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Deposition Method Developed for Telescope Mirrors

Monday, October 9, 2017

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Researchers based out of the University of California Santa Cruz have begun developing a protective coating for telescopes by reworking a method commonly used in microelectronics.

"It turns out that improving the performance of mirrors is all about thin-film materials, and that's what I do. So then I got hooked," said Nobuhiko Kobayashi, a professor of electrical engineering in the Baskin School of Engineering at UC Santa Cruz.

Working alongside astronomers Joseph Miller, Andrew Phillips and Michael Bolte, Kobayashi received funding for the research project from the National Science Foundation, along with support from UC Observatories director Claire Max. All of this is geared toward the development of protective coatings for large, silver-based telescope mirrors.

Telescope Coatings

Most telescopes use aluminum instead of silver for the reflective layer of the astronomical mirrors, despite silver’s superior reflective properties.

Images: UC Santa Cruz

Researchers based out of the University of California Santa Cruz have begun developing a protective coating for telescopes by reworking a method commonly used in microelectronics.

"Silver is the most reflective material, but it is finicky to work with, and it tarnishes and corrodes easily," Phillips said. "You need barrier layers on top that can keep anything from getting through to the silver without messing up the optical characteristics of the mirror."

Telescopes already in operation could become more efficient and cost-effective if their mirrors were recoated in silver, noted Bolte. He went on to add that the new coating would effectively make the mirror bigger.

"The reason we want bigger telescopes is to collect more light, so if your mirrors reflect more light it's like making them bigger,” he explained.

Coating Development

The new coating being developed at UC Santa Cruz could make this possible, according to the university, using the technique called atomic layer deposition. The process builds a thin film of material over time with consistent uniformity, thickness control and substrate surface conformity. After a pilot study, the use of ALD resulted in better protective coatings for samples of silver mirrors, rather than more traditional physical deposition methods.

"Atomic layer deposition performs significantly better," Phillips said. "The problem is that the systems used in the electronics industry are designed for silicon wafers, so they're too small for a telescope mirror."

Sizing Up Technology

During the pilot study, the team used an ALD system at Kobayashi’s lab that was designed for microelectronics, which later inspired them to design a larger setup that could accommodate telescope mirrors. After filing for a patent and finding Structured Materials Industries, a Piscataway, New Jersey-based thin-film deposition systems manufacturer, the team had its new system in the lab by July of this year.

Working alongside astronomers Joseph Miller, Andrew Phillips and Michael Bolte, Kobayashi received funding for the research project from the National Science Foundation, along with support from UCO director Claire Max. All of this is geared towards the development of protecting coatings for large silver-based telescope mirrors.

Currently, the system can accommodate a mirror that is 0.9 meters in diameter, but, Phillips noted, there is no reason that the system cannot be scaled up to cater to larger mirrors. The mirror segments for the Thirty Meter Telescope will be 1.4 meters across, for example. The twin Keck Telescopes in Hawaii, where UC Obersvatories is a managing partner, are comprised of hexagonal segments that are 1.8 meters across, totaling in 10-meter primary mirrors.

Research Motivation

Using silver on TMT mirror segments has been a major motivation in the team’s research on new coating technologies, Bolte said. The researcher also anticipates that the coating will be used on existing telescopes, given that traditional aluminum-coated mirrors last about three to five years before recoating becomes necessary.  

"We hate to lose telescope time, and we lose a lot of nights recoating segments at Keck," Phillips said. "We'd like to have a silver coating that could last five to 10 years."

Currently, researchers are using physical deposition to coat mirror blanks in silver, along with an initial barrier layer to protect the silver while it is transferred to the ALD system. From there, ALD is used to form final barrier layers.

"Right now, it's a hybrid process, but we're following the development of atomic layer deposition for the silver coating as well," Phillips said.

This research could have an impact on astronomy, according to Bolte.

"This is the last trick we have to make existing telescopes more efficient," he said. "It could really make a big difference."

Previous Research

A team of researchers at NASA’s Goddard Space Flight Center has been working to develop the perfect coating to protect highly reflective aluminum telescope mirrors that the agency said are sensitive to three bands of wavelength: infrared, optical and far-ultraviolet.

Aluminum has been the key to developing the mirrors themselves, but the problem the team encountered was that the aluminum substrate easily oxidizes, leading to a loss of reflectivity.

NASA said that one approach involved the physical vapor deposition of a thin layer of xenon difuoride gas onto the aluminum surface. The fluorine ions that are formed then prevent the aluminum from oxidizing.


Tagged categories: Aerospace; Colleges and Universities; Research; Research and development

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