22 October 1999 Aging studies of LiF-coated optics for use in the far ultraviolet
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The FUSE is an astrophysics mission especially designed to access the quite rich spectral region between 90.5 nm and 118.7 nm with a high spectral resolving power. The FUSE instrument contains four identical off-axis paraboloid telescope mirrors and four spherical diffraction gratings. Two mirrors and two gratings are coated with silicon carbide (SiC) and have a bandpass of 90.5 nm to 110.0 nm. The remaining two mirrors and gratings are coated with lithium fluoride (LiF) over aluminum (Al) providing about twice the reflectivity of the SiC at wavelengths larger than 105.0 nm but very little reflectivity below 102.0 nm. The Far UV reflectivity of the Al + LiF coated FUSE optics is very sensitivity to moisture and molecular hydrocarbon contamination. To avoid degradation of the reflectivity all optics testing and handling has been carefully controlled to minimize the exposure of the coating to ambient air. In general the optical surfaces were kept in nitrogen purged enclosures. We report on a simple test program in which small Al + LiF witness mirrors were stored in different relative humidity (RH) environments in order to study the degradation of their reflectivity between 92.7 nm and 121.6 nm as a function of time. The result of this study were used to establish guidelines for storage and test environments for FUSE optics prior to launch. Our methods and results are then compared to a similar aging study performed by the NASA/GSFC Optical Thin Film Laboratory.
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Cristina M. Oliveira, Cristina M. Oliveira, Kurt Retherford, Kurt Retherford, Steven J. Conard, Steven J. Conard, Robert H. Barkhouser, Robert H. Barkhouser, Scott D. Friedman, Scott D. Friedman, "Aging studies of LiF-coated optics for use in the far ultraviolet", Proc. SPIE 3765, EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy X, (22 October 1999); doi: 10.1117/12.366542; https://doi.org/10.1117/12.366542

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