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.

Mirror, mirror: Inside the Keck II telescope, a 10-meter-wide mirror (bottom) collects starlight and reflects it to a smaller secondary mirror (top), which in turn directs it to other instruments, including the adaptive-optics system. Credit: Keck Observatory

Multimedia   Watch a video on adaptive optics and see an animation of the Thirty Meter Observatory.

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.

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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.