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6 July 2018 Performance and design of MgF2 + Au coatings on aluminum mirrors: enabling far-ultraviolet solar occultation measurements for Europa-UVS
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Abstract
Europa-UVS plans to characterize the Europan exosphere by performing solar occultation measurements at key points during the Europa Clipper mission. Observing the Sun from Jovian space requires a system with high dynamic range. The high end of the dynamic range of the Europa-UVS MCP-XDL detector is limited by dead time effects and is rated at ~ 300 kHz, corresponding to a <1.2 μs dead time requirement. The global count rate is estimated to exceed this upper limit by a factor of two during solar occultation measurements, due to the dramatic increase in solar photon flux towards the high end (> 150 nm) of the Europa-UVS bandpass (55-210 nm). To reduce the input photon flux in the Europa-UVS optical train, we propose applying a thin MgF2 overcoat on the heritage bare gold solar port mirror adopted from the New Horizons Alice solar occultation channel and the JUICE-UVS solar port. The MgF2 layer, with the distinct advantage as the standard coating on the other heritage optics for Europa-UVS, suppresses the reflectance of the solar port mirror, especially above 140 nm, compensating the solar photon flux increase. The decrease in reflectance has been modeled using Fresnel reflectance theory and verified experimentally by comparing reflectance of Au and MgF2-Au mirrors. With the implementation of the MgF2 layer together with a reduction in the solar port aperture size, we predict global count rates that are well-matched to the 300 kHz threshold of the Europa-UVS detector.
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Ujjwal Raut, Preston L. Karnes, Kurt D. Retherford, Michael W. Davis, G. R. Gladstone, Thomas K. Greathouse, Brandon Walther, and Elizabeth Czajka "Performance and design of MgF2 + Au coatings on aluminum mirrors: enabling far-ultraviolet solar occultation measurements for Europa-UVS", Proc. SPIE 10699, Space Telescopes and Instrumentation 2018: Ultraviolet to Gamma Ray, 106992Z (6 July 2018); https://doi.org/10.1117/12.2312928
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