Stars Untwinkled

Since the days of Galileo, astronomers have been plagued by one consistent drawback: the Earth’s atmosphere. Starlight can travel undisturbed through empty space for hundreds of light-years. But in the last few nanoseconds before it hits Earth and its telescopes, the light gets distorted by swirling winds and air currents.

“It gives you a very fuzzy, dancing image,” said Fred Chaffee, who directs the world’s largest telescope, the Keck, and has long been frustrated by the inability of the powerful machine to create sharper images. “The reason stars twinkle is because of the atmosphere.”

Now, four centuries after Galileo lifted his handmade “spyglasses” toward the moons of Jupiter, astronomers have a high-tech fix for their earthbound telescopes. Adaptive optics systems look at guide stars to measure atmospheric distortions. The system then uses those measurements to change the shape of a mirror that can be bent up to a thousand times per second to correct for the rapid fluctuations of the atmosphere.

“It’s like putting eyeglasses on a telescope,” said Andrea Ghez, a UCLA astronomer who is using adaptive optics to probe a supermassive black hole that appears to reside at the center of the Milky Way.

Ghez’s work requires measuring the stars rotating around the edge of the black hole. “You just can’t see the stars without adaptive optics,” she said. “And if you can’t see the stars, you can’t do the work.”

The idea to correct telescopes was first proposed by California astronomer Horace Babcock in the 1950s. The military developed some of the technology in its “Star Wars” program and declassified it in 1991. High-speed computing, the development of glass mirrors so thin they can be easily deformed and a lot of hard work in recent years have made it a reality.

“It’s actually very hard to do,” said Jerry Nelson, the designer of Keck’s 30-meter mirror and the director of the Center for Adaptive Optics, a National Science Foundation center based at UC Santa Cruz. “It’s a very sophisticated procedure.”

The first adaptive optics systems have been in place on some of the world’s biggest telescopes, including Keck and Gemini on Mauna Kea, Hawaii, for less than two years. Already, the system has produced impressive results, including several highlighted this month when astronomers gathered for their annual meeting in Washington, D.C.

UC Berkeley astronomer Imke de Pater has been using adaptive optics to look at objects closer to home--the planets in our solar system. Adaptive optics has been able to focus images of Neptune to such an extent that the images are as good or better than those taken by spacecraft flying by the planet, she said.

She has been able to make out storms in Neptune’s atmosphere, suggesting mysterious activity on a body that was previously seen as something of a planetary dud.

Another victory for adaptive optics came in 1999, when astronomers turned Keck toward Io, one of the moons of Jupiter, because the night was too cloudy to observe more distant objects. They happened to catch a volcano in the act of erupting. In an image unblurred by adaptive optics, the volcano was a large, bright blip on the edge of the moon.

Ray Jayawardhana, a research fellow at UC Berkeley, is trying to understand the details of how planets form from the dusty disks that surround young stars. Using adaptive optics on the 8.2-meter Gemini telescope, he led a team that captured an image of a protoplanetary disk circling an extremely young star. The image was so detailed, the team could infer the shape of the disk and the size of dust particles within it--particles that could one day grow into planets like Earth.

Astronomers are now racing to capture an image of a Jupiter-sized planet circling a young star. Jupiter-like planets have been detected indirectly by other astronomers, but no images have been captured because current telescopes had not been able to discern the dimmer planets from the bright stars they orbit. “It’s possible now only because of adaptive optics on large telescopes,” Jayawardhana said.

Space telescopes such as Hubble avoid the vagaries of the atmosphere by orbiting above it, but they are costly to build and maintain and therefore much smaller than telescopes on the ground, limiting their use for tasks such as planet hunting.

A UCLA team used adaptive optics to probe some of the most distant reaches of the universe in a hunt for young galaxies that could help explain how galaxies evolve. Because adaptive optics systems require a guide star to correct fluctuations in the atmosphere, the team could probe only areas of the sky that were near relatively bright stars.

“We call it the looking-around-the-street-lamp technique,” said Tiffany Glassman, an astronomy graduate student who conducted the search with her advisor, James Larkin. Luckily, there were plenty of galaxies in those regions--and plenty of detail to capture with the new technology. “Without adaptive optics, we can barely tell they’re galaxies and not stars,” she said. With adaptive optics, the two could distinguish disks and bulges and even spiral arms within the distant galaxies.

Those using adaptive optics have been restricted in what part of the sky they could search because of their need for guide stars. Stars bright enough to serve that purpose exist in only 1% of the sky. Astronomers are now moving past that barrier by creating their own guide stars with powerful lasers.

A team of engineers from Lawrence Livermore National Laboratory spent a weekend atop the summit of Mauna Kea, installing a 200-watt dye laser on the Keck telescope. The $7-million laser shoots a beam of yellowish-orange light 60 miles into the sky. The laser is the precise wavelength of light needed to chemically excite a layer of sodium atoms--leftovers from burning meteors--that hovers in the Earth’s atmosphere. When the laser hits them, the sodium atoms light up and send that light back to Earth. It’s light that “looks for all intents and purposes like a star,” Chaffee said.

The instrument is “literally bolted to the side of the telescope, so when the telescope moves, the laser goes with you,” said Deanna Pennington, a laser physicist from Livermore now working in Hawaii to install the new laser. “You create your own reference star.” Once the laser is tested and fully working this summer, it should open up most of the sky to adaptive optics, except close to the horizon or near spy satellites or planes--places where shining the laser is taboo.

Those who run telescopes aren’t stopping at just one laser. There are plans to outfit the Gemini telescope with five lasers to create a system that could apply suitable corrections to each of the atmosphere’s different levels and create even clearer images. And Nelson’s center is working on micromechanical sensors that could make deformable mirrors smaller, cheaper and more responsive.

“It’s a real revolution,” he said. “And it’s just starting.”