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NASA Investigates Optical Coatings for Far-UV Spectral Range

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To meet the projected goals for its next generation of space telescopes, NASA is taking on a new optical challenge — the fabrication of protective coatings for mirrors to be used for astrophysics studies in the Lyman Alpha range.

So far, no one has developed a coating that effectively protects and maintains an aluminum mirror’s high reflectivity in the 90- to 130-nm range, also known as the Lyman Alpha range. At this spectral regime, scientists can observe a rich assortment of spectral lines and astronomical targets, including potentially habitable planets beyond Earth’s solar system.

“The low reflectivity of coatings in this range is one of the biggest constraints in far-ultraviolet telescope and spectrograph design,” said principal investigator Manuel Quijada.

NASA team developing optical coatings for aluminum mirrors.
Principal investigator Manuel Quijada is shown here with the type of optic he and his team would coat with a fluoride film to provide maximum reflectance over a wide spectral range. Courtesy of NASA/W.Hrybyk.

Quijada is leading a team to investigate techniques and materials for creating and applying protective coatings on highly reflective aluminum mirrors sensitive to the IR, optical and far-UV wavelengths — the spectral range that NASA envisions for proposed space telescopes after the James Webb Space Telescope and Wide Field Infrared Survey Telescope. The goal is to develop a coating and process that not only improves reflectance in the far UV, but also allows observations in the other wavelength bands.

The team is investigating the use of physical vapor deposition to apply a thin layer of xenon difluoride gas to an aluminum sample. According to Quijada, studies have shown that the treatment of xenon difluoride creates fluorine ions that tightly bind to the aluminum surface, preventing further oxidation.

The team is experimenting with two other thin-film deposition techniques, ion-assisted physical vapor deposition and atomic layer deposition, to use for applying thin films of aluminum trifluoride, an environmentally stable coating.

“Traditional coating processes have not allowed the use of aluminum mirrors to their full potential,” Quijada said. “The new coatings we’re investigating would enable a telescope covering a very broad spectral range, from the far ultraviolet to the near-infrared in one single observatory. NASA would get more bang for the buck.”

NASA team developing optical coatings for aluminum mirrors.
Principal investigator Manuel Quijada is dwarfed by the two-meter coating chamber where he applies thin films onto telescope mirrors that can be as large as one meter in diameter. The ability to coat large mirrors is key to enabling astronomical instruments of the future. Courtesy of NASA/W. Hrybyk.

The team has previously developed a coating for another region of the UV spectral band. In 2016, a validation test proved that a protective coating developed by the team provided 90 percent reflectance in the 133.6- to 154.5-nm range — according to the team, the highest reflectance ever reported for this UV band. To achieve this level of performance, the team developed a three-step physical vapor deposition process to coat aluminum mirrors with protective magnesium fluoride or lithium fluoride films.

These high-reflectance coatings are enabling new types of instruments, Quijada said. Two heliophysics missions that will study the interactions between Earth's ionosphere and solar winds will employ this coating technology.

“We need to push further down in the ultraviolet spectrum,” Quijada said, referring to the targeted far-UV spectral range. “We need to get access to the whole ultraviolet to infrared range. We are blazing a trail in mirror coatings.”

This story originally appeared on the NASA web site

Photonics Spectra
Aug 2017
The scientific observation of celestial radiation that has reached the vicinity of Earth, and the interpretation of these observations to determine the characteristics of the extraterrestrial bodies and phenomena that have emitted the radiation.
Research & TechnologyAmericasNASAcoatingsopticsmirrorsinfraredaerospaceastronomyLyman Alpha limitoptical coatingsTech Pulse

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