Advanced Al mirrors with passivated LiF for environmentally stable 1-meter class UV space telescopes

The recently released 2020 Decadal Survey on Astronomy and Astrophysics has prioritized a large (≈ 6 m aperture) infrared/optical/ultraviolet (IR/O/UV) space telescope as the next NASA flagship mission with the scientific goal of searching for signatures of life on planets outside of the solar system in addition to perform other critical astrophysical observations using the same platform. The report observes that the outstanding science discoveries that this mission is proposed to accomplish will only be feasible if critical technologies reach TRL 6 or above in the next 5 years. Among the technology needs mentioned, the IR/O/UV telescope requires improvements in the mirror optical coatings. This proposal will address these needs. Our team at the Goddard Space Flight Center (GSFC) has recently developed a new reactive Physical Vapor Deposition (rPVD) process (Quijada et al., Mirror Tech Days 2021) that consists in fluorination/passivation of thermally evaporated Al mirrors protected with a LiF overcoat. These coatings offers the highest ever reported Far-Ultraviolet (FUV) reflectance performance while exhibiting a remarkable environmental stability. This new coating technology also satisfies all the reflectivity requirements as specified in the Large UV-Optical-Infrared Surveyor (LUVOIR) Science and Technology Definition Team (STDT) report. Moreover, the entire coating process is carried out at ambient temperature and a technology readiness level (TRL) of up to 3 has been demonstrated at a small scale. This proposal effort will advance the TRL of this mirror coating process to 5-6 by performing two tasks. First, we will transfer this rPVD process to our largest 2-meter chamber at GSFC in order to enable the coating of a 1-meter diameter class mirror. Secondly, we will validate this process by coating the primary mirror (0.5 meter diameter) and gratings (150 x150 mm2) of the Far-UV Off Rowland-circle Telescope for Imaging and Spectroscopy (FORTIS) sounding rocket (PI: S. McCandliss) that is being developed at John Hopkins University. This instrument concept will be operating in the FUV (90-150 nm) spectral region to carry out laboratory experiments and design studies. A parallel effort in this proposal will be to conduct a comprehensive study to better understand the environmental stability observed in mirror coatings produced with the rPVD process. Although the outstanding stability of these coatings have been demonstrated in the lab, the fundamental physics that ultimately drive this behavior is not fully understood to enable optimization and an informed strategy for preflight hardware processing. Thus, we propose to combine the optical coatings expertise at GSFC with the expertise in materials analysis at Brigham Young University (Prof. David Allred), which has recently studied the optical, microscopic and structural changes of bare LiF thin films exposed to humid air. We will establish a collaboration to perform a similar study on these Al+xeLiF mirrors fabricated through the rPVD process. A better understanding of the degradation mechanisms on Al+LiF mirrors will help to further improve its performance, particularly in the range of 91.2-100 nm.

The Strategic Astrophysics Technology program (SAT) supports focused development efforts for key technologies to the point at which they are ready to feed into major missions in the three science themes of the Astrophysics Division: Exoplanet Exploration, Cosmic Origins, and the Physics of the Cosmos. This program is specifically designed to address middle technology readiness level (TRL) "gaps" between levels 3 and 6: the maturation of technologies that have been established as feasible, but which are not yet sufficiently mature to incorporate into flight missions without introducing an unacceptable level of risk. NASA does not require a data management plan for proposals to SAT.

This is early/mid-stage (TRL 3) — the most realistic path in is NASA SBIR/STTR, which funds small businesses and research institutions to develop technology aligned with NASA's needs (equity-free, phased funding). Check whether a current SBIR/STTR solicitation topic overlaps with this project's technology area, or contact the project directly (above) to ask.

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