A bundle of laser beams creates five artificial stars in the night sky above Mount Hopkins in Southern Arizona. Laser light reflected by air molecules is analyzed by a computer that drives the actuators on the adaptive mirror.
Credit: Thomas Stalcup. [Full Story]
Scientists have successfully tested a new type of laser-corrected vision for telescopes that takes the widest starry-sky views ever seen from the ground while eliminating blur caused by the atmosphere.
Now astronomers can see entire single star clusters or many distant galaxies within the same field of view. That allows for more efficient use of expensive telescopes and observing time to tackle challenges such as examining thousands of early, distant galaxies. [Photo of the telescope laser in action.]
"You need to look at large patches of sky within one shot, and you need to do it at high resolution," said study leader Michael Hart, an astronomer at the University of Arizona in Tucson.
Sharp wide views of space
The method developed by Hart's group cancels out atmospheric turbulence across a telescope view about one-fifteenth the diameter of a full moon. Its success will likely spread to the new class of 98-foot (30-meter) telescope giants such as the Giant Magellan Telescope planned for development in Chile.
Such work represents a major update of adaptive optics technology that has been around for decades. Ground telescopes use adaptive optics to adjust for the ever-changing blurry effect that comes from peering at space through Earth's atmosphere, but can erase the blurriness only in a tiny view of the sky.
In adaptive optics, computers analyze the light from a natural or artificial guide star as a baseline to figure out the blurriness. Hundreds of actuators can then warp the surface of the telescope mirrors thousands of times per second to cancel out the blurry effect.
The new ground-layer adaptive optics system used five lasers mounted on the 21-foot (6.5-meter) MMT telescope at Mount Hopkins in Arizona. Past systems on other telescopes have used just one laser to create a single artificial guide star.
Each laser points in a different direction so that they end up spread out in a pentagon pattern as they punch more than 15 miles (24 km) into the sky.
But the lasers are angled so that the light reflected back to the telescope aperture is just from the lowest layer of atmosphere, within one- third of a mile (500 meters) from the ground. Software can then pick out the common blurry signal from that part of the atmosphere and adjust for it.
"If you correct the deleterious effects of the first few hundred meters of atmosphere, you go an awfully long way to fixing everything," Hart told SPACE.com. "You also get a wide field of view, because the ground layer is close to telescope."
The study is detailed in the August 4 issue of the journal Nature.
Wider view, lower resolution
That wide-view success came at the cost of resolution, so that the images, while sharp, don't appear quite as sharp as those seen with traditional adaptive optics.
But the tradeoff often becomes worthwhile. For instance, researchers want a wide enough field of view to see an entire star cluster, as well as enough resolution to pick out the motions of individual stars.
Next up, Hart's team wants to correct for a much greater layer of atmospheric interference by creating a 3-D model of the turbulence. Telescopes would also need a stack of several adjustable mirrors to fix the blurriness in 3-D and get back some of the higher resolution ? an approach known as multi-conjugate adaptive optics.
"What we're doing is shining laser beams through the atmosphere in all directions, so we can build an instantaneous snapshot of the atmosphere millisecond by millisecond," Hart explained.
The first-ever use of multiple lasers to create many guide stars also becomes crucial for the new giant telescopes under development, including the Giant Magellan Telescope, the Thirty-Meter Telescope and the European Extremely Large Telescope. Each instrument costs well within the range of $1 billion.
"If you're going to spend that amount of money, you better be darn sure that the telescope is able to produce the best science it can, or it's not worth the money," Hart said.
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