All-dielectric reflective coating is the use of Optical interferometry, selecting two species with characteristics of high and low refractive dielectric materials, through multiple sets of high and low refractive index materials composed of multilayer overlay, achieve the goal of wide-band and high reflectivity. normally, all-dielectric reflective coating contains dozens, even hundreds of film according to coating performance requirements. compared with traditional metal mirrors, all-dielectric reflective coating have some advantages of a long life, small film absorption, high reflection efficiency within the reflective band. All-dielectric reflecting film has high requirement for coating uniformity. Film stress control is also very important, especially the mirror diameter is big, otherwise it is difficult to realize its application on large aperture and high accuracy optical mirror. 

According to the application demand of astronomical optical telescope, NIAOT make significant progress in blue zone all-dielectric reflective coating and have coated for LAMOST spectrometer. Spectral reflectance is better than 97.6% from 350nm to 590nm, and is significantly higher than silver film. After coating mirror is able to maintain very high surface accuracy: before coating mirror RMS is 0.0149λ, after coating mirror RMS is 0.0135λ(λ=632.8nm). 

Now there are more than 50 layers in blue zone all-dielectric reflective coating  ,so it is expected to increase the service life of more than five years, and the original silver film reflector is only 2 years. in order to further test its performance, the mirror was installed in the LAMOST located on Xinglong Astronomical Observatory for the actual observation on 2 Feb. After the success it will gradually replace the spectrometer blue mirror, and decrease exponentially  maintenance work. 

The next step NIAOT will carry out LAMOST spectrometer all-dielectric mirror in red zone and explore the feasibility of all-dielectric mirror coatings in the whole band (360-900nm). 

result of surface before coating 

result of surface after coating