Adaptive Optics

Adaptive Optics[1]

How did the world’s largest telescopes conquer the tempestuous atmosphere?

Many early scientists, even Isaac Newton, wrestled with the problem of atmospheric distortion; but the real advances didn’t begin until the 1970s. That’s when the Pentagon was working on something seemingly unrelated to astronomy; it needed a way to focus a laser beam on a distant target, which meant protecting the beam from choppy wind. At the same time, the Defense Advanced Research Project Agency (DARPA), an agency within the Department of Defense, wanted to identify satellites launched by the Soviet Union.

Even at a good location, atmospheric turbulence smears out details smaller than 1 arcsecond across. That’s good enough to see the cylinder-shaped Hubble telescope, which is similar in size to most spy satellites, but not good enough to make out details. The military needed a way to do better.

If scientists could accurately measure how the atmosphere is moving, they could send that information along to a flexible secondary (or tertiary) mirror. In principle, this deform able mirror would exactly cancel distortions introduced by the atmosphere into the primary mirror’s image, sculpting the rays of light (from a satellite or any other target) back to near-perfect alignment.

One of the first AO demonstrations was installed in 1980 on DARPA’s telescope in Maui. Today the technology has advanced an amazing amount.

Compensating for the atmosphere is no easy task. The simplest method calls for a star in the field of view, which would look like a small point if its light could travel undisturbed to the telescope. The atmosphere introduces an extra blur. So keeping a telescope eye on the star gives a measure of atmospheric turbulence.

The figure above shows how adaptive optics (AO) works. As starlight shines through Earth’s atmosphere, turbulent air distorts is wavefront. A blurry image results. In a laser guide-star AO system, a sodium laser shoots up to the mesosphere, scattering among the sodium atoms there to create what appears to be a bright yellow star. Computer algorithms measure this artificial star’s wavefront, which is similarly jangled by the time it reaches the ground. The computer then deforms a flexible mirror to return both wavefronts to their undisturbed forms.

Obviously vital in astronomy, this technology is absolutely essential in contemporary intelligence work.

[1] See Shannon Hall, “Untwinkling the Stars,” Sky and Telescope (131, 5, May 2016, pp. 30-36)

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