Reflecting on mirrors: UV telescopes

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  • Published: Jun 1, 2017
  • Author: David Bradley
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Telescopic coatings

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. CREDIT Credits: NASA/W.Hrybyk

NASA is developing techniques that will allow it to create highly reflective aluminium mirrors that are sensitive to a broad spectrum of electromagnetic radiation from the infrared, through the visible to the far-ultraviolet regions.

Following on from work to make the most reflective telescopic mirrors, Manuel Quijada and his team at the National Aeronautics and Space Administration's Goddard Space Flight Center in Greenbelt, Maryland, USA, are now investigating broadband mirrors destined for the envisioned space telescopes after the James Webb Space Telescope and Wide Field Infrared Survey Telescope. The new instruments and the missions in which they will play a role should be able to tackle a wide range of astrophysics studies, from gaining new insights into the epoch of reionization across the universe, through galaxy formation and evolution, to star and planet formation.

Specifically, Quijada and his colleagues are looking into three different techniques and materials for creating and applying protective coatings to low-density aluminium mirrors that should prevent these sensitive components from being ruined by the formation of an oxidized layer on their surface when they are exposed to oxygen and so losing their sharp reflectivity. "Aluminium is a metal that nature has given us the broadest spectral coverage," Quijada explains. "However, aluminium needs to be protected from naturally occurring oxides with a thin film or substrate of transparent material."

Broadband mirrors

The problem facing the NASA technologists is that nobody has ever been able to develop a way to protect a piece of aluminium from oxidation processes without interfering with the high reflectivity of the mirror. High reflectivity in the 90 to 130-nanometre range, which is known as the Lyman Alpha range is critical to the success of such a telescopic mirror. In this spectral region, astronomers can observe a rich assortment of spectral lines and astronomical targets, which might even include potentially habitable exoplanets. "The low reflectivity of coatings in this range is one of the biggest constraints in far-ultraviolet telescope and spectrograph design," Quijada said.

One of the more recent NASA missions was dedicated entirely to far-ultraviolet observations, the Far Ultraviolet Spectroscopic Explorer, or FUSE. However, this was decommissioned in 2007 after a successful prime mission. It acquired some 6000 observations of almost 3000 separate astronomical objects over its eight years in orbit, but FUSE's lithium fluoride substrate coating was not stable enough and began to degrade. Quijada's goal is to develop a coating and process that not only improves reflectance in the far ultraviolet, but also allows observations in the other wavelength bands.

Bringing mirrors into range

"Traditional coating processes have not allowed the use of aluminium mirrors to their full potential," Quijada adds. "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."

For instance, the team has investigated the efficacy of the physical vapour deposition method to apply a thin layer of xenon difluoride gas to an aluminium sample. According to Quijada, studies have shown that the treatment of xenon difluoride creates fluorine ions that bond tightly to the aluminium surface, which precludes oxidation. The team is also looking at whether one of two other thin-film deposition techniques - ion-assisted physical vapour deposition and atomic layer deposition - might also be used to apply thin films of aluminium trifluoride to the metal. This substance is thought to be even more environmentally stable than other known and experimental coatings. The researchers have so far demonstrated success with a coating for another region of the ultraviolet spectral band. They could maintain 90 percent reflectance in the 133.6-154.5 nm range. This was the highest reflectance reported for that particular portion of the UV. The coating preparation involved a three-step physical vapour deposition process to coat aluminium mirrors with protective magnesium fluoride or lithium fluoride films.

The team suggests that such protective and high-reflectance coatings are already facilitating the construction of new types of instruments. Two new heliophysics missions that will study the interactions between Earth's ionosphere and solar winds - the Ionospheric Connection Explorer and the Global-scale Observations of the Limb and Disk -will use this coating technology.

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

Related Links

NASA 2017, online: "NASA Team Takes on a New Optical Challenge — the Lyman Alpha Limit"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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