Astronomical telescope mirrors typically use aluminum for
their reflective layer rather than silver, due largely to aluminum’s superior
corrosion resistance. But researchers say new protective coatings could enable
the use of silver, which offers better reflective properties.
The research is a collaborative effort between Nobuhiko Kobayash, a professor of electrical engineering, and astronomers Andrew Phillips and Michael Bolte, all of the University of California, Santa Cruz (UC Santa Cruz) (Santa Cruz, California).
With funding from the U.S. National Science Foundation (Arlington, Virginia) and support from multi-campus research unit UC Observatories (Santa Cruz, California), the researchers are developing new protective coatings for large silver-based telescope mirrors by adapting a technique widely used in the microelectronics industry.
“Silver is the most reflective material, but it is finicky to work with, and it tarnishes and corrodes easily,” Phillips says. “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.”
“It is by far the cheapest way to make our telescopes effectively bigger,” Bolte says of using silver instead of aluminum. “The reason we want bigger telescopes is to collect more light, so if your mirrors reflect more light, it’s like making them bigger.”
Atomic Layer Deposition
The new coating technology being developed at UC Santa Cruz could make that feasible. The researchers are using a technique called atomic layer deposition (ALD), which gradually builds a thin film of material, one molecular layer at a time. The research team says this provides 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 says. “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.”
Pilot Study Results
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 (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 says.
The new system was delivered to his laboratory in July and has performed well in initial testing, the researchers say. The team adds that they plan to use the system to demonstrate that it works for telescope mirrors and other large substrates, as well as to continue perfecting the coatings.
The system can currently accommodate a mirror up to 0.9 m in diameter, and there is no reason the design could not be scaled up to accommodate even larger mirrors or mirror segments, Phillips says. The 10-m primary mirrors of the twin Keck Telescopes in Hawaii are composed of hexagonal segments 1.8-m across.
Reduced Recoating Requirements
The researchers say a traditional 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 says. “We’d like to have a silver coating that could last five to 10 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 says.
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 says. “It could really make a big difference.”
Source: UC Santa Cruz, www.ucsc.edu.