Study of scattered light generated by silicon carbide mirrors

Sammanfattning: Silicon carbide (SiC) shows promise as an increasingly widespread material for aerospace mirror systems. It has already been successfully used in recent space programs, with and without Chemical Vapor Deposition (CVD). The additional processes and costs generated by CVD make the alternative of SiC without CVD more practical, as long as stray light specifications are met. Nonetheless, theoretica lmodels remain underequipped to describe properly the scattering effects generated by the porous surface of SiC. The present study, performed at EADS Astrium in Toulouse, aimed at developing an analytical model to relate SiC roughness to Bidirectional Reflectance Distribution Function (BRDF). Series of measurements of roughness maps and BRDFs were carried out enabling to assess the abilities of Astrium subcontractors to measure SiC properties. A fast numerical tool was developed to compute BRDFs from roughness maps. A comparison between computed and measured BRDFs was thus made possible. The K-correlation model proved to successfully describe scatter properties of SiC CVD and an adjustment of this model was proposed for SiC without CVD. Because of the limitations of optical profilometry in measuring pores of low dimensions and remnant difficulties in BRDF measurements, reliable data is still required to continue these first steps and eventually develop models that will take SiC pores into account.