diffraction-limited optical performance is kept even down to ~80K as built in the room temperature.