Establish the effectiveness of modern lithium-fluoride coatings under integration conditions

Establish the final efficacy and environmental robustness of modern lithium-fluoride mirror coatings for HWO ultraviolet instrumentation, particularly their degradation under humidity exposure during integration, testing, and handling.

Background

Ultraviolet optical systems are highly sensitive to particulate, hydrocarbon, and humidity contamination. The proposed HWO ultraviolet instrument relies on lithium-fluoride-based mirror coatings, whose performance can degrade under humid conditions.

The paper reports that newer coatings appear less hygroscopic than conventional coatings, but the authors identify the final effectiveness of that development as unresolved. This uncertainty affects contamination-control procedures, human-safety practices, integration planning, and the expected end-to-end ultraviolet throughput of the observatory.

References

While modern LiF coatings have reduced sensitivity to hygroscopic effects, the final efficacy of that development is not fully understood.

— An Ultraviolet Instrument on the Habitable Worlds Observatory: Considerations and Capability Trades on the Multi-Object, Integral-field, and High-resolution modes of a potential UVI  (2610.12974 - Fleming et al., 9 Oct 2026) in Section 4.2, subsection “UV Hardware Protection During Integration and Testing”