New coats for telescopes

Silver makes better telescope mirrors. It reflects more visible and infrared light than aluminum, the metal most telescope mirrors are made from. But while aluminum forms a natural protective layer when exposed to air, silver doesn’t—that’s why it tarnishes.

UCSC researchers worked with Structured Materials Industries (SMI) to design and build a prototype atomic layer deposition system large enough to fit telescope mirrors (l to r: graduate student Brian Giraldo, SMI’s Gary Provost, and postdoc David Fryauf). Credit: Nobby Kobayashi.

To combat this degradation, telescope makers coat the silver with a protective layer using physical vapor deposition, spraying, for instance, aluminum oxide vapor onto the silver. Even so, the mirrors still corrode after a couple years.

The problem is that this coating method leaves pinhole-like gaps. “All kinds of stuff—oxygen, water, sulfur—those elements go through the pinholes to reach the silver,” said Nobby Kobayashi, professor of electrical engineering.

With collaborators UC Observatories astronomers Andrew Phillips and Michael Bolte, UCSC postdoc David Fryauf, and Structured Materials Industries (SMI), Kobayashi designed a new instrument that employs atomic layer deposition (ALD) to coat the silver. Widely used in making semiconductor chips, ALD creates a dense, even film by coating surfaces one atomic layer at a time.

Because ALD systems are designed for small, thin silicon wafers, the researchers had to build a much larger one to accommodate telescope mirrors. While the prototype is still being tested, proof-of-principle experiments have shown promising results, Kobayashi said.

Marcus Woo