Polishing, coating, and integration of SiC mirrors for space telescopes

Abstract

In the last years, the technology of SiC mirrors took an increasingly significant part in the field of space telescopes. Sagem is involved in the JWST program to manufacture and test the optical components of the NIRSpec instrument. The instrument is made of 3 TMAs and 4 plane mirrors made of SiC. Sagem is in charge of the CVD cladding, the polishing, the coating of the mirrors and the integration and testing of the TMAs. The qualification of the process has been performed through the manufacturing and testing of the qualification model of the FOR TMA. This TMA has shown very good performances both at ambient and during the cryo test. The polishing process has been improved for the manufacturing of the flight model. This improvement has been driven by the BRDF performance of the mirror. This parameter has been deeply analysed and a model has been built to predict the performance of the mirrors. The existing Dittman model have been analysed and found to be optimistic. IntroductionThese last years, the technology of SiC mirrors took an increasingly significant part in the field of space telescopes replacing gradually more classical solutions based on lightweighted glass mirrors. The remarkable properties of SiC in terms of hardness, stiffness, thermal stability combined with a reasonable density are indeed of primary importance for all space applications and recent success of flight mission based on SiC mirror telescope (like ROCSAT…) tends to confirm all its usefulness.SAGEM/REOSC works on the polishing, coating and integration technologies of SiC mirrors since ten year through various space programs and for various customers: Scan mirror INSAT 3D, ROCSAT and THEOS telescope , Spirale Telescope, GAIA Primary Mirror Demonstrator …).This paper presents the manufacturing processes and the most recent results obtained by SAGEM/REOSC on the polishing, coating and integration of the three TMAs of the NIRSpec's instrument (ESA contribution of the JWST). The performances of the qualification model of the FOR TMA is presented both at ambient temperature and in operational conditions of use at 20K. Sagem contribution to NIRSpec projectIn the frame of the James Webb Space Telescope (JWST) program, the European Space Agency is in charge of the development of the NIRSpec instrument. The prime contractor of this instrument is EADS Astrium in Ottobrunn, Germany and thanks to its 10 years experience in SiC mirrors manufacturing, Sagem has been awarded the contract for the polishing, coating, integration and qualification of most of the optical components of the NIRSpec instrument.

Process Chain for Ultra-Precision and High-Efficiency Manufacturing of Large-Aperture Silicon Carbide Aspheric Mirrors

“…Silicon carbide (SiC) material has the characteristics of low density, high modulus, high specific stiffness, high dimensional stability, and good thermal properties and has become the preferred material for the new generation of space reflectors [3]. Currently, large aperture silicon carbide (SiC) aspheric mirrors have been used in space optical remote sensing detection systems, high-resolution ground observation systems, space telescopes, and other space optical systems such as the SPICA infrared astronomical telescope [4], the ASTRO-F astronomical telescope [5], and the JWST space telescope [6].…”