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Focus on Space
February 06, 2001
This infrared image of the Arches Cluster, taken at the Gemini Observatory, is the sharpest image ever taken of this cluster, which is located less than 100 light years from the center of our galaxy. Adaptive optics allowed the telescope to image this cluster at an unprecedented clarity. The Arches Cluster is one of the largest clusters of stars in our galaxy and not very well understood due to the 25,000 light years of obscuring dust that lies between us and our galaxys center. The cluster is about 10 times larger than most other clusters in our galaxy and is destined to be ripped apart by the intense gravitational influence of the nucleus of our galaxy. image: Gemini Observatory, National Science Foundation and the University of Hawaii Adaptive Optics Group
Never before seen features of planets and images of stars at the center of our galaxy are now coming into focus thanks to recent advances in adaptive optics.
The technology allows researchers to eliminate distortions caused by the Earth’s atmosphere so that large ground-based telescopes can pick up objects that are very distant and, until now, were invisible.
A clearer view
Imagine you were near-sighted and were finally able to see by putting on a pair of glasses. That’s essentially what adaptive optics (AO) has done for the astronomy world. "It’s a tool that enables people to see roughly ten times better from a telescope on the ground than they would ordinarily be able to," says Claire Max, associate director for advanced technology at the National Science Foundation (NSF - funders of this research) Center for Adaptive Optics and physicist at the Lawrence Livermore National Laboratory (LLNL). "So that lets us use the world’s largest telescopes, which are 30 feet across, and which are so heavy that they can’t be launched into space, to see things as clearly as if they were in space."
Until recently, ground-based telescopes were limited in the images they could pick up due to turbulence in the Earth’s atmosphere that causes them to blur. Astronomers solved that problem by sending telescopes, such as the Hubble Space Telescope, into space where they had an unimpeded view. But such telescopes are not only expensive to build and operate, they need to be relatively small so that they’re not too heavy. By using AO to compensate for the distortion caused by the atmosphere, scientists have made ground telescopes, like the Keck Telescope in Hawaii, as powerful as if they were in space.
"So for example the Keck Telescope, the diameter of the mirror is four times bigger than the Hubble, so that means it can collect 16 times as much light, so it can see things that are 16 times fainter," says Max. "What that means in practice is that the place where that’s really important is looking way back into the early universe at things that are very very very far away and that are therefore very faint."
This animation shows what happens inside adaptive optics telescopes at places like the Gemini Observatory in Hawaii. The big green box represents part of the telescope, which gets images and sends them to a computer. As it zooms in, notice the red and blue streams of data coming in. The blue area represents where the information regarding turbulence in the Earth’s atmosphere goes, is routed back to the lens, which then constantly changes shape based on the information. This causes clearer images to be seen at the end of the line.
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courtesy Gemini Observatory
AO systems work by using a deformable mirror to cancel out the distortion that light undergoes as it passes through our atmosphere. To do this, astronomers normally focus on a bright source of light as a reference, which is then used to calibrate the surface of the mirror. In order to function properly, the mirror must be constantly changed to compensate for even small changes in the atmosphere. This means its shape is updated several hundred times a second.
New improvements and discoveries
But what happens if there is no bright light to focus on, as is often the case? Scientists have found a way around this by using laser beams to create a light source. "The idea is if you shine a laser beam up into the upper parts of the Earth’s atmosphere and make an artificial star using that laser beam, then you can point the laser anywhere. And that means that any object in the universe should be available to adaptive optics, not just ones that happen to lie near a bright star," says Max, who is also leader of LLNL’s Laser Guide Star Project. Max has been involved in building lasers at both the Keck Observatory and the Lick Observatory in California.
Although AO systems have been used for over a decade, recent refinements have led to some spectacular views. Scientists at Keck were recently able to see that Neptune’s atmosphere is full of giant storm systems, which may be associated with the reappearance of a great dark spot. They plan on tracking these features over time to find clues about the planet’s structure.
Thanks to AO, recent observations of Titan, Saturn’s largest moon, have allowed astronomers to peak through its orange haze and map its surface. "In particular, what’s exciting is that there’s been a prediction that there might actually be lakes on the surface that are made of liquid hydrocarbons, like the oil lakes in Kuwait, and we see very dark regions on the surface of Titan that might correspond to those liquid ethane lakes," says Max.
The Next Big Thing
AO has a lot of other applications, besides astronomy. It might be used to help improve human vision beyond 20/20 (see the STN2/Popular Science co-production for details on that). LLNL is nearing completion of its National Ignition Facility, which will use AO to focus 192 laser beams onto a tiny target. AO is also being used to improve laser communication systems that could replace fiber optics in computer networks.
The first images from the Gemini North telescope in Hawaii using AO have shown by far the clearest images yet of the stars at the core of our galaxy, which are thought to center around a huge black hole. Such images have been elusive due to the distance and the amount of gas and dust that light has to travel through to reach us. Most of the light that is detectable is in the infrared (heat) part of the spectrum, which is what the AO system looks at.
Even with such increased visibility from telescopes on the ground, however, scientists still need some help from outer space. "They’re actually very complementary to each other because there are some wavelengths of light that don’t get through the atmosphere and that means the only way that you can see those colors of light is by flying telescopes in space," explains Max. "There’s actually a very nice sort of ping pong match where the ball bounces back and forth between space-based and ground-based telescopes."
AO will eventually play a role in space in a project called the Next Generation Space Telescope, which is scheduled to launch in 2009 and which will replace the Hubble. The telescope will feature an eight meter mirror. "Its going to have petals so that it unfolds like a flower," says Max. "And when it unfolds they don’t know whether it’s going to go into the right shape, so theyre planning to use some form or other of adaptive optics to tweak it up and make it exactly the shape that they need after it’s unfolded."
According to Max, future AO systems will have more components so that they’ll be able to see both infrared and visible light. They’ll also be specially tuned to be able to see even the faintest of planets, and they’ll be able to cover a much wider field of view by using several deformable mirrors. If she’s right, the sky’s the limit for astronomers studying the universe in the years ahead.