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.
The Southwest Research Institute Ultraviolet Reflectance Chamber (SwURC) is a highly capable UV reflectometer
chamber and data acquisition system designed to provide bidirectional scattering data of various surfaces and
materials. The chamber provides laboratory-based UV reflectance measurements of water frost/ice, lunar soils,
simulants, and analogs to support interpretation of UV reflectance data from the Lyman Alpha Mapping Project
(LAMP) Lunar Reconnaissance Orbiter (LRO). A deuterium lamp illuminates a monochromator with a nominal
wavelength range of 115 nm to 210 nm. The detector scans emission angles -85° to +85°in the principal plane. Liquid
nitrogen passed through the sample mount enables constant refrigeration of tray temperatures down to 78 K to form
water ice and other volatile samples. The SwURC can be configured to examine a wide range of samples and
materials through the use of custom removable sample trays, connectors, and holders. Calibration reference standard
measurements reported here include Al/MgF2 coated mirrors for specular reflection and Fluorilon for diffuse
reflectances. This calibration work is a precursor to reports of experiments measuring the far-UV reflectance of water
frost, lunar simulants, and Apollo soil sample 10084 in support of LRO-LAMP.