The Ultraviolet Imager (UVI) is one of the instruments of the ESA-CAS Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) joint mission to image Earth's northern auroral regions over the 160-180 nm UV waveband with a 10° × 10° field of view. The UV light is guided to the detector with four thin film-coated mirrors that ensure most of the signal filtering, crucial to achieve a high out-of-band rejection and limit contributions from solar diffusion, dayglow and unwanted atomic spectral lines. In this paper, we present the design and performances of the spectrally selective reflective coating, which is based on an interferential MgF2/LaF3 multilayer stack deposited by ion-assisted electron-beam evaporation. Its peak reflectivity is above 85 % and with an adjustable central wavelength within 1 nm, whereas the out-of-band reflectivity between 120 nm and 155 nm and between 200 nm and 1100 nm remains below 6 % on average. The coating has been space qualified (thermal cycling under vacuum, radiations, UV exposure...) and shows stable performances in conditions representative of the instrument operation environment.

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…).