ARTH-BOUND astronomers have long sought to diminish the effects of the atmosphere on their observations. Stars that appear as sharp pinpricks to the eye become smeared "blobs" by the time they are imaged by large ground-based telescopes.
At the University of California's Lick Observatory on Mount Hamilton near San Jose, California, Laboratory researchers and their UC colleagues are installing a system on the 3-m Shane telescope that will correct these troublesome distortions. The system includes a dye laser that will create a "guide star" in the upper atmosphere and very sensitive adaptive optics that will measure and correct for atmospheric distortions. According to Scot Olivier, project scientist for the adaptive optics subsystem, the Shane is the first major astronomical telescope with such a laser system.
Other groups have been using adaptive optics systems with natural guide stars. However, it turns out that not just any star will do. It must be bright enough, that is, generate enough light to serve as a reference. When observing at visible wavelengths, astronomers using adaptive optics require a fifth-magnitude star, one that is just bright enough to be seen unaided. For near-infrared observations, only a tenth-magnitude star is needed, which is 100 times fainter.
The problem, Olivier noted, is that even though there may be hundreds of thousands or even a million stars bright enough to be guide stars, they only cover a small fraction of the sky. "Many times, there just isn't a natural guide star in the area you want to observe," he said. "This is the kind of situation where a telescope equipped with a laser guide star comes out ahead." (See box.)


Creating a Guide Star
Laser guide star efforts have generally focused on two methods of creating artificial stars. The first method uses visible or ultraviolet light to reflect off air molecules in the lower atmosphere from fluctuations (Rayleigh scattering), creating a star at an altitude of about 10 km. The other method uses yellow laser light to excite sodium atoms at about 90 km. The sodium-layer laser guide star turns out to be crucial for astronomy, because astronomers need large telescopes to see objects that are very far away and therefore very dim. These large telescopes require the laser guide star to be as high as possible so that the light from the laser star and the observed object pass through the same part of the atmosphere. With a guide star at the lower elevation, the system senses and corrects for only about half of the atmosphere affecting the light from a distant object.
The Laboratory's key contribution to this field has been the introduction of the sodium-layer laser guide star based on AVLIS dye laser technology. Claire Max, the project's principal investigator and the current Director of University Relations at LLNL, was a co-inventor of the idea of using a laser guide star in the sodium layer of the atmosphere for astonomical telescopes.




The guide star is created by a dye laser system, which is a small, closely related version of the system used by the Laboratory's Atomic Vapor Laser Isotope System (AVLIS) program. At Lick, green light from solid state lasers beneath the main floor of the telescope travels through fiber optics to a compact dye laser mounted on the side of the Shane telescope. A beam projector then directs the yellow dye laser light up through the atmosphere. At about 100 km, the laser beam hits a layer of sodium atoms created by micrometeorites, which vaporize as they enter the upper atmosphere. The yellow laser light, tuned to 0.589 micrometers, excites the sodium atoms, which then emit this yellow light in all directions, creating a glowing guide star in the upper atmosphere wherever the astronomer needs it.
Some of the light from this artificially created star travels back through the atmosphere into the Shane telescope. There, an adaptive optics system measures and corrects the guide star image for atmospheric distortions caused by air turbulence and temperature changes. Small sensors continuously monitor changes in the direction of light waves from the guide star. The sensors send this information to a computer, which in turn controls the movements of hundreds of tiny actuators attached to the back of a flexible mirror. Moving hundreds of times a second, the actuators deform the surface of the mirror to "smooth out" the image of the guide star.
When the telescope is viewing a celestial object, light from the guide star and the object travel through the same turbulence and receive the same corrections from the deformable mirror. The result is a clearer image of the object as well.
Last year, Livermore scientists, operating the adaptive optics system on Lick's 1-m telescope, observed objects at visible wavelengths. Using natural stars as guides, they corrected images to the diffraction limit of the telescope. At the end of last year, they moved the adaptive optics to the 3-m telescope and, again using natural guide stars, corrected to its diffraction limit in the near-infrared. The last step was to install the laser under the guidance of Herb Friedman, project scientist for the laser subsystem.
According to Olivier, the adaptive optics on the 3-m telescope allow astronomers to resolve objects more than 10 times smaller than before, when viewing in the near-infrared. With the addition of the laser guide star, astronomers are now able to perform these high-resolution observations over a large fraction of the sky. "This combination," notes Olivier, "makes this system arguably the world's most powerful tool for high-resolution, near-infrared astronomy."
Now that the Lick system is up and running, the Livermore team and other UC astronomers are beginning high-resolution, near-infrared observations of star-forming regions, quasars, and other interesting astronomical objects. Preliminary results from this research will be available later this fall.
In addition, based largely upon experience gained in building the Lick system, the Livermore team was recently awarded a contract to build the major components of a laser guide star adaptive optics system for the largest telescope in the world, the 10-m Keck telescope in Hawaii, owned by UC and the California Institute of Technology. This system, scheduled for completion in 1997, will become the world's most powerful tool for high-resolution near-infrared astronomy as we enter the 21st century.


For further information contact Scot Olivier (510) 423-6483 (olivier1@llnl.gov).



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