A Study of Technologies of Coating on the Primary Mirror of the 2.4m Optical Telescope of the Yunnan Observatories

In this paper we study technologies of adding aluminium coatings on the primary mirror of the 2.4m optical telescope of the Yunnan Observatories. These are for operations with a large vacuum evaporation coating machine of the type ZZS3200, which is the first vacuum evaporation chamber to coat objects downward in China. We discuss some difficult issues of coating operations, including approaches to control the environment, arrange the evaporation sources of the coating machine, choose the protection film, and remove old coatings. Using these technologies we completed the coating of the primary mirror of the 2.4m telescope with an aluminum reflection layer and an MgF₂ protection layer. The average reflectivity of an accompanying plate in the coating (as the witness sample) is 87.16% in the wavelength range from 350nm to 1100nm. After the coating the limit magnitude of the telescope reaches 23.5 magnitudes, which is about 1 magnitude fainter than before the coating.