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Traditional mirror coatings for the “Lyman UV” (wavelengths less than 115 nm) do not reflect light very well. The last NASA mission with sensitivity to the Lyman UV was known as FUSE and it used lithium fluoride protected aluminum mirror coatings (LiF+Al). That’s a complicated way of saying that a thin film of aluminum on top of the glass mirror is a great reflector of ultraviolet light, but aluminum oxidizes within seconds when exposed to air, and aluminum oxide is a poor ultraviolet reflector. To prevent the formation of this aluminum oxide layer, we immediately overcoat the aluminum with an even thinner layer of a clear material that lets the light through to the aluminum, but doesn’t let the air through to oxidize the surface. This is the lithium fluoride, which is the clearest material known for Lyman UV photons (it’s possible beryllium fluoride is clearer, but beryllium is toxic!)

Unfortunately, lithium fluoride is a poor material for evaporating into a thin film. Mirrors made in this fashion rarely reflect more than 65% of the ultraviolet light that hits them, and when you have 7 mirrors in your instrument, like the LUVOIR UV spectrograph, then you throw away a lot of light! Including the losses associated with the gratings and detectors, a LUVOIR UV spectrograph with LiF+Al optics would reject 99 out of every 100 photons that enter into the telescope. On top of all that, LiF is sensitive to humidity and degrades slowly over time when exposed to ambient air.

These problems, which have existed for over 50 years, have finally shown some progress thanks to breakthroughs at NASA Goddard Space Flight Center and Jet Propulsion Laboratory, as well as the University of Colorado, in creating better LiF films with ultra-thin humidity resistant protective layers. This new form of “enhanced lithium fluoride”, or “eLiF” is roughly 85% reflective in the Lyman UV and has increased resistance to humidity (see Fleming et al., 2017). This advancement improves the throughput of LUVOIR-LUMOS by nearly 600%! These coatings were baselined in the LUVOIR study, but are considered low “Technology Readiness Level” (TRL) because they have limited heritage on smaller missions. For all we know, the protective coating could degrade under the harsh radiation of space, or perhaps the deposition process itself, which involves heating the bare glass to over 250 degrees Celsius, could have a long term impact on the telescope optics. Until the TRL can be raised, eLiF is not viable for large NASA missions. SPRITE will be the first mission to test eLiF coatings in an orbital environment, qualifying eLiF mirror coatings to TRL 8, opening up massive new potential for exciting UV science!