Guide star helps Keck see the light

Lab scientists in collaboration with the W.M. Keck Observatory have created a "virtual" guide star over Hawaii. The virtual guide star will be used with adaptive optics on the Keck II telescope to greatly increase the resolution of 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 unprecedented 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 1 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.

The virtual guide star, which achieved "first light" on Dec. 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 a meter (39 inches) in diameter, which allowed the adaptive optics system to measure the distortions of the atmosphere. Two more commissionings are scheduled for later this month.

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 same color of common low-pressure sodium vapor streetlights. The virtual guide star system was developed in collaboration with the W.M. Keck Observatory, with additional support provided by the National Aeronautics and Space Administration (NASA) and the National Science Foundation’s Center for Adaptive Optics (CfAO) at UC Santa Cruz.

Scientists and engineers from the Laser Science & Technology Division of the National Ignition Facility Directorate at LLNL played a key role in preparing the laser that creates the guide star. For two and a half years, Deanna Pennington, Curtis Brown, Pam Danforth and Holger Jones worked in Hawaii redesigning, demonstrating and commissioning the laser system on the telescope so that the virtual guide star could achieve "first light."

The team hopes to integrate the system with adaptive optics in June when they will be able to see a real star.

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 Keck adaptive optics, for which LLNL scientists developed the fast real-time control system, astronomers are obtaining infrared images with four times better resolution than 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 virtual guide star is expected to lead to many more.

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

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. It is based on a concept originally implemented by LLNL scientists at the UC Lick Observatory at Mount Hamilton.

"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 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 team. 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 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.

The Keck virtual guide star system is the world’s most powerful laser currently in use at an astronomical telescope. The laser was developed by LLNL staff who also played a key role in the deployment of the laser at the telescope.

For images of the virtual guide star, see http://www.llnl.gov/llnl/06news/NewsMedia/keck_images.html For further images, go to http://www2.keck.hawaii.edu:3636/realpublic/gen_info/kiosk/news/laser.html