Recent achievements with a cryogenic ultra-lightweighted HB-Cesic mirror

During the past two years, ECM, Germany, together with Mitsubishi Electric Corporation (MELCO), Japan, developed a new carbon-fiber-reinforced SiC material, called HB-Cesic®, which possesses superior mechanical and thermal cryogenic properties compared to traditional Cesic®. This combination makes HB-Cesic® an excellent choice for large cryogenic mirrors, which will be required for future scientific space missions, such as SPICA and DARWIN. ESA contracted Thales Alenia Space (TAS), France, to design a super-lightweighted HB-Cesic® mirror with a diameter of 600 mm, isostatic fixations, and a special astigmatism compensation device (ACD) for mirror shape control. The mirror was manufactured by ECM, polished and coated by Société Européenne de Systèmes Optiques (SESO), France, and tested to cryogenic temperatures by TAS. The measured wave-front error at ambient and cryogenic temperatures demonstrated the excellent homogeneity of HB-Cesic® and TAS' expertise in mirror mounting. Furthermore, when thermally actuated, the ACD exhibited perfect control of the mirror shape. This success confirmed HB-Cesic®'s superior material properties and its applicability to future cryogenic space mirrors. In this paper we describe the design and fabrication process of this cryogenic mirror and give test results at ambient and cryogenic temperatures.