Advanced Environmentally Resistant Lithium Fluoride Mirror Coatings for the Next-Generation of Broadband Space Observatories

TL;DR

This paper discusses the development and qualification of advanced lithium fluoride-protected aluminum mirror coatings with enhanced durability and reflectivity for next-generation space observatories operating in the far-ultraviolet spectrum.

Contribution

It introduces a new ultrathin capping layer to protect LiF-based coatings, improving their durability for space applications.

Findings

Enhanced reflectivity of LiF-protected aluminum mirrors.

Development of a protective ultrathin capping layer.

Assessment of coating performance for space mission qualification.

Abstract

Recent advances in the physical vapor deposition (PVD) of protective fluoride films have raised the far-ultraviolet (FUV: 912-1600 {\AA}) reflectivity of aluminum-based mirrors closer to the theoretical limit. The greatest gains, at more than 20%, have come for lithium fluoride-protected aluminum, which has the shortest wavelength cutoff of any conventional overcoat. Despite the success of the NASA FUSE mission, the use of lithium fluoride (LiF)-based optics is rare, as LiF is hygroscopic and requires handling procedures that can drive risk. With NASA now studying two large mission concepts for astronomy, Large UV-Optical-IR Surveyor (LUVOIR) and the Habitable Exoplanet Imaging Mission (HabEx), which mandate throughput down to 1000 {\AA}, the development of LiF-based coatings becomes crucial. This paper discusses steps that are being taken to qualify these new enhanced LiF-protected aluminum (eLiF) mirror coatings for flight. In addition to quantifying the hygroscopic degradation, we have developed a new method of protecting eLiF with an ultrathin (10-20 {\AA}) capping layer of a non-hygroscopic material to increase durability. We report on the performance of eLiF-based optics and assess the steps that need to be taken to qualify such coatings for LUVOIR, HabEx, and other FUV-sensitive space missions.