Extreme Photonics: Mega Mirrors

Dan Drollette, Senior Editor

When Mary Edwards wanted to look for interior flaws in the glasswork her colleagues were making, she took off her jewelry, removed her shoes and belt buckle, and emptied her pockets of sharp objects. The senior product engineer at Corning Inc. in Canton, N.Y., then stepped very carefully onto the surface of a mirror 8.3 m in diameter.

The glasswork was destined to become the largest single (monolithic) mirror ever made and the heart of Japan's new Subaru telescope. ("Subaru" is Japanese for the Seven Sisters, a cluster of stars that Westerners call the Pleiades.) Like all telescope mirrors of extreme size, the Subaru mirror is a one-of-a-kind item: It was too expensive for the manufacturer or the customer to cast a backup. Consequently, there was little margin for error.

Handling an 8-m-class primary mirror is difficult enough, but astronomers already are sketching plans for telescopes 12 times as large as the Subaru. There are plans for telescopes with mirrors that are 30, 50, even 100 m across. A mirror the size of a football field may make some astronomers scoff, but Roberto Gilmozzi, director of the European Southern Observatory's Paranal Observatory telescope in Chile and promoter of the proposed 100-m telescope, is undeterred: "If we build this, it will have 10 times the collecting area of every telescope ever built put together."

If astronomers can make such quantum jumps in size, they will overcome the forces of history. Every time the size of a conventional mirror is doubled, the complexity and the cost increase eight times; consequently, it used to take 20 to 30 years between each jump. Now engineers hope to shorten the cycle.

They expect to do so by using optical interferometry, in which the light beams from several mirrors, tens or hundreds of meters apart, combine at a central detector, and by using adaptive optics, in which thin, flexible mirrors move hundreds of times per second to compensate for the distorting effects of turbulence in the Earth's atmosphere.