You're camping out in the mountains on a clear summer night. The velvet-black sky sparkles with millions of flickering dots. The starry twinkle, though, which has driven generations of poets to rapture, is the bane of astronomers bent on capturing clear, sharp images of the galaxies, stars, and planets that populate the universe. Viewed through large Earth-based telescopes, that twinkle is seen as blur, which reduces astronomers' ability to see finely detailed structure. Sir Isaac Newton identified the problem 300 years ago [see "Early Days"]. Writing less than a century after the invention of the telescope, he declared: "If the theory of making Telescopes could at length be fully brought into Practice, yet there would be certain bounds beyond which Telescopes could not perform. For the air through which we look upon the stars is in perpetual Tremor." The "tremor" arises from turbulent mixing of air at different temperatures, which continually changes the speed and direction of starlight as it passes through the atmosphere. The same effect distorts the view of distant objects seen through the shimmer above a hot parking lot.
Today, a new technology called adaptive optics is, in effect, removing the atmospheric tremor. And the improvements that it brings to today's telescopes represent an advance at least as great as the invention of the telescope itself. The technique brings together the latest in computers, material science, electronic detectors, and digital control in a system that warps and bends a mirror in the telescope to counteract, in real time, the atmospheric distortion.
The advance promises to let ground-based telescopes reach their fundamental limits of resolution and sensitivity, outperforming space-based telescopes and ushering in a new era in optical astronomy. Most alluringly, using this technology, it will finally be possible to see gas-giant type planets in nearby solar systems in our Milky Way galaxy. Although about 100 such planets have been found in recent years, all were detected through indirect means, such as their gravitational effects on their parent stars; none has actually been seen directly.
An adaptive optics system recently installed on the 6.5-meter-diameter telescope, called the MMT telescope, on Mt. Hopkins, just south of Tucson, Ariz., takes the technology a step further [see photo]. It reduces the thermal background "noise" that comes from the telescope itself to below that of any other conventional telescope using an adaptive optics system. With less "noise" astronomers can see fainter objects than they would be able to see otherwise.
Reaching for the limits
In theory, a telescope's resolving power is directly proportional to the diameter of its primary light-gathering mirror or lens. But in practice, images from large telescopes are blurred to a resolution no better than would be seen through a 20-cm aperture with no atmospheric blurring. At scientifically important infrared wavelengths, atmospheric turbulence degrades astronomers' ability to resolve fine detail by at least a factor of 10.
Space telescopes avoid problems with the atmosphere, but they're enormously expensive and the limit on aperture size of telescopes that are currently launchable is quite restrictive. The Hubble Space Telescope, the world's largest unclassified telescope in orbit, has an aperture of 2.4 meters; terrestrial telescopes can have a diameter four times that size.
One can turn instead to larger telescopes on the ground, equipped with adaptive optics systems to compensate in real time for the atmospheric aberration. With this setup, the image quality that can be recovered is close to what that same telescope would deliver if it were in space.