AUSTIN, TEXAS -- Given the world's largest telescope, the Keck I, what astronomer would throw away more than 95 percent of the light it collects?

Yet that is what a team of astronomers at the University of California, Berkeley, has done in order to obtain the best interferometer in the infrared region of the spectrum, the invisible wavelengths just beyond red.

They placed a mask over the telescope that reduced the 10-meter (400-inch) diameter telescope to a set of 21 small slits. Interference between the light coming through pairs of holes can paint a detailed picture of distant stars that appear as mere points of light through the unmasked Keck.

"This is a new use for the Keck Telescope and a new technique that makes the Keck still more versatile and spectacular," says Charles H. Townes, a professor of physics in the Graduate School at UC Berkeley and a Nobel Laureate in Physics. Townes will discuss the interferometer results in his Russell Prize Lecture on Jan. 6 at 11:40 a.m. CST at the 193rd national meeting of the American Astronomical Society. The meeting is being held January 5-9 at the Austin Convention Center in Austin, Texas.

In his talk, entitled "The Quest for High Spatial Resolution," Townes also will discuss high-resolution imaging with UC Berkeley's Infrared Spatial Interferometer of stars in the thermal or mid-infrared region of the spectrum. This unique instrument was built by Townes and Senior Space Fellow William C. Danchi of UC Berkeley's Space Sciences Laboratory and is parked on the side of Mt. Wilson in Southern California.

"Both these instruments give unprecedented high resolution results in the infrared," Danchi says. "They are revolutionizing our understanding of stars that hitherto appeared to be point-like, many of which are cooler than our Sun and heavily obscured by dust, making imaging at infrared wavelengths all important."

Graduate student John D. Monnier also is presenting a paper at the meeting on Jan. 8 in which he describes use of the Keck aperture masking technique to detail the structure and composition of dust envelopes surrounding stars both young and old. The work was done by postdoctoral fellow Peter Tuthill, Monnier and Danchi.

Interferometry is a technique more regularly used in radio astronomy, where signals from separate telescopes are combined as if from one very large telescope. An example is the Very Large Array in Socorro, New Mexico, which has 27 telescopes separated by distances as large as tens of kilometers.

Interferometric techniques are much less developed at infrared and optical wavelengths, Danchi says, due in part to the very tight optical tolerances needed and the effects of the atmosphere, which causes the twinkling of stars.

The atmospheric problems can be partly eliminated, though, by combining signals from three separate telescopes to form "closure phases."

In the case of the Keck I Telescope, an aperture mask consisting of 21 slots in an aluminum plate is placed in front of the secondary mirror, blocking all but about three percent of the light. The Keck then behaves like a collection of 21 small 35-centimeter telescopes.

"This mask, designed by Tuthill, provides 210 separate interferometers all at once, yielding a very good angular resolution and a detailed picture in the infrared," Townes says.

Fringe patterns from the interference of these smaller "telescopes" are recorded with the facility's near-infrared camera (NIRC) at the focus of the Keck I telescope and reconstructed into an image using software originally developed for radio interferometry.

So far, complex images with interesting structure were observed for every type of star looked at, Danchi says. Among these are AGB stars, Wolf-Rayet stars and Young Stellar Objects or YSOs.

AGB stars are intermediate mass stars near the end of their lives, losing material to the interstellar medium through stellar winds. Over the course of time this material, along with material from supernovae, is recycled into new stars, planets and other objects.

Using the Infrared Spatial Interferometer, Danchi, Townes, and their associates discovered recently that many of these AGB stars do not lose material continuously as had been previously thought. Instead they emit clouds of gas and dust episodically.

"Some kind of episodic instability causes them to puff off material on a time scale of tens to hundreds of years," Townes says. The physical origin of the time scales is not understood and is still under investigation.