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Joe's Big Idea
For Sharpest Views, Scope The Sky With Quick-Change Mirrors
Originally published on Mon June 24, 2013 11:55 am
It used to be that if astronomers wanted to get rid of the blurring effects of the atmosphere, they had to put their telescopes in space. But a technology called adaptive optics has changed all that.
Adaptive optics systems use computers to analyze the light coming from a star, and then compensate for changes wrought by the atmosphere, using mirrors that can change their shapes up to 1,000 times per second. The result: To anyone on Earth peering through the telescope, the star looks like the single point of light it really is.
The reason the atmosphere blurs light is that there are tiny changes in temperature as you go from the Earth's surface up into space. The degree to which air bends light depends on the air's temperature.
With adaptive optics systems, telescopes on Earth can see nearly as clearly as those in space. What's more, you can build bigger telescopes on Earth than can be sent into orbit. The bigger the telescope, the smaller and fainter the objects it can see.
"Adaptive optics has really revolutionized so many fields of astronomy," says Andrea Ghez, an astronomer at the University of California, Los Angeles. But such systems did not start out as tools for astronomers. "It was part of the strategic defense thinking of the nation, of what we could do to get better images of what was out in space," says Robert Duffner, author of The Adaptive Optics Revolution: A History.
During the Cold War the United States became concerned that the Soviet Union might be developing weapons that would be put into orbit. "The Air Force was interested in using telescopes on the ground to look up through the atmosphere to get clearer images of space objects — mainly satellites and missiles," says Duffner.
The Air Force had other ideas for adaptive optics besides looking at satellites. One of them involved shooting down missiles.
The notion was to aim a laser beam from the ground toward a relay mirror in space. "The mirror could then deflect the laser beam and send it to an incoming missile," says Robert Fugate, a scientist with the Air Force Research Laboratory at Kirtland Air Force Base in Albuquerque, N.M.
The hitch with such a plan, Fugate says, was that the atmosphere would smear out the laser, diluting its destructive power. Adaptive optics offered a solution. You can think of it as the reverse of compensating for the atmosphere in a way that makes starlight appear to be a single point. In this case, instead, the scientists would smear out the laser light so the countering distortions in the atmosphere would then bring it back to a narrow beam. That was the theory. In practice, the system was never built.
In 1991, the military agreed to declassify most of the work it had been doing with adaptive optics, so astronomers could take advantage of what the Air Force had learned. In the last two decades, the technology has brought some remarkable achievements.
"One of the most exciting recent ones is the study of planets outside our own solar system," says UCLA's Ghez. "Just 15 years ago, we didn't know about any planets around stars outside our sun. Now, not only do we know about them, but we can take a picture of them with this technology."
The technology is also valuable for looking at objects closer to Earth. "It's really interesting to look at planets within our own solar system. We send satellites out to study these planets in detail. And yet if we can point a telescope from the ground at these planets, like Saturn, or the moons of Jupiter, we can study them in equal detail to what the satellites might be doing," says Ghez.
She doesn't study planets. Ghez studies the giant black holes that exist in the center of galaxies. Adaptive optics has blown that field wide open, too. "You can actually see the stars that reside right around the black hole, and you can see matter falling into the black hole thanks to this technology," she says.
There's just one problem. For adaptive optics to work, you need a bright enough star to make the corrections on. So, until recently, if you wanted to explore a patch of sky with no bright star, you were out of luck. But scientists have figured out a workaround — they create artificial stars using a laser. "We shine a laser up into the atmosphere, and there's conveniently a very thin layer of sodium atoms up at 90 kilometers," Ghez says. "And this laser can stimulate those atoms to shine like a star. And then we can look at that star — that artificial star — and make the corrections."
The use of adaptive optics is also transforming vision research. Austin Roorda is at the optometry school at the University of California, Berkeley, and says that the cornea, lens and fluid inside the eye distort light, just as the atmosphere does. By analyzing that distortion, he says, scientists can use optics to "un-distort" the light, so the cells at the back of the eye no longer appear blurry during eye exams.
Adaptive optics could let a doctor see individual damaged cells at the back of the eye, Roorda says, and offer an important new tool for diagnosing and treating eye diseases like macular degeneration and retinitis pigmentosa. And there's more, he says. "We may have a tool that will allow us to measure the efficacy of a treatment, [and] that may slow the degeneration of those cells, and even restore those cells' function."
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DAVID GREENE, HOST:
You know that song. A twinkling star might be a child's delight, but it's an astronomer's nightmare. You see, stars don't twinkle. They only appear to twinkle because the earth's atmosphere distorts and blurs their light. One way to beat the twinkle is to put your telescope in space. NASA plans to launch the James Webb space telescope in 2018 to replace the aging Hubble telescope that's in orbit right now.
But the James Webb telescope costs nearly $9 billion. A technology called adaptive optics is actually a much cheaper option. It lets telescopes on the ground see almost as well as those in space. NPR's Joe Palca has been doing a series of stories about inventors and their inventions, a series that we call Joe's Big Idea. And today, Joe looks at the remarkable things that you can do with adaptive optics.
JOE PALCA, BYLINE: Have you ever looked down a long straight road on a hot day and off in the distance the road appears to be shimmering? Just about any hot surface will cause the same kind of blurring. The shimmering occurs because hot air just above the road bends light a little bit differently than cooler air higher up. Well, that's what twinkling is all about; tiny fluctuations in the temperature of the air between you and the star you're looking at.
Adaptive optics compensates for those fluctuations so an astronomer like Andrea Ghez of UCLA no longer has to ask of stars how I wonder what you are.
