Reflectivity degradation study of 0.7-m Growth India Telescope (GIT), Hanle

Abstract

The reflectivity of the optical components of the 0.7-m GROWTH India Telescope (GIT) at the Indian Astronomical Observatory (IAO), Hanle, was measured using a CT7 reflectometer at seven discrete wavelengths ranging from 365 to 970 nm. The measurements were carried out to assess the present condition of the reflective coatings and establish baseline data for future monitoring. The primary mirror was measured at four accessible positions and showed an average reflectivity of approximately 78%, with a scattering loss of around 18.2%. The reflectivity varied with wavelength, ranging from 72.7% at 464 nm to 82.4% at 624 nm. Measurements of the secondary mirror at two positions yielded an average reflectivity of approximately 90.2%, with a scattering loss of about 2%. The results indicate comparatively higher dust accumulation and lower reflectivity for the primary mirror. The measurements provide baseline data for future assessment of coating degradation and optical performance through periodic monitoring under the environmental conditions of the Hanle site.