Advanced Al/eLiF mirrors for the SPRITE CubeSat

The development of high reflectivity, protective mirror coatings for the Lyman UV (lambda < 120 nm) is essential for enabling both CubeSats and large UVOIR missions to tap this rich bandpass. Aluminum mirrors with fluoride-based protective layers are the baseline UV coating technology; these mirrors have been proven to be stable, reliable, and with long flight heritage. However, despite their overall acceptable optical performance, the efficiency of current state-of-the-art Al coatings is still insufficient in the 100-120 nm wavelength range for optical systems in which several reflections are required. Optimizing the efficiency down to 100 nm is essential for the viability and scientific return of the next generation of future FUV-sensitive space telescopes, as was called out in the 2020 astrophysics decadal survey. In this proceeding, we will present recent advances in the development of environmentally stable Al+eLiF+MgF2 mirrors deposited on substrates with increasing thicknesses, developed for the SPRITE CubeSat. SPRITE is projected to be the most sensitive instrument to date in the 100 - 120 nm range and the first orbital imaging spectrograph for this bandpass with sub -arcminute resolution.