Silicon carbide-based functional components in the Visible Emission Line Coronagraph on-board the ADITYA-L1 mission

A state-of-the-art Visible Emission Line Coronagraph (VELC) payload on-board India's solar mission ADITYA-L1 was designed to study various solar phenomena. To maintain the thermal stability of the system, VELC design recommends silicon carbide (SiC)-based components because of their outstanding mechanical, thermal and optical properties. In particular, a SiC-based tertiary mirror (M3) was used for the collection of undesired sunrays and reflecting them out from the system, and a SiC radiator plate (popularly known as a cold finger) for efficient heat dissipation from the mirror and, in turn, from the system. This article describes the processing and evaluation of SiC-based M3 mirror and cold finger for VELC. The substrates for M3 mirror and cold finger were processed through dry pressing of SiC powder with the required formulation, followed by machining and temperature-assisted densification under an inert atmosphere. SiC components developed using powder metallurgical technique exhibited about 98.4% relative density (RD) and achieved the structural and thermal requirements of M3 mirror and cold finger. The optical requirement of M3 mirror was achieved through a coating of SiC substrate with 100% RD employing chemical vapour deposition followed by surface grinding and polishing. The final mirror achieved a surface flatness better than 20 nm, and microroughness data showed less than 5.1 angstrom root mean square surface roughness in a spatial scale of 0.02 to 0.9 mm.