In the summer of 2022, the Subaru Telescope suspended observations for about two months for telescope maintenance and the aluminum recoating of the primary mirror. This was the 9th coating for the Subaru Telescope primary mirror, and it has been five years since the 8th one in 2017.
A high mirror reflectivity is essential for efficient observations. However, the reflectivity decreases over time due to the dust and dirt accumulation on the mirror surface. Before the recoating, the reflectivity of the primary mirror at 400 nanometers (violet light) was about 17 percent lower than in 2017.
The recoating process started with removing the primary mirror and the Cassegrain focus instrument from the telescope. The primary mirror was then washed with acid to remove the old coating. The surface was inspected and repaired. Then the mirror was recoated with aluminum in a vacuum chamber. Because the work is intensive and heavy-industry-scale, but at the same time requires precision, we prepared for more than one year. COVID-19 prevention measures were implemented while working at the summit. Actual summit work constituted only about 20% of the entire process; the other 80% being preparation.
Mirror Washing
The primary mirror was washed twice. In the primary washing, hydrochloric acid dissolved the old aluminum coating. After the inspection, the secondary washing removed dust and dirt from the mirror surface. Then the mirror was placed in the vacuum chamber.
Aluminum Coating
Inside the vacuum chamber, which is just big enough to accommodate the 8.3-meter primary mirror, 288 tungsten filaments pre-wetted with aluminum had been installed. When the filaments were fired (heated by an electric current running through them), the aluminum evaporated. Due to limits on the available electrical current, the filaments were fired in three groups of 96. Aluminum vapor released from the filaments created a thin film on the mirror surface. To create a homogeneous film, the process needed to be conducted in vacuum, free from disruptions by the air.
Finish and the Recovery of the Mirror Reflectivity
After the primary mirror came out of the chamber Satoshi Miyazaki, director of Subaru Telescope, inspected its finish together with engineers and scientists. The vacuum chamber is located on the base floor of the telescope dome, and washing and coating were done there. After the inspection, the mirror was lifted by a crane back to the third floor (observation floor), where the telescope is located and then attached to the telescope. The reflectivity measurements showed that just after the recoating, the mirror returned to the same values as in 2017 (Figure 3).
For about two months, multiple staff members took photos and videos every day to document the work process. These will be used for reference for the next, 10th, coating.