Improved Lyman-Ultraviolet Astronomy Capabilities through Enhanced Coatings

The  Large Ultraviolet ,Visible, Optical, Infrared (LUVOIR) telescope concept is one of the  NASA mission concept studies for the 2020 decadal study that will merge ultraviolet (UV) astrophysics and visible exoplanet science. This proposal aims at developing broadband reflecting mirror coatings with high performance that could be used on the primiary mirror of LUVOIR in order to enable wavelength coverage from the infrared and down to the Far-Ultraviolet (FUV) spectral regions. Improved reflective coatings for optics, particularly in the FUV region (90-120 nm), could yield dramatically more sensitive instruments and permit more instrument design freedom. The coating performance developed through this work will be evaluated both theoretically and experimentally in the context of meeting requirements for exoplanet research.

Pure Aluminum exhibits a high reflectance over the proposed spectral range of the LUVOIR observatory (90-5000 nm). However, the Al has to be protected from the naturally occurring Al₂O₃ oxide layer (when exposed to oxygen) with a thin film of a transparent material for use below 130 nm. Aluminum protected with fluorides such as LiF or MgF₂ have been the most commonly used solutions [4]. But below 102 nm down to 90 nm, no transparent material is available to protect Al and coating mirror reflectance stays well below 30%. But even above 102 nm, the reflectance of protected Al is limited by the residual absorption of the fluoride overcoats and the hygroscopic nature of the LiF overcoat.  The low reflectivity of coatings in the Lyman Ultraviolet (LUV) range of 90-130 nm is one of the biggest constraints on FUV telescope and spectrograph design, and it limits the science return of FUV-sensitive space missions. In fact, to achieve high-reflectance in broadband coatings has been identified as an “Essential Goal” in the technology needs for the LUVOIR) surveyor observatory. Improved reflective coatings for optics, particularly in the LUV spectrum, could yield dramatically more sensitive instruments and permit more instrument design freedom. Furthermore, investigations in improving reflectance performance of coatings in this region has the potential to offer unprecedented return on investment given that the LUV contains the highest density of line observations in the UVVIS-IR spectrum.