Reflectivity degradation study of mirror coupons at NLST site Merak

Abstract

The reflectivity degradation study of mirror coatings at the proposed National Large Solar Telescope (NLST) site at Merak, Ladakh, was initiated to assess the performance of different optical coating configurations under the site-specific environmental conditions. The study was carried out using a CT7 reflectometer, which measures specular reflectivity at seven discrete wavelengths ranging from 365 to 970 nm. Four mirror samples, namely M1, H1, S1, and S2, were characterized prior to installation and again at the Merak site to establish baseline reflectivity data. At Merak, the mean reflectivity values of M1 and H1 were measured as 86.5% and 89.1%, respectively, while the bare-aluminium-coated coupons S1 and S2 exhibited mean reflectivities of 89.2% and 88.9%, respectively. The corresponding scattering losses (due to any dust, pinhole effect, or any other contamination that leads to scattering effect) were estimated to be approximately 1.3% for M1, 0.4% for H1, and 0.9% for both S1 and S2. Following these baseline measurements, the mirrors were installed on the dome surface for environmental exposure and long-term monitoring. S1 was placed in an exposed location, whereas S2 was retained in a protected environment to serve as a reference coupon. This arrangement will facilitate the identification of changes specifically associated with environmental exposure. The measurements and installation carried out during this study establish the baseline required for subsequent monitoring of coating performance. Periodic measurements at the same locations and wavelengths will enable the assessment of reflectivity degradation and provide information on the long-term behaviour and suitability of the different mirror coating configurations under the environmental conditions prevailing at Merak.