Development of VUV multilayer coatings for SMILE-UVI instrument

The Ultraviolet Imager (UVI) instrument is a very challenging imager developed in the frame of the SMILE-ESA mission. The UV camera will consist of a single imaging system targeted at a portion of the Lyman-Birge-Hopfield (LBH) N2 wavelength band. The baseline design of the imager meets the requirements to record snapshots of auroral dynamics with sufficient spatial resolution to measure cusp processes (100 km) under fully sunlit conditions from the specified apogee of the spacecraft. To achieve this goal, the UVI instrument utilizes a combination of four on-axis mirrors with an intensified FUV CMOS based camera. The mirrors will be coated with spectral selective interferometric layers to provide most of the signal filtering. The objective of these filters is to select the scientific waveband between 160 and 180 nm. The combined four mirrors have to give an out-of-band rejection ratio as high as possible to reject light from solar diffusion, dayglow and unwanted atomic lines in a range of 10-8 – 10-9. Different multilayer coatings are considered and optimized according to the π- multilayer equation for different H/L ratio and for different angles of incidence. Our theoretical evaluation shows a least a modification of the reflectance spectrum as a function of the angle of incidence, so that the optical beams hitting the different mirrors can have different optical properties depending on the optical fields and the distribution of the rays on the pupil. In this paper the effect of fields and coating homogeneity on the spectral throughput of the UVI instrument will be assessed and described.