UA: Taking the Twinkle out of the Night Sky

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UA: Taking the Twinkle out of the Night Sky

Post by bystander » Wed Aug 04, 2010 7:55 pm

Taking the Twinkle out of the Night Sky
University of Arizona | 04 Aug 2010
A breakthrough in adaptive optics allows astronomers to obtain space-telescope quality images over a wide field of view – here on Earth.

If you are like most people, you probably enjoy the twinkling of stars that blanket the sky on a clear summer night. If you are an astronomer, chances are you find it extremely annoying.

A team of University of Arizona astronomers led by Michael Hart has developed a technique that allows them to switch off the twinkling over a wide field of view, enabling Earth-based telescopes to obtain images as crisp as those taken with the Hubble Space Telescope, and much more quickly.

They describe the technique, called laser adaptive optics, in the Aug. 5 issue of Nature.

Atmospheric turbulence blurs the light from celestial objects by the time it reaches the mirror of a ground-based telescope. Most of the distortion happens less than a half mile above ground, where heat rising from the surface ruffles the air.

Think of laser adaptive optics as noise-canceling headphones, only for light waves instead of sound waves. A bundle of laser beams and a pliable mirror in the telescope optics form the heart of the system.

From their observatory on Mount Hopkins south of Tucson, Ariz., Hart and his group point a bundle of green laser beams into the night sky. Some of the laser light bounces off oxygen and nitrogen molecules high up in the atmosphere, creating five artificial stars spread across the field of view.
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The turbulence data are then fed into a computer that controls the adaptive mirror, whose back side is studded with so-called actuators, small magnetic pins surrounded by coils.

When the computer sends electric currents through the coils, the actuators move, not unlike a loudspeaker translates electric signals from an amplifier into movements of the sound membranes. Hart's adaptive mirror has 336 actuators glued to its back side that cause the mirror to warp just enough to cancel out the flickering caused by the atmosphere. The corrective movements are too tiny for the human eye to see and happen a thousand times each second.

The difference between a telescope with adaptive optics and one without is similar to a camera with a built-in image stabilizer compared to one without.

According to Hart, astronomers and engineers have advanced adaptive optics considerably over the past 15 to 20 years, but until now, the technology was fraught with a fundamental limitation: Atmospheric blurring could only be removed along a very narrow line of sight.
A ground-layer adaptive optics system with multiple laser guide stars - M Hart et al

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