|
A new material designated C/SiC is the basis for a current ultra-lightweight scan mirror (ULSM) ESA/ESTEC technology development contract for the METEOSAT Second Generation (MSG) Program (Ref. {1}). It consists of random- oriented carbon fibers as 'greenbody' which is shaped to the designed configuration, and is infiltrated in the liquid phase with Si resulting in a SiC enriched surface layer, with a CVD grown SiC optical polishing layer. The reflective coating finally is silver under a dedicated protection layer. The requirements for this mirror are rather stringent: during 7 years in a geostationary orbit of life it will permantly be exposed to centrifugal forces induced by the satellite spinning at 100 rpm. Due to high image quality requirements for the imager - designated SEVIRI (Spinning Enhanced Visible IR Imager) - the MTF degradation caused by the scan mirror alone are defined to <EQ 3%, resulting in rather high surface quality requirements. The main contribution is the spin induced mirror tip deflection which turned out to be the design driver. The technologically most challenging requirement is to develop a scan mirror with 800 X 500 mm2 effective aperture and a central cutout - the telescope is a Gregory concept - to withstand tight environmental requirements at a mass which was specified as < 6 kg. The ESA contract covers a baseline concept with the subsequently presented design of a monolithic C/SiC mirror, mated to a CFRP structure by isostatic mounts. At the end of this technology program, a fully tested 1:1 scale mirror with silver and protection coating will be rendered to ESA. Since this contract is a technology contract, applications beyond the MSG scan mirror are borne in mind, including higher optical surface qualities such as (lambda) /30 or curved surfaces, also aspheric mirrors of possibly even larger diameters. The manufacturing and optical process implement subcontractors which are not listed among the authors, i.e. SGL Carbon/Meitingen, FRG, for the Si infiltration; Schunk Kohlenstofftechnik/GieBen, FRG, for the CVD SiC coating; REOSC Optique, Ballainvilliers, France for the optical polishing, and Jenoptik, Jena, FRG, for the silver and protection coating.
|
