Telescope tunes in to the guiding light.

Telescope tunes in to the guiding light

Since the days of Isaac Newton, opticalastronomers have had to live with ground-bound telescopes and be satisfied with cursing the turbulent atmosphere that caused their images to blur.

Even today, with recent advances inimaging that have improved resolution, anything short of a telescope sent into space is unable to obtain a resolution at visible wavelengths of more than about 1 arc second. Certain kinds of telescopes that can measure and adapt to the irregularities in the earth's atmosphere are one solution, but they, too, face their own set of demons: Such telescopes require a bright source to correct for turbulence, and most deep-sky viewing is without such sources.

Now Laird A. Thompson of the Universityof Hawaii at Manoa and Chester S. Gardner of the University of Illinois at Urbana-Champaign have taken the first step to solve that problem by using a laser to create an artificial guide star. Although successful in creating the star, Thompson still refers to the image--which measured some 1 to 2 arc minutes across--more as a "blob' than a real point source. "We have a long way to go to actually make a usable star,' he told SCIENCE NEWS.

But the incentive to succeed is there. Ifadaptive telescopes with electrically deformable mirrors and artificial guide stars can work together to overcome the problems of atmospheric turbulence, resolution could improve to 1/10 of an arc second, Thompson says, the same resolution promised by the Hubble Space Telescope scheduled for launch in 1988.

Thompson's experiment, reported inthe July 16 NATURE, builds on the work of French researchers R. Foy and Antoine Labeyrie, who first suggested that lasers might be used to create artificial stars for adaptive imaging systems. In general, such imaging systems are equipped with sensors that constantly monitor the atmosphere and then feed the information to a computer, which alters the shape of the mirror or makes slight changes in orientation of other optics (SN: 1/3/87, p.10). But to do that, these telescopes need a reference source at least as bright as a 10th-magnitude star, Thompson says, which is too faint to be seen with the naked eye but is more than bright enough by an astronomer's standards.

Last January, Thompson and Gardnerbecame the first to turn Foy and Labeyrie's theory into data by shining a flashlamp-pumped dye laser 60 miles high into the sodium layer of the earth's mesosphere. Tuned to the same wave-length as the sodium, 5,890 angstroms, the laser scattered the alkali metal in such a way as to create a light source. Researchers then used the University of Hawaii's 2.2-meter telescope at the Mauna Kea Observatory to measure and photograph the predicted return flux from the artificial source. "We just wanted to make sure we knew how much light was going to come back,' Thompson says. The light they measured corresponded with what they expected from the laser guide star.

Once it is honed to look like a real star,the purpose of the reference point would be to lie directly in front of the star being imaged. Astronomers would then accommodate for the light being emitted from the reference point. To save energy, the laser would be pulsed so that it would go on for 10 to 20 microseconds 200 times every second, which is how often the atmosphere can vary. Thompson and Gardner also are considering other types of lasers and scattering methods, such as Rayleigh scattering, in their effort to create an artificial star.

Even if the right laser and method arefound, though, researchers still have to make sure that the transmission of the energetic source itself won't distort the image and that the guide star behaves as a normal star would. After that, the rest is written in the stars.