VUV bandpass reflective coatings for the SMILE-UVI instrument

The Ultraviolet Imager (UVI) is a challenging instrument developed in the frame of the SMILE mission, a collaboration between ESA and CAS. The UVI instrument is a CMOS-based ultraviolet camera developed to image Earth's northern auroral regions. It is centered on the 160-180 nm UV waveband, with a 10° × 10° field of view. At the core of the instrument, four thin film-coated mirrors guide light into its detector and ensure most of the signal filtering, crucial to achieve an out-of-band rejection ratio as low as possible to reject light from solar diffusion, dayglow and unwanted atomic lines. We developed an interferometric coating based on an MgF₂/LaF₃ multilayer stack deposited by ion-assisted electron-beam deposition. We gradually improved our evaporation setup to reach a high degree of homogeneity, precision and repeatability on the material thicknesses, over the entire mirrors surface. The reflectivity maximum is above 85% and the wavelength at which it occurs is adjustable within 1 nm, while the out-of-band reflectivity between 120 and 155 nm and between 200 nm and 1100 nm is kept below 6% on average never exceeding 8 %. The coating has been space qualified and shows stable performances in conditions representative of the instrument operation environment (thermal cycling under vacuum, radiations, UV exposure…).