In-situ monitoring of Subaru Telescope’s optical performance using a portable spectrophotometer

We report the reflectivity of the Subaru Telescope's mirrors and these time evolutions measured with the Subaru Portable Spectrophotometer (SPS). Thanks to the capability of SPS, the absolute, spectroscopic reflectivity has been measured in-situ on the telescope since October 2017, and it becomes possible to understand and forecast the time evolution of the reflectivity degradation. We established a simple two factor model for the reflectivity degradation of the primary mirror which has coated with aluminum in 2017. From a study of CO2 cleaning with SPS, a part of dust on the mirror surface was found to be removed with CO2 cleaning, on the other hand, the roughness of the surface was found to become larger than before cleaning. The time evolution of the reflectivity of the primary mirror is now able to be forecasted. In parallel, we have applied SPS measurement to the infrared secondary mirror of the Subaru Telescope (IR M2) and found a significant loss of reflectivity in the visible wavelength in November 2018. IR M2 had been coated with silver in 2008 and used for over ten years. Although the original reflectivity of silver mirror is ~98% at 589 nm, there was ~50% in November 2018, and it was ~30% at in November 2019. One of the causes of the significant loss could be due to volcanic gas from the explosion of Kilauea in May 2018; however, it was hard to explain the continuing degradation in reflectivity through the following year. The reflectivity could not be recovered by any quick cleanings. We carried out recoating of IR M2 in November 2019. A three-factor model to explain the reflectivity degradation of IR M2 was considered. The model would help us to understand what happens on silver mirrors. On the other hand, unknown localized phenomenon such as a white spot was seen on the IR M2 mirror surface.