Adapt or die
It's the maximum maxim of biology. Adapt or die. In astronomy, matters are not quite so extreme. Still, "adaptive optics" is a new way to compensate for turbulence in the atmosphere. The technology, which derives from the military's early Star Wars projects, samples turbulence many times a second, and then varies the surface of a mirror to correct for it.

Turbulence is what causes stars to twinkle. Avoiding turbulence is a major justification for forking over billions of dollars for orbiting observatories. Thus the allure of adaptive optics, which can be placed on easy-to-build telescopes right here on the ground.

You can see the benefit of adaptive optics in the photos, above, from the Gemini North telescope in Hawaii. The technique is so precise, said astronomer Ray Jayawardhana last year, while he was at the University of California at Berkeley, that it could make images of hot young planets the size of Jupiter. (At least 80 planets have been discovered outside the solar system, but they are all many times larger than that.)

Adaptive optics is scheduled to be at the heart of the Panoramic Survey Telescope and Rapid Response System (PAN STARRS), now being built in Hawaii. Working together, a series of telescopes will be able to survey the entire sky several times a night. A computer will analyze the data, searching for fast-moving asteroids. The telescope is part of a larger effort to find asteroids that could slam Earth.

This is NASA's conception of Earth getting whacked by an asteroid. The remote threat of catastrophic asteroid impact is a major justification for building new, high-capacity telescopes. Illustration by Don Davis for NASA's Asteroid Comet Impact Hazards Website. Copyright© NASA.

Stay active or die
It sounds like one of those physical-fitness slogans. But in fact, active optics is a way to compensate for two banes of giant, ultra-precise telescopes. First, their mirrors sag under their own weight. Second, they change size as the night cools down at mountaintop observatories.

Active optics compensates for these slow changes with small movement devices on the mirror. Unlike adaptive optics, they needn't move hundreds of times a second, and thus are simpler to design and build.

And while adaptive optics may be the sexiest new telescope technology on the horizon, it's not for everybody, says Kenneth Nordsieck, who is leading the design team for a spectroscope that will be SALT's primary instrument. On SALT, the University of Wisconsin-Madison professor of astronomy says, "We're deliberately not doing adaptive optics, because it is hugely expensive and ... very scary from the standpoint of complexity." The mirror will, however, use active optics to hold a perfect spherical shape. Each of the 91 segments has sensors to detect its exact relationship to its neighbors, and actuators to correct misalignments, Nordsieck says.

Despite the complexity of adaptive optics, it's possible to retrofit an existing telescope- "simply" by putting an extra mirror in the light's path, and controlling that mirror by computer and actuators. You don't have to replace the entire mirror - the most costly single part of a large telescope.

A complicated feedback system analyzes light from a reference beacon (or a bright star). When the system detects atmospheric turbulence, it moves the mirrors in response. Courtesy Center for Adaptive Optics

Beyond adaptive and active optics, other, more obscure factors help explain the recent surge in the building of large and ever-more accurate telescopes, says Nordsieck. Advances in optical and mechanical design, some of it developed for orbiting telescopes, leads to lighter, more durable instruments. Improved computer technology has also played a role. "Modeling technology in general has made [the advances] possible," Nordsieck says. "The fact that we can now buy a Linux machine for $1,000 that will do an optical design with 18 lenses ... is really enabling."

The whole point of pushing telescope technology, Nordsieck says, is to open our eyes to the universe. "In the last 100 years, the entire advance in astronomy has been pushed by instruments, better detectors, not by theoreticians coming up with new ideas. That's why I'm in this business. By building new instruments, in a better and different way, you can change what people are thinking about."

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