As sky-watchers marvel at the beauty of the annual Perseid meteor shower — set to peak late Sunday and early Monday — astronomers will be thanking their shooting stars for another reason. Airborne pollution from vaporized meteors is proving crucial for the next generation of super-sharp snapshots of exotic moons and distant galaxies.
Pollution of any kind is usually bad news for telescopes, which thrive on clear skies. But shooting stars — flaming bits of debris from comets — leave behind traces of an element astronomers are harnessing to sharpen the focus of their telescopes.
That element is sodium. Traces of it waft in a band encircling the Earth about 55 miles above ground. American astronomer Vesto Slipher of Arizona discovered this sodium layer in 1929, but only recently have astronomers found a use for it — by zapping it with lasers.
“We can excite the sodium [with a laser] and make an artificial star,” said Chad Trujillo of the Gemini Observatory, a multinational consortium that operates telescopes in Hawaii and Chile.
Sensors watching these artificial “guide stars” see them twinkle — the effect of atmospheric turbulence. This wavy air usually renders distant targets fuzzy. But by watching the guide star, sophisticated computers on the ground can calculate the turbulence and send that data to the telescope. Sophisticated mirrors in the telescope then deform hundreds of times per second to compensate for the wavy air.
It’s like adding a pair of glasses with rapidly changing prescriptions to the telescope.
This technique — adaptive optics — has been called the biggest advance in ground-based astronomy since Galileo’s first telescope some 400 years ago. “You get better resolution from the ground than you can from space,” Trujillo said.
Astronomers began to tinker with crude adaptive optics in the 1950s. But the technology did not mature until the military became interested in artificial guide stars in the 1980s, said Claire Max, director of the center for adaptive optics at the University of California Observatories. “At that point, it was all highly classified,” said Max, who was working at the Lawrence Livermore National Laboratory at the time. The military wanted to image Soviet satellites, but Max had other ideas: “I said, ‘Astronomical objects move a lot slower that satellites, so I think it will be a great idea for astronomy.’ ”
Max and her colleagues pushed the Defense Department to declassify the technology, which it did in the early 1990s. “There was a famous meeting of the American Astronomical Society when the veil was lifted and everybody heard what the Air Force was doing,” Max said.
The first laser guide-star system came online at the Lick Observatory in 1995. Max used the system to bring Saturn’s moon Titan into focus. A hazy blob sharpened into view, allowing study of the storms raging in the moon’s methane atmosphere.
Earlier this year, the most sophisticated laser guide-star system to date came online at the Gemini South telescope in Chile. The $25 million system sharpens the image provided by the telescope’s 25-foot mirror onto a wider swath of sky than ever before.
“We’re looking deep into the cores of colliding galaxies now and watching black holes. It’s really fun,” Max said. “You couldn’t see any of the detail if you looked without these systems.”
It took five years to build the Gemini guide-star system, but the experience will aid the next generation of giant telescopes. Laser guide stars will be crucial for maximizing their views.