Reflectivity degradation study of 2m Himalayan Chandra Telescope mirror

Abstract

The reflectivity degradation study of the optical components of the 2-m Himalayan Chandra Telescope (HCT) at the Indian Astronomical Observatory (IAO), Hanle, was carried out using a CT7 reflectometer. The measurements were performed at seven discrete wavelengths ranging from 365 nm to 970 nm. Reflectivity measurements of the HCT primary mirror were carried out at eight predefined positions, both before and after optical cleaning, to assess the effect of cleaning on the mirror reflectivity. Before cleaning, the primary mirror exhibited an average reflectivity of 74.8%, with scattering loss of 22.01%. Following optical cleaning using distilled water and Labolene solution, the average reflectivity increased to 84.9%, while the scattering loss decreased to 8.9%, corresponding to an improvement in average reflectivity of approximately 13.5%. Reflectivity measurements were also carried out for the secondary and tertiary mirrors, with average reflectivities of approximately 89.1% and 87%, respectively. A 30-mm-diameter bare-aluminium-coated S3 coupon installed on the elevation side member of the HCT was also characterized. Its reflectivity was measured at Bengaluru and Hanle prior to installation to establish baseline data for future coating degradation studies. In addition, visual inspection of the primary mirror revealed numerous pinholes, scratches, stains, and patch-like cosmetic defects across its surface, which were documented as part of the assessment of the mirror coating condition. The measurements presented in this report establish baseline reflectivity data for the HCT optical components and the S3 aluminium-coated coupon. Periodic measurements at six-month intervals, using the same measurement positions and wavelengths, are planned to monitor long term reflectivity degradation under the environmental conditions of the Hanle site.