ANDREA GHEZ: It's just like the curtain opening and you can see things that you could never see before.
PALCA: Seeing things clearly is essential for progress in science. It's true for biologists looking inside cells. It's true for astronomers looking at stars and planets.
GHEZ: Adaptive optics has really revolutionized so many fields of astronomy. One of the most exciting recent ones is the study of planets outside our own solar system. Just 15 years ago, we didn't know about any other planets or stars outside out sun. And yet now, not only do we know about them, but we actually can take a picture of them with this technology.
PALCA: Ghez doesn't study planets. She studies the giant black holes that exist in the center of galaxies. Adaptive optics has transformed her research, too.
GHEZ: You can actually see the stars that reside right around the black hole and we can see matter falling onto the black hole thanks to this technology.
PALCA: And it's not just scientists who want to see things clearly.
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UNIDENTIFIED MAN: The Russians chalk up another victory in the Space Race as they put two...
PALCA: At the start of the Cold War, the Pentagon decided to gamble on the then unproven technology of adaptive optics so it could see what the communists might be threatening us with from space. Robert Fugate is a scientist at the Air Force research laboratory at Kirtland Air Force Base in New Mexico. Back in the late 1980s, he designed one of the first really successful adaptive optic systems.
I asked him if he remembered the day he first knew the system worked.
ROBERT FUGATE: Like it was yesterday.
PALCA: Oh, wow. Tell me about it.
FUGATE: Well, it was, you know, like 1 o'clock in the morning. We were at the one and half meter telescope and we were looking at a star, you know, a big blobby mass. So I said, well, you know, everything looks OK to me. I think we should try it. So we hit the return key on the computer so to speak and, wow, you know, we had this huge blob on the screen. It went to a point. And holy cow.
PALCA: Of course, the military is interested in looking at things besides stars, but once you have a way to compensate for atmospheric blurring, you can look at anything you want. There's a modern version of the system Fugate built installed on the Keck telescope on the top of the Mauna Kea volcano in Hawaii. Peter Wizinowich has driven to the top of Mauna Kea to show me what these adaptive optic systems are really like. Wizinowich has been involved with the Keck system from the start.
PETER WIZINOWICH: So if you go to your right...
PALCA: He takes me up in an elevator to a platform surrounding the telescope and we go into a small structure attached to the telescope frame.
WIZINOWICH: So this is the adaptive optics enclosure. What you're hearing is we turned on a HEPA filter when we came in.
PALCA: Wizinowich takes the panel off a large box. Inside are all kinds of mirrors. This is the guts of the adaptive optic system.
WIZINOWICH: Light from the telescope comes in that far end there.
PALCA: Wizinowich says the atmosphere is doing two things to the light as it comes down from the stars. One, it's making the light move around or jiggle.
WIZINOWICH: And it's also smearing it.
PALCA: Adding to the blur.
WIZINOWICH: And what adaptive optics is trying to do is taking out that image motion.
PALCA: So it uses one mirror to that and then, after a computer calculates how the light is being smeared, an adjustable mirror unsmears it. Wizinowich points out the small round mirror that does the unsmearing. It has 349 adjustable elements that move up to 1,000 times per second to restore the starlight to the single point it really is. That's amazing. It looks like a bathroom mirror.
WIZINOWICH: Well, except its coating's on the front surface and I hope it's a lot better quality than your bathroom mirror, otherwise we'll buy from the same vendor, because this was expensive.
PALCA: Now, there's one problem with adaptive optics. You need to have a bright enough star to make the corrections on. It used to be if you wanted to look at a patch of sky with no bright star, you were out of luck. But Andrea Ghez says scientists have figured out a way around that problem. They create artificial stars using a laser.
GHEZ: So we shine a laser up into the atmosphere and there's conveniently a very thin layer of sodium atoms up at 90 kilometers and this laser can stimulate those atoms to shine like a star and then we can look at that star, that artificial star and make the corrections.
PALCA: That's how Bob Fugate's system worked. That's how the Keck system works. Although, the Keck adaptive optic system has been incredibly successful, Wizinowich says Keck is planning upgrades. Now that astronomers have a taste of what's possible, they want even better adaptive optic systems to see even finer details. So do eye doctors. Yes, I said eye doctors.
Not to see stars, but to see the fine structures at the back of the eye. Austin Roorda is at the optometry school at the University of California, Berkeley. He says just like the atmosphere, the cornea lens and fluid inside the eye also distort light so engineers have developed an adaptive optic system for the eyeball.
AUSTIN ROORDA: What it actually represents is really a paradigm shift in how one would use ophthalmoscopy to study the eye.
PALCA: Oh, wait a minute. Say that word again. Ophthalmoscopy?
PALCA: Ophthalmoscopy is the act of taking pictures of the back of the eye. Roorda thinks adaptive optics could have an important role in diagnosing and treating eye diseases like macular degeneration and retinitis pigmentosa because adaptive optics gives you a way to see individual cells at the back of the eye, cells that are damaged by diseases.
ROORDA: We will have a tool that will allow us to measure the efficacy of a treatment that may slow the degeneration of those cells or even restore those cells' function.
PALCA: Now, Roorda is looking at the outside of individual cells. He says the next frontier is looking into the cells themselves, a sort of adaptive optics for microscopes. Anytime there are new technologies for seeing the world more clearly, scientists make important discoveries. Remember Galileo and his telescope? He advanced scientific knowledge quite a bit with his new technology.
Who knows what Andrea Ghez and other modern scientists will do with theirs. Joe Palca, NPR News. Transcript provided by NPR, Copyright NPR.