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Quiet revolution on the ground
FOR MORE than a decade the unblinking eye of the now-doomed Hubble space telescope offered the best views of the cosmos. Floating above the distorting effects of the Earth’s atmosphere, Hubble has taken pin-sharp snapshots of objects way out in the depths of space that have delighted astronomers and the public alike.
But while attention has been focused on Hubble, a revolution has been happening back on Earth. The enormous new optical telescope in South Africa is one of a new breed that astronomers will use to probe the universe while keeping their feet firmly on the ground.
“If you put a telescope in space it’s true that you’ll get a clearer view but it has to be a big telescope and that may not be a practical proposition,” says Paul Murdin, a researcher at Cambridge University’s institute of astronomy. “You can have the situation where a big telescope on the ground can outperform a small telescope in space.”
And if there’s one thing that telescopes on the ground can be it’s big. The primary mirror of the Hubble telescope is 2.4 metres wide, the largest that could be squeezed into the cargo bay of the space shuttle that carried it into orbit. The mirror of the new South African telescope is 11m across, and there are plans for instruments with mirrors as big as 100m —more than enough, in theory, to take astonishing new images that will make Hubble look like a Box Brownie.
One problem remains though: however big, ground-based telescopes will have to peer through the murky mess that is our atmosphere. “The atmosphere is full of bubbles of hotter and colder air,” Murdin says.
“These bubbles float about across the telescope beam and divert the incoming radiation so pictures of the stars beyond are a bit blurry.” More poetically, this atmospheric distortion is what puts the twinkle into starlight.
The interference can be reduced somewhat by placing ground-based telescopes as high up as possible, typically on the mountain tops of Chile and Hawaii. But it can never be avoided completely. “The challenge of having things like Hubble, which are producing these pin-sharp images, forced astronomers to think of technology to get around the blurriness,” Murdin says.
In the end, the astronomers didn’t have to invent new technology. The military did that for them. Handily for telescope scientists, the secret to taking the twinkle from starlight is the same as taking the rough edges from a photograph of a terrorist training camp taken by a spy satellite. It’s called adaptive optics and it works by effectively fitting the telescope with a contact lens to correct its defective vision.
Adaptive optics has been around for about a decade, but it’s only now that astronomers are preparing to test its potential on full-scale, ground-based telescopes. It works by bouncing the light collected by the primary mirror off a separate, much smaller mirror that can account for the atmospheric disruption and so sharpen the image. Typically, this is a piece of glass or silicon less than 1mm thick. Its shape can be subtly changed more than a thousand times a second by applying an electric current.
“You just push behind it at discrete points with actuators,” says Richard Myers, an astronomer working in adaptive optics at Durham University. “As each one pushes you get a bump and the bumps are engineered so they almost touch. That’s the trick, you can essentially get any shape you like.”
To counteract distortion, the computer controlling the bumps needs to know how much atmospheric interference there is, so scientists usually look at its effect on well-characterized light arriving from very bright stars. The technique works pretty well, but it has two limitations.
Firstly, it stops astronomers pointing telescopes at areas that don’t have a bright “guide” star nearby, restricting them to about 1 per cent of the available sky. Second, once the telescope is set up to correct its blurred images according to the disturbances in the light from the guide star, it can’t look at anything else.
“The first job is to take the 8m class of telescopes and to try to overcome these problems,” Myers says. The results could be spectacular. Myers says the new class of telescopes have the potential to produce the first direct image of a planet outside our solar system — a trick way out of Hubble’s league.
— Dawn/The Guardian News Service
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