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12 September 2019 ZERODUR® substrates for application of high-temperature protected-aluminum far ultraviolet coatings
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Abstract
Recent development in coating deposition processes for aluminum (Al) mirrors that are protected with a metal-fluoride overcoat (such as LiF, MgF2, or AlF3) have improved reflectance performance particularly in the far- ultraviolet (FUV) part of the optical spectrum. The active research in this area is motivated by the fact that these gains in reflectance are expected to significantly increase the throughput of any future FUV sensitive NASA missions into the Lyman Ultraviolet. These reflectance improvements are attributed, in part, by performing the metal-fluoride overcoat depositions with the substrates at an elevated temperature as high as 250 °C. ZERODUR® is a widely used material as a mirror substrate because, among other things, it exhibits a low coefficient of thermal expansion (CTE) over a wide range of temperatures. Moreover, ZERODUR® has recently been proposed for several future NASA concept missions where this improved FUV mirror coating may be used. Given the elevated temperature at which these improved FUV coatings are produced, it is imperative to make sure that heating of the substrate will not significantly impact the final figure of the coated mirror. In this paper, we will study and report the effects of heating ZERODUR® up to the highest temperature mentioned above (250 °C) during a simulated coating process. These studies are relevant since it has been reported the CTE will change if ZERODUR® is cooled down from application temperatures between 130°C and 320°C with rates that differ from the initial production annealing rate of 3°C/hr.
© (2019) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Manuel A. Quijada, David A. Sheikh, Javier G. Del Hoyo, and J. Gabriel Richardson "ZERODUR® substrates for application of high-temperature protected-aluminum far ultraviolet coatings", Proc. SPIE 11116, Astronomical Optics: Design, Manufacture, and Test of Space and Ground Systems II, 111160T (12 September 2019); https://doi.org/10.1117/12.2530585
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