Ben Bova: Things are looking up for ground-based telescopes

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Mirror, mirror on the wall, who will make the biggest mirror of them all?

When I was a teenager with a keen interest in astronomy, the Hale Telescope at the Palomar Observatory in California began to give astronomers marvelous new detail about the starry universe.

It was the biggest telescope in the world, with a primary mirror 200 inches (about five meters) in diameter. The common wisdom among astronomers was that it would be pointless to try to build anything bigger, because atmospheric turbulence would blur the telescope’s “seeing” too much to make it worthwhile.

You see, thin air isn’t so thin. And it isn’t still. To an astronomer, we live at the bottom of a thick bowl of bubbling soup. Seeing through is difficult.

Our atmosphere absorbs a lot of the visible light from the stars, and blocks altogether many other wavelengths of electromagnetic energy. Most ultraviolet, infrared and radio wavelengths don’t make it through the atmosphere. X-ray and gamma radiation are completely blocked.

Worse still, the atmosphere is in constant turbulent motion. That’s what makes the stars twinkle. It may seem charmingly romantic to you and me, but to an astronomer trying to get a good, hard look at the stars, this atmospheric turbulence is excruciatingly troublesome.

The stars are so far away that even in the largest telescopes they appear as nothing more than mere pinpoints of light. The bigger a telescope is, the smaller area of the heavens it focuses upon. And the smaller its field of view, the more atmospheric turbulence causes problems. The stars wink in and out of focus; they might even jump out of the telescope’s field of view altogether.

The bigger the telescope, the more acute this problem becomes.

Thus astronomers thought it would be pointless to build a telescope larger than Palomar’s five meters. They wanted to move their instruments into orbit, above the Earth’s soupy, restless atmosphere.

So astronomy moved into space, where specialized telescopes could see in the infrared, ultraviolet and other regions of the electromagnetic spectrum that were totally blocked by the atmosphere.

What’s more, instruments that gathered visible light could work without being bothered by atmospheric turbulence. The Hubble Space Telescope is the biggest telescope to be placed in orbit so far: its main mirror is 94 inches in diameter (about 2.3 meters), which would make an impressive instrument even on the ground.

But while astronomy moved into space in a major way, there were a few thinkers who believed they could build ground-based telescopes much larger than Palomar’s — and allow them to see even more clearly than the Hubble, up in orbit.

They key to all this came out of the Strategic Defense Initiative, the “Star Wars” missile-defense program.

Part of SDI involved using high-power lasers to shoot down ballistic missiles. But atmospheric turbulence broke up the laser beams, making it unlikely that a laser could stay on its target long enough to destroy the missile.

To counter this problem, scientists invented “rubber mirrors.” That is, they developed mirrors that were so thin they could be flexed by actuators attached to their backs. With high-speed computers controlling them, the mirrors could flex microsecond by microsecond and cancel out the fluctuations in the atmosphere caused by turbulence.

Astronomers took note. With adaptive optics, as the “rubber mirrors” are more correctly called, they could get much, much sharper images of the heavens. That meant that they could build bigger mirrors, bigger “light buckets” than ever before.

At the University of Arizona, mirrors of six, eight and 10 meters have been built. With such big primary mirrors gathering in light, and adaptive optics correcting for atmospheric turbulence, new observatories on the ground are seeing farther and more clearly than the Hubble, in space.

Plans are afoot to build even bigger instruments. Arizona’s Roger Angel is working on the Giant Magellan Telescope, which will consist of seven 8.4-meter mirrors, giving it an effective primary mirror 24.5 meters in diameter.

The Bible says, “And they shall beat their swords into ploughshares.” Astronomers are using SDI technology to give us better, deeper views of the universe.

Naples resident Bova is the author of more than 120 books, including “The Return,” his latest futuristic novel. Bova’s Web site address is www.benbova.com

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