Design and production of VUV bandpass coatings for the mirrors of the SMILE-UVI instrument

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 outof-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.