Scientists from the Lawrence Livermore National
Laboratory, 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 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.

The virtual guide star, which achieved "first light" on Dec. 23, 2001,
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, that 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 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).

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 will 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, 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 University of California’s Lick Observatory at Mount
Hamilton, CA.

"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 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 and LLNL staff played a key role in the deployment of the laser
at the telescope.

For images of the virtual guide star, see

For further images, go to

Founded in 1952, Lawrence Livermore National Laboratory is a national
security laboratory, with a mission to ensure national security and
apply science and technology to the important issues of our time.
Lawrence Livermore National Laboratory is managed by the University
of California for the U.S. Department of Energy’s National Nuclear
Security Administration.

Photo of the Keck "virtual" guide star, showing the orange laser beam
emerging from the dome of the Keck II Telescope atop 14,000 foot Mauna
Kea volcano in Hawaii. In this 20 minute time-exposure, motion of the
stars has made "trails" or streaks in the sky, and the Keck Observatory
hazard lights can be seen on the road in the foreground. Lights from
hotels on the Kona Coast can be seen in the background to the lower
right. Photo credit: John McDonald, Canada France Hawaii Telescope