The two telescopes are a pair of identical twins called Keck 1 and Keck 2 atop Mauna Kea on the island of Hawaii. Each individual Keck has a segmented mirror array measuring about 30 feet (10 meters) across. When the two telescopes' light is combined though, it's as if it was a single unit as large as the 275 feet (85 meters) separating them.

"Successfully combining the light from the two largest telescopes on Earth is a fabulous technical advancement for science," said Dr. Anne Kinney, who directs NASA's Astronomical Search for Origins program.

"Using them in this way gives us the equivalent of an 85-meter telescope. This will open the possibility of obtaining images with much greater clarity than ever before possible," she said.

The performance of powerful telescopes is most dependent on the device's opening, or "aperture." The more light a telescope gathers, the more it can be magnified without losing sharpness. Since magnifying light after a telescope has collected it is easy, telescope designers have been struggling to maximize the apertures of their instruments.

Until recently, 30 feet (10 meters) seemed to be about the upper limit, and even the two Kecks had to slightly "cheat" to reach that size. A single mirror or lens that large would distort under its own weight. So the builders combined 36 individually mounted, computer-driven hexagonal mirror segments to form a larger reflector.

But to create an even bigger telescope, engineers resorted to Cold War technology. First, to minimize the interference from Earth's atmosphere, the telescopes are equipped with anti-distortion devices called adaptive optics -- a science developed for spy planes and satellites. Then the light from the two composite mirrors is combined in a technique called "interferometry" so they create a single image -- brighter and with higher resolution than the sum of its parts. The resulting virtual aperture is as large as the distance between the telescopes.

"This is a major step in the creation of a whole new class of astronomical telescopes that will have an enormous impact on future knowledge," said Dr. Paul Swanson, the Keck interferometer project manager at NASA's Jet Propulsion Laboratory (JPL).

"Historically, breakthrough technologies like the Hale 200-inch (5-meter) and the Hubble Space Telescope have made discoveries way beyond the purpose for which they were originally built," he said.

The Keck interferometer will most likely continue testing until fall of 2001. In the meanwhile, JPL will be soliciting project proposals from scientists interested in using the giant telescope.