Lasers Take the Twinkle Out of the Night Sky

If you are a hopeless romantic, you probably love to see a nighttime sky filled with twinkling stars. But if you are an astronomer, probably not so much. Now a team of University of Arizona astronomers led by Michael Hart has developed a technique that allows astronomers to stop the twinkling effect over a wide field of view, enabling Earth-based telescopes to obtain images that are as crisp as those made using the Hubble Space Telescope, and much faster. The technique is called laser adaptive optic and the team describes it in the August 5 issue of Nature.

Normally, light from celestial objects is blurred by atmospheric turbulence by the time it reaches the optics of a ground-based telescope. Most of that distortion happens less than a half mile above the ground, where heat rises from the surface and disturbs the air.

The new technique can be thought of as noise-canceling process, only for light waves instead of for sound waves. The heart of the process is formed by a bundle of five green lasers and a pliable mirror.

Hart and his team demonstrated the process from their observatory on Mount Hopkins, south of Tucson, Arizona. The five lasers are used to detect turbulence in the atmosphere. Any light reflected back from each laser, and the amount reflected back, indicates the amount of turbulence in the telescope’s field of view. The turbulence data is then fed into a computer which control’s the telescope’s adaptive mirror.

The back of the mirror is covered with 336 actuators, or small magnetic pins surrounded by coils. When the computer sends electric current through the coils, the actuators move, causing the mirror to warp just enough to cancel out the turbulence which causes the twinkle in the atmosphere. The corrective movements are too tiny for the human eye to see and happen a thousand times each second.

Astronomers and engineers have advanced adaptive optics over the past 15 to 20 years, but the technology was limited in that it could only be applied to a very narrow portion of the telescope’s field of view. According to Hart, this new technology can be applied over the telescope’s entire field of view. There is some trade-off in the new technique, in that it sacrifices some of the very high resolution in order to gain a larger field of view. Hart expects that this trade-off is well worth it because of the many scientific uses that it makes possible.

One use could be to study very old galaxies that formed around 10 billion years ago. These are known to astronomers as high red-shift galaxies and are thought to be billions of light years away.

The new technique would allow astronomers to study the spectral characteristics and chemical composition of these galaxies. Until now, such a study was difficult because the light from these galaxies was so faint.

For more information check out these links:

University of Arizona, Department of Astronomy and Steward Observatory
http://www.as.arizona.edu/

University of Arizona article: Taking the Twinkle Out of the Night Sky
http://uanews.org/node/33078

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