Laser beams help take the twinkle out of starlight

If you have ever peered down a highway on a sunny day, you have probably seen the rising, wavelike ripples of heated air that distort the appearance of objects near the horizon. Similar disturbances in the atmosphere above us make stars twinkle as their light is distorted on the way down to Earth.

Although twinkling stars inspired a well-known nursery rhyme, the effect hampers astronomers’ attempts to study the heavens. Scientists at Lawrence Livermore National Laboratory are now building systems, known as a synthetic guide stars, to help astronomers accurately account for atmospheric distortions wherever they choose to point their telescopes. Pictures collected by large terrestrial telescopes equipped with such systems often exceed the quality of Hubble Space Telescope images.

Guide stars have long played an important role in correcting atmospheric distortion. Astronomers pick a bright, stable star near a region of the sky that they hope to study and monitor distortions in the guide star image to deduce the optical properties of the atmosphere. They then correct their images with adaptive optics, which distort telescope components to offset atmospherically induced errors. Generally, adaptive optics corrections involve warping light-collecting telescope mirrors with computer controlled motors that respond to changes in the guide star image.

Guide stars and adaptive optics combine to provide stunning pictures of planets, galaxies, and other objects. Unfortunately, astronomers find suitable natural guide stars in only one percent of the sky. At the 2002 Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference (May 19-23, www.cleoconference.org) meeting in Long Beach, CA, Deanna Pennington explained that she and her colleagues at Lawrence Livermore National Laboratory have now opened up a much larger portion of the heavens with a manmade analog to guide stars. Pennington estimates that the synthetic guide star system can correct for atmospheric turbulence in about three fifths of the sky.

The researchers create their synthetic guide star by beaming a laser through the atmosphere in the direction of an object they wish to study. The frequency of the laser is specifically chosen to excite sodium atoms, causing them to emit yellow light. Sodium is relatively scarce in most of our atmosphere, but incoming meteors deposit a concentrated layer of the atoms in the mesosphere, about ninety kilometers above the earth. The laser paints an artificial star in the sky when it strikes the sodium rich layer.

Pennington points out that natural stars are best for correcting atmospheric distortion. Stars are so far away that they appear to be point light sources effectively located at infinity. Synthetic guide stars, in comparison, are too close to completely eliminate atmospheric errors, but they provide astronomers with seventy-five percent of the benefit offered by natural guide stars.

Any telescope equipped with adaptive optics can benefit from the addition of a synthetic guide star system, but the technology will be indispensable for future, earth-based observations. “Very large telescopes,” says Pennington,” must use adaptive optics to approach their theoretical performance. Atmospheric turbulence primarily dictates the ultimate resolution a telescope can achieve.” Telescope designs being considered for future observatories may sport mirrors thirty to a hundred meters across, and will dwarf even cutting-edge telescopes such as the ten-meter telescope at Keck Observatory in Hawaii. Larger telescopes collect more light and could provide ever high resolution images, provided we correct for the hazy blanket of gas that surrounds our planet.

Although the Keck telescope already uses adaptive optics that rely on natural guide stars, Pennington’s group installed a new sodium guide star system at the observatory last year. Keck’s adaptive optics successfully locked on to the laser-generated guide star in tests performed last December, but modifications to the system interfaces prevented astronomers from using it to collect images. If the final installation stages go as planned, a synthetic guide star shining over Hawaii should help astronomers peer into the heavens by Christmas 2002.

For more information contact:

Deanna Pennington pennington1@llnl.gov
Lawrence Livermore National Laboratory
P.O. Box 5500, L-477
Livermore, CA 94551
925-423-9234