Reflectivity degradation of the Subaru Telescope primary mirror

The reflectivity degradation of a telescope’s mirrors is broadly known; however, due to lack of suitable instrumentation, it has not been sufficiently studied. To overcome this situation, we developed a portable spectrophotometer named the “Subaru Portable Spectrophotometer” (SPS) to measure the absolute spectral reflectivity in-situ for a large optical component, such as a telescope’s primary mirror. The reflectivity of the Subaru telescope primary mirror was measured with SPS right before its eighth coating, and it showed larger degradation at shorter wavelength. Since then, the reflectivity degradation has been monitored with SPS. To explain the wavelength-dependence of the reflectivity degradation, we developed a two-factor model of a simple achromatic loss and scattering. The two-factor model shows an excellent match to real degradation. The time evolution of the achromatic loss and scattering are shown, and these are about 2.3% achromatic loss per year, and the color-dependent loss from root-mean-square (rms) surface roughness, which is the cause of scattering, increases from an initial small roughness after coating by about 40 Å per year. From these results, we extrapolated the model to a long-elapsed time to estimate the reflectivity in the future. The reflectivity of the Subaru Telescope primary mirror is expected to decrease about 22% at 400 nm by 1200 days after coating, and by 10% at 950 nm. We deduced the most efficient coating frequency of the Subaru Telescope primary mirror that maximizes the number of photons that can be acquired, and it is once every three years.