While many materials could be used as IR mirrors, only aluminum has the potential of reaching far into the ultraviolet, as low as 85 nm. Unfortunately, Al oxidizes rapidly when it is exposed to the atmosphere, diminishing the reflectance of bare aluminum mirrors below 200 nm. For terrestrial observations, this is not a large problem, since the Earth’s atmosphere blocks radiation below this wavelength. However, mirrors used in space could use the whole range of aluminum’s reflectance, if a bare, or nearly bare mirror, could be deployed. The compromise to date has been to use a thin UV-transparent, protective barrier composed of magnesium fluoride, lithium fluoride and/or aluminum fluoride. These allow the observation of the Lyman alpha line but light below 110 nm is effectively blocked. We report on our studies of ultrathin fluoride barrier layers and of removable layers which would lie on ultrathin fluoride barriers or directly on the aluminum surface. Removable barriers could be removed shortly before launch or at the mirrors point of use. These removable barriers consist of organic layers like First Contact™ or parylene, inorganic layers like amorphous silicon (a-Si), or a combination of both. We have observed, for example, that ultrathin films of AlF3 (<2.5 nm) are capable of blocking the oxidation of aluminum for several hours in air, long enough to have a removable barrier like First Contact™ applied on their surface. We also report on the effectiveness of ultrathin a-Si as a long-term barrier that may be removable via atomic hydrogen.
David D. Allred, Yhoshua Wug-Jerez, R. Steven Turley, J. Gabriel Richardson, and James P. Hamilton, "A study of ultrathin fluoride and removable barrier films on aluminum for space-observatories with far UV observations. (Conference Presentation)," Proc. SPIE 10699, Space Telescopes and Instrumentation 2018: Ultraviolet to Gamma Ray, 1069903 (Presented at SPIE Astronomical Telescopes + Instrumentation: June 10, 2018; Published: 10 July 2018); https://doi.org/10.1117/12.2314382.5807166322001.
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