Light scattering due to particulate contamination over the primary mirror of Visible Emission Line Coronagraph on board Aditya-L1 mission

Abstract

To meet the proposed science goals of the Visible Emission Line Coronagraph on board the Aditya-L1 mission, the total scattered light should be less than 5 ppm (parts per million) of solar disk brightness over the field of view (FOV). Major contributors to the scatter light are microroughness and contamination over the surface of the primary mirror (M1). A detailed study to derive the microroughness specifications of the M1 provides a fair idea of the effect of the contamination on the mirror in terms of the scatter generated from the surface: 1.53 Å band-limited RMS microroughness on the M1 surface contributes toward a total scatter of 4 ppm over the FOV. This restricts the scatter over the FOV due to contamination being less than 1 ppm, and it is equally apportioned to scatter due to both particulate and molecular contamination. Hence, the scatter over the FOV due to particulate contamination should be limited to 0.5 ppm. Our paper provides a detailed explanation of the studies carried out to determine the maximum allowable particulate contamination levels on the M1.