A major milestone in astronomical history
took place recently at the W.M. Keck Observatory when scientists, for the
first time, used a laser to create an artificial guide star on the Keck II
10-meter telescope to correct the blurring of a star with adaptive optics
(AO).

Laser guide stars have been used on smaller telescopes, but this is their
first successful use on the current generation of the world’s largest
telescopes.

The resulting image, captured by the NIRC2 infrared camera, was the first
demonstration of a laser guide star adaptive optics (LGS AO) system on a
large telescope. When complete, the LGS AO system will mark a new era of
astronomy in which astronomers will be able to see virtually any object in
the sky with the clarity of adaptive optics.

“This is one of the most gratifying moments in all my years at Keck,”
remarked Dr. Frederic Chaffee, director of the W.M. Keck Observatory the
evening the observations were made. “Like any positive first light result,
there is much to be done before the system can be considered operational.
But also like any positive first light result, it shows that it can be done,
and gives us great optimism that our goals are not impossible dreams, but
are instead attainable realities.”

Adaptive optics is a technique that has revolutionized ground-based
astronomy through its ability to remove the blurring of starlight caused by
the earth’s atmosphere. Its requirement of a relatively bright “guide star”
in the same field of view as the scientific object of study has generally
limited the use of AO to about one percent of the objects in the sky.

To overcome this restriction, in 1994 the W.M. Keck Observatory began
working with Lawrence Livermore National Labs (LLNL) to develop an
artificial guide star system. By using a laser to create a “virtual star,”
astronomers can study any object in the vicinity of much fainter (up to 19th
magnitude) objects with adaptive optics and reduce its dependence on bright,
naturally occurring guide stars. Doing so will increase sky coverage for the
Keck adaptive optics system from an estimated one percent of all objects in
the sky, to more than 80 percent.

“This new capability of using a laser guide star with a large telescope has
invited astronomers to start exploring the night sky in a much more
comprehensive manner,” said Adam Contos, optics engineer at the W.M. Keck
Observatory. “In the future, I would expect most major observatories to be
installing similar systems to take advantage of this incredible enhancement
to their AO capabilities.”

In January 2001, after more than seven years in development, the Keck and
LLNL teams celebrated