On February 7, the National Astronomical Observatory of Japan (NAOJ) Hawaii Subaru Telescope announced that it had suspended observations with the Subaru Telescope for about two months, and carried out re-deposition work to re-coat the primary mirror with aluminum and maintenance of the telescope. This is the ninth time that the primary mirror has been vapor-deposited, and the first time in five years since the previous one in 2017.
Even with a large telescope, no matter how advanced the technology is, it is necessary to maintain the reflectance of the primary mirror in order to make efficient and detailed observations. However, dirt gradually accumulates on the surface of the primary mirror, and the reflectance decreases. For example, in the case of the Subaru Telescope, five years have passed since the last aluminum coating deposition work was performed. For example, a decrease of about 17% was confirmed.
The re-deposition process begins with the removal of the primary mirror and Cassegrain focus instrumentation from the telescope. The primary mirror is then washed to remove the aluminum coating, and the surface of the primary mirror, which has been returned to a single piece of glass, is inspected for scratches and repaired. Since the vapor deposition work is a precise and large-scale work, it was carried out after careful preparation for more than a year. It is no exaggeration to say that about 80% of the work is preparation before the actual performance.
Cleaning of the primary mirror is performed twice. The primary cleaning uses hydrochloric acid to dissolve the old aluminum coating. After being inspected for scratches, the primary mirror, which has had its surface dust and dirt removed in a secondary cleaning, is placed in an evaporation tank for re-evaporation.
The deposition pot is large enough to fit the Subaru Telescope’s primary mirror, which has a diameter of about 8.3m. Inside is a vacuum with 288 specially made tungsten filaments pre-melted with aluminum. In the vacuum pot, an electric current is applied to the filament, and the aluminum that shoots straight out of the filament evaporates, which is called “firing” (due to power consumption, it was carried out in three batches of 96 filaments each). This firing causes aluminum to be re-deposited (coated) on the primary mirror surface. The reason for evacuating the chamber is that the presence of air interferes with the formation of a highly pure and uniform film on the surface of the primary mirror by the evaporated aluminum.
After that, the deposition check is performed on the primary mirror taken out from the deposition pot. This time, careful confirmation was carried out by Satoshi Miyazaki, director of the Subaru Telescope, and others. As a result of the inspection, it was confirmed that the reflectance of the primary mirror had recovered to the value immediately after the previous re-deposition.
After confirming the finish, the primary mirror was lifted by crane from the first floor, where cleaning and re-deposition work was carried out, to the third floor (observation floor) where the telescope is located, and then installed in place. The effect of this re-evaporation will definitely be demonstrated in future observations.