Lightweight Cold Mirror And Fixation

This paper presents the main results of a feasibility study (ESTEC contract n° 6122/84/NL/GM with AEROSPATIALE and REOSC) of a cold lightweight mirror. This is the pri-mary mirror of a RITCHEY-CHRETIEN telescope for the ISO satellite, diffraction limited at 5 μm ( λ/14 RMS). The following points are the main difficulties which must be taken into account to define the mirror : - Choice of mirror material, based on the best optical suitability, homogeneity of the thermal expansion coefficient and straylight characteristics. The available materials are ZERODUR optical grade and fused silica HERASIL 1 grade. Fused silica seems to be the best material because there is no deformation of a sample under radiations and no hysterisis during temperature cycles, opposite to ZERODUR. - Lightweighting, REOSC know-how with numerical machine tools allows to reach a weight of 20 kg (instead of 57 kg) and in terms of aspherical coefficient, the wave front er-ror is about λ/86 in RMS value at 5 μm. - Mirror fixation device, based on a patented fixation for ambient temperature, does not introduce torque in the mirror and take up the differential thermal expansion between the mirror and the optical support structure. A calculation gives a wave front error of λ/70 RMS at 5 μm. - Mirror cooling, temperature of the mirror in space has to be less than 10° K. It is necessary to bond thermal straps at the rear of the optical surface of the mirror in lightweighting holes. The number of straps necessary to cool down to 10° K the mirror in 48 hours must be over 40. The number of holes in the mirror is 57.