Mirror reflectivity is an important indicator of the telescope performance, and it is an important factor to determine the efficiency of astronomical observation. In order to maintain the reflectivity of the LAMOST telescope, cleaning mirror and recoating is required. The coating of large aperture optical elements, especially the high reflective film, first need to have a clean high vacuum environment. 

Recently, the staff of LAMOST technical maintenance has carried out the transformation of the oil free system of 1600 coater. Prior to the transformation, the coating equipment use the diffusion pump system, and is a kind of oil vacuum system. It is very low for water vapor extraction rate, film quality is affected by oil vapor pollution and environmental humidity. After many tests and analysis, technical maintenance personnel have successfully transformed into a non oil vacuum pump system.

After testing, the transformation of the equipment run stable, eliminating the oil vapor pollution and environmental humidity on the film, while the limit vacuum degree increased by an order of magnitude, from the original 5x10E-4 Pa to 6.5x10E-5 Pa. And aluminum mirror coating under the oil-free vacuum environment is effectively improved due to the removal of oil pollution and the improvement of the degree of vacuum, the UV band reflector efficiency after the transformation has been improved significantly.

Appearance of the modified coater

Appearance of the coater before modification