World's largest telescope creates a virtual star
Posted: January 5, 2002

The W. M. Keck Observatory, home to the world's two largest telescopes, and the Lawrence Livermore National Laboratory (LLNL) have created a "virtual" guide star that will greatly increase the ability of the Keck II telescope using adaptive optics to resolve fine details of astronomical objects. Installed in 1999, the Keck adaptive optics system has enabled astronomers to minimize the blurring effects of the Earth's atmosphere, producing images with unsurpassed detail and resolution. The adaptive optics system uses light from a relatively bright star to measure the atmospheric distortions and to correct for them, but only about one percent of the sky contains stars sufficiently bright to be of use. The new virtual guide star will enable Keck astronomers to study nearly the entire sky with the high resolution of adaptive optics.

A very faint beam from the Keck sodium laser appears in this 20-minute exposure from the Keck II telescope dome. The laser creates a "virtual" star high above the Earth's surface, which is not visible to the human eye, but is bright enough to guide high resolution adaptive optics at Keck. Photo: John McDonald, Canada-France-Hawaii Telescope Corp.
The virtual guide star, which achieved "first light" on December 23, was created using a 20-watt dye laser to illuminate a diffuse layer of sodium atoms that exists 60 miles (95 km) above the Earth's surface. When activated by the laser, the sodium atoms produced a very small source of light, less than 3 feet (1 meter) in diameter, which allowed the adaptive optics system to measure the distortions of the atmosphere. The resulting virtual star was measured at 9.5 magnitude, about 25 times fainter than anything that can be seen by the unaided eye, but bright enough to operate the adaptive optics system. The star appeared orange, the familiar color of common low-pressure sodium vapor street lights. The laser guide star system was funded by the W. M. Keck Foundation, with additional funding provided by the National Aeronautics and Space Administration (NASA) and the National Science Foundation's Center for Adaptive Optics (CfAO). It was developed in collaboration with the Lawrence Livermore National Laboratory (LLNL).

Adaptive optics refers to the ability to compensate or adapt to turbulence in the Earth's atmosphere, removing the blurring of starlight. Adaptive optics systems measure the distortions of the light from a star and then remove the distortions by bouncing the light off a deformable mirror that corrects the image several hundred times per second. With the Keck adaptive optics system, astronomers are obtaining infrared images with four times better resolution than those produced by the Hubble Space Telescope, which orbits high above the Earth's atmosphere. Many significant discoveries have already been attributed to Keck adaptive optics, and the Keck laser guide star system will lead to many more.

The world's largest telescope creates a virtual star over Hawaii, bringing Keck astronomers one step closer to being able to use high resolution adaptive optics on almost any object of scientific interest. Photo: Adam Contos, W. M. Keck Observatory
"We asked for an early present this year, and just before Christmas we were given a virtual star that will dramatically increase the research capabilities of the world's largest telescope," said Dr. Frederic Chaffee, director of the W. M. Keck Observatory. "This effort could not have been possible without the talent and dedication of our adaptive optics and laser guide star teams. We couldn't be happier with these results, and we look forward to fully integrating the laser with our adaptive optics system by the middle of 2002."

The Keck virtual guide star system consists of a dye laser that is used to produce light with the wavelength of the atomic sodium resonance line at 589 nm. The 20-watt output of the dye laser is projected out of a 20-inch (50 cm) lens attached to the side of the 10-meter Keck II telescope.

"We have seen lasers develop into powerful tools in fields ranging from medicine, to laser printers to compact disc players. Our new virtual guide star marks the start of a new era, when we'll see lasers contributing to astronomy as well," said Claire Max of LLNL, principal investigator for the Keck laser project.

The main components of the Keck adaptive optics system are a wavefront sensor camera, a fast control computer and a deformable mirror. The wavefront sensor camera measures distortions due to atmospheric turbulence using light from the guide star. A control computer computes the wavefront distortion up to 670 times a second and sends commands to the deformable mirror. The deformable mirror, about six inches (15 cm) in diameter, is made out of a thin sheet of reflective glass controlled by 349 actuators that can adjust the shape of the mirror by several microns, a distance large enough to correct for atmospheric distortions.