City `light pollution' can't cloud sky view of mobile telescopes

ASTRONOMICAL telescopes almost always have the same problems: They are extremely expensive; once you put them someplace they can't be moved; and if a big city springs up next door the urban lights eventually interfere with the view and run you out of business. But researchers at the University of California at Berkeley, led by Nobel laureate Charles Townes, have built a relatively inexpensive pair of telescopes small enough to be carted around on a truck and totally immune to ``light pollution.'' The device will be particularly adept at peeking into the nurseries of stars to watch their birth and early growth.

As if to prove the virtue of the instrument, Dr. Townes plans to first set them up atop Mt. Wilson in the mountains east of Los Angeles, where one of the world's most famous observatories is being scaled down because it cannot see through the glare from Los Angeles.

``The light won't bother us at all,'' said Townes, whose work in the 1950s led to the development of the laser, and a Nobel Prize in 1964. Even regular air pollution ``doesn't make much difference,'' Townes said in an interview.

The telescopes are immune to city lights for the same reason they are particularly good at peeking into the darkest places in the universe: They use the infrared part of the spectrum. Working in tandem, they can detect the shapes, motion, and sizes of young stars even when they are hidden behind clouds of interstellar dust.

Townes expects the telescopes to work with far more precision than the best conventional optical telescopes.

The mobility of the device will enable astronomers to take it almost any place in the world where there are roads. Townes already is talking about sending the telescopes to South America to help with the exploration of the southern sky, which is much less known than the sky over the Northern Hemisphere.

Currently, Townes said, one telescope is in a parking lot at Berkeley, the other is being finished up in the Space Sciences Laboratory.

When deployed, the two telescopes will be laid on the ground instead of sticking skyward. A flat 80-inch mirror will be tilted to reflect incoming starlight onto a 65-inch parabolic mirror, which will aim the light back onto the flat mirror's focal plane. The infrared frequency will be read electronically.

Infrared radiation, which lies between visible red and microwaves in the spectrum, is emitted by any body with a temperature above absolute zero. That includes baby stars. Many newborn stars are invisible to optical telescopes because they are not bright enough to shine through the clouds of dust that gather between galaxies, or they may be still too cool to glow with visible light.

The Berkeley telescope will use two techniques to watch stars being born: It first spots the stars with heterodyne detection. In this method, the signal reaching the telescope is mixed with another signal of a known frequency. The result is a third frequency that can be analyzed with all the delicacy of modern electronics, Townes said.

Naturally, Townes uses a laser to generate the known frequency signal.

The telescopes measure the size of the stars by using an interferometer that ``reads'' the incoming light. The two signals overlap slightly, causing interference with each other. The astronomers read the pattern of this interference.

``We measure the differences in time of arrival of the signals from the star,'' he said. Since the two telescopes can be anywhere from 15 to 1,000 feet from each other, the difference in arrival time would have to be measured in millions of billionths of a second.

Because the telescopes are watching a part of the spectrum below that of visible light, the instruments are oblivious to city street lighting, which blinds more telescopes than air pollution.

The Lick Observatory, atop Mt. Hamilton east of San Jose, Calif., went so far as to persuade the city to install special yellow streetlights, which interfere less with astronomical observation. But Townes's invention makes that rather extreme step unnecessary. The prototype was built in the mid-1970s by Townes and three graduate students and tested for seven years at Kitt Peak National Observatory in Arizona. The total cost of the device was $3 million (cheap by modern standards) and was funded by the US Office of Naval Research. Townes is particularly eager to use the telescopes to watch some puzzling signals coming from the center of the Milky Way galaxy that he thinks may be a black hole.