Telescopes See Farther
Advances in adaptive optics are bringing the universe into focus.
Astronomers have spotted the most distant, oldest galaxy they’ve ever seen, using optical tricks both celestial and man-made. While the observation of the galaxy as it existed just two billion years after the Big Bang is scientifically significant in its own right, it also serves as an early peek at what’s to come as astronomers adopt a sophisticated technique called adaptive optics to peer much deeper into the night sky.
The team of astronomers, from Caltech and Durham University, in England, announced their findings in the journal Nature last week. Using the Keck telescope in Hawaii, they examined a galaxy 11 billion light-years from Earth. Previously, astronomers had been able to see no farther than seven or eight billion light-years. Because looking across astronomical distances is the equivalent of looking back in time, the observation brings astronomers much closer to the birth of the universe, approximately 13 billion years ago.
To spot a galaxy at such a great distance, the astronomers used two optical tricks. One is a naturally occurring phenomenon called a cosmic lens, which exploits gravity’s ability to bend light. A galaxy that’s precisely aligned between the astronomers and the object they want to look at will bend the light from the object around itself, refocusing it toward the astronomers. That gives them an image about eight times sharper than if they’d tried to look at the distant object alone.
But when the object is a galaxy that’s only a few thousand light-years across (as opposed to the 100,000-light-year diameter of the Milky Way) and 11 billion light-years away, eight times the sharpness still yields little more than a point of light. Astronomers use adaptive optics to make the image clear enough to get some useful information.
Light can be thought of as a wave, with a series of wave fronts moving through space, much like the fronts of ocean waves rolling ashore. Ordinarily, the front of a light wave is flat. But as it passes through Earth’s turbulent atmosphere, it becomes distorted–more like unevenly corrugated cardboard. This turbulence is what makes stars twinkle, and it reduces a telescope’s resolution. So the Keck uses an adaptive-optics system that measures that turbulence and corrects for it.
To make the measurement, a ground-based laser fires a beam of light into the air, where it strikes a thin layer of sodium deposited by meteors burning up in the atmosphere, about 90 kilometers up. The sodium reflects the laser light toward the telescope’s main mirror, which directs it to a series of wave-front sensors that measure how much the atmosphere has distorted the light wave. Based on these measurements, a computer causes a series of actuator arms to push and pull on a set of small, deformable mirrors. The actuators bend the mirrors roughly a micrometer (about one-hundredth the thickness of a human hair) many times each second, canceling out the atmosphere’s turbulence. The corrected wave front is then registered by a camera. Caltech astronomer Richard Ellis says that the result is an image of higher quality than astronomers get with the Hubble Space Telescope, which has no atmospheric distortion to contend with.
The scientists discovered that the distant galaxy was spinning, just as galaxies spin today, but that it had not yet formed the spiral arms that our own Milky Way galaxy displays. For Ellis, who’s trying to understand how the universe evolved and is one of the paper’s authors, the observation is significant. “It’s telling us the universe was really fairly organized when it was only 10 to 15 percent of its current age,” he says.
Although the concept of adaptive optics has been around for decades, it’s only in the past few years that it’s become sophisticated enough and easy enough to use to become a routine part of astronomy. The system was installed on the Keck II telescope in 2004, and it was the first for a telescope that big. It has since been used to provide clearer views of astronomical objects, but nothing as distant as the galaxy seen last week. The Keck II’s adaptive-optics system, however, pales in comparison to what’s planned for the new Thirty-Meter Telescope (TMT), which a U.S.-Canadian team that includes Caltech, the University of California, and the Association of Canadian Universities for Research in Astronomy will build over the next decade. A decision on whether to place it on Mauna Kea, in Hawaii, where the Keck is, or in Chile is expected next year.
The TMT will have nine times the light-collecting area of the Keck II, whose primary mirror is 10 meters across. And according to Brent Ellerbroek, the adaptive-optics group leader for the TMT, the new telescope’s optics system will be far more sophisticated. It will use about six lasers to measure atmospheric turbulence. While a single laser measures turbulence at only one small spot in the telescope’s line of sight, an array of lasers can provide a three-dimensional picture of distortions over a wider area and at different heights in the atmosphere. The wave-front sensors will also have smaller apertures, to make more-precise measurements, and there will be thousands of actuators, up from hundreds in the Keck, to control the larger number of mirrors. All that measuring and moving presents a computing challenge. “We have to use more-complex algorithms,” Ellerbroek says.
“It’s a great engineering challenge,” says Scott Uebelhart, a postdoctoral associate studying space policy in MIT’s Program in Science, Technology, and Society. But he thinks that the effort is worthwhile. “TMT pretty much puts everything else to shame,” Uebelhart says.
With the advanced system in the TMT, Ellis says, astronomers won’t have to get lucky and find a cosmic lens in order to see far away. Exactly how close to the birth of the universe they’ll get is a question that’s yet to be answered, he says. “We’re almost at the very beginning.”