Better Telescope Mirrors With Silver

UC Santa Cruz researchers worked with Structured Materials Industries to design and build an atomic layer deposition (ALD) system large enough to accommodate telescope mirrors. Andrew Phillips, Nobuhiko Kobayashi, and David Fryauf (l to r) examine the deposition chamber. (Source: T. Stephens, UCSC)

Materials scientist Nobuhiko Koba­yashi wasn’t quite sure why the astro­nomer he met at a wine-tasting several years ago was so interested in his research, but as he learned more about tele­scope mirrors it began to make sense. “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 Kobayashi, a pro­fessor of electrical engi­neering at UC Santa Cruz.

Joseph Miller, former director of UC Obser­vatories (UCO), were interested in a thriving colla­boration between Kobayashi and UC Santa Cruz astro­nomers Andrew Phillips and Michael Bolte. With funding from the National Science Foun­dation and support from current UCO director Claire Max, the researchers are developing new protective coatings for large silver-based tele­scope mirrors by adapting a technique widely used in the micro­electronics industry. According to Phillips, most astro­nomical tele­scope mirrors use aluminum for the reflec­tive layer, despite the superior reflec­tive pro­perties of silver. “Silver is the most reflective material, but it is finicky to work with, and it tarnishes and corrodes easily,” he 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.”

Existing telescopes could substan­tially increase their effi­ciency by recoating their mirrors with silver instead of aluminum. “It is by far the cheapest way to make our tele­scopes effec­tively bigger,” said Bolte. “The reason we want bigger tele­scopes is to collect more light, so if your mirrors reflect more light it’s like making them bigger.” The new coating tech­nology being developed at UC Santa Cruz could make that feasible. The researchers are using atomic layer depo­sition (ALD), which gradually builds a thin film of material, one molecular layer at a time, with excellent uniformity, thickness control, and conformity to the surface of the substrate. In a pilot study, ALD provided much better pro­tective coatings for silver mirror samples than tradi­tional physical depo­sition techniques.

“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.” The results of the pilot study, which used an ALD system in Kobayashi’s lab designed for micro­electronics, convinced the team to design a larger system that could accom­modate telescope mirrors. They filed for a patent on their concept and found an equipment vendor willing to work with them to build the system. The vendor, Structured Materials Indus­tries (SMI) in Piscataway, New Jersey, makes thin-film deposition systems for the micro­electronics industry. “We gave them the concept and our require­ments, and they did the engineering design work and fabri­cation,” Kobayashi said.

The new system was delivered to his labora­tory in July and has performed well in initial testing. The researchers will use the system to demonstrate that it works for tele­scope mirrors and other large substrates and to continue perfecting the coatings. The system can accom­modate a mirror up to 0.9 meter in diameter, and there is no reason the design could not be scaled up to accom­modate even larger mirrors or mirror segments, Phillips said. The 10-meter primary mirrors of the twin Keck Tele­scopes in Hawaii are composed of hexagonal segments 1.8 meters across, and the mirror segments for the Thirty Meter Telescope (TMT) will be 1.4 meters across.

According to Bolte, the desire to use silver on the TMT mirror segments is a major driver of their research on new coating technologies. But he expects the tech­nology will also be used to recoat the mirrors of existing telescopes. An aluminum-coated mirror lasts about three to five years before it needs recoating, a process that puts the tele­scope tempo­rarily out of action. “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 ten years.”

At this point, the researchers are using a physical deposition process to put the silver coating on the mirror blanks along with an initial barrier layer to protect the silver while the mirror is trans­ferred to the ALD system. Atomic layer deposition is then used for the 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. Bolte said the new tech­nology could have a big impact in astronomy, in the same way that the advent of digital detectors to replace photo­graphic plates gave new life to small tele­scopes throughout the world several decades ago. “This is the last trick we have to make existing tele­scopes more effi­cient,” he said. “It could really make a big dif­ference.” (Source: UCSC)

Reference: D. M. Fryauf et al.: Corrosion protection of silver-based telescope mirrors using evaporated anti-oxidation overlayers and aluminum oxide films by atomic layer deposition, SPIE Proc. 99240S (2016); DOI: 10.1117/12.2238749

Link: Jack Baskin School Of Engineering, University of California, Santa Cruz, USA


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