At the summit of Mauna Kea, Hawaii, NASA astronomers have linked the two 10-meter (33-foot) telescopes at the W.M. Keck Observatory. Photo: NASA/JPL

The two largest telescopes in the world were linked together this week for the first time, a key step forward for efforts to directly observe extrasolar planets.

Astronomers used a sophisticated interferometry system to combine light from the two 10-meter (33-foot) telescopes of the W.M. Keck Observatory atop Mauna Kea, Hawaii, on Monday night, effectively creating a virtual telescope 85 meters (280 feet) across.

"Successfully combining the light from the two largest telescopes on Earth is a fabulous technical advancement for science," said Anne Kinney, director of NASA's Astronomical Search for Origins program. "This will open the possibility of obtaining images with much greater clarity then ever before possible."

The resolution, or minimum detectable separation, of an astronomical image is governed by the wavelength of light used to take the image and the diameter of the telescope, when other factors like atmospheric distortions are removed. To improve the resolution -- decrease the minimum separation -- requires either taking the image at shorter wavelengths or using a larger telescope. Since changing the wavelength is often impractical or undesirable, astronomers are left with increasing the size of telescopes. That, too, causes problems, as very large telescopes are very expensive and difficult to build and use.

A solution to this problem that astronomers have been pursuing for a number of years is interferometry. Light from two or more smaller telescopes is carefully combined so that the light waves collected by the telescopes exactly match. This creates a single image from a "virtual" telescope whose diameter is the separation between the actual telescopes, with a corresponding improvement in resolution.

This method was used Monday night at Keck Observatory, when the two 10-meter telescopes observed HD 61294, a faint star in the constellation Lynx. Light from each telescope was directed though an advanced optical system into a tunnel between the two telescopes, where instruments combined the light into a single image with a resolution equivalent to one taken by a telescope 85 meters across, the distance between the two Keck telescopes.

Such work is technically very challenging, as it requires manipulating light to a precision of less than one wavelength, or less than a millionth of a meter. The Keck interferometer system is able to adjust the light path between the telescopes to within a few millionths of a centimeter. In addition, adaptive optics systems mounted on each Keck telescope remove distortions caused by the Earth's atmosphere that would otherwise prevent the interferometer from accurately combining the light.

The linked telescopes, which together are called the Keck Interferometer, make up the world's most powerful optical telescope system. Photo: NASA/JPL

The test is a major step for efforts to discover extrasolar planets. Such planets are typically discovered today indirectly, usually as the planet's gravity tugs at its parent star, creating periodic Doppler shifts in spectral lines observed by astronomers. The increased resolution provided by interferometry, however, would allow astronomers to directly observe planets that would otherwise be too close to the stars they orbit to be seen. Also, a related technique known as "nulling interferometry" can combine images such that light from the star is cancelled out, making it much easier to detect the faint reflected light from planets.

Tests of the interferometer will continue for several more months, and limited science observations, including searches for extrasolar planets, are scheduled to begin in about half a year. NASA will also be accepting proposals for other uses of the interferometer. "Historically, breakthrough technologies like the Hale 200-inch and the Hubble Space telescopes have made discoveries way beyond the purpose for which they were originally built," said Paul Swanson, interferometer project manager.

The Keck system is not the only astronomical interferometry system being developed. The technology used in the Keck system was first tested with the Palomar Testbed Interferometer, a system of three small telescopes with a separation of 110 meters at Palomar Observatory in California. Other interferometer systems include the Naval Prototype Optical Interferometer outside of Flagstaff, Arizona and the CHARA Array, a project of Georgia State University's Center for High Angular Resolution Astronomy located on Mount Wilson in California.

Astronomers eventually plan to establish interferometry systems in space, where telescopes can be set up hundreds of meters to kilometers apart from one another, improving resolutions to the point where even terrestrial planets like the Earth can be directly observed. NASA has proposed one such mission, the Terrestrial Planet Finder, that would use four 3.5-meter telescopes on a baseline up to a kilometer long. Such a mission would not be launched for at least a decade, however.

Until then, astronomers will use tools like the new Keck interferometer as part of NASA's Origins program, an effort to understand how life got started on Earth and whether it may exist elsewhere. "This first light from the Keck Interferometer marks a dramatic step forward and will help us accomplish the ultimate goal of the Origins Program -- to search for signs of life beyond by examining the light from 'Earths' orbiting nearby stars," said project scientist Charles Beichman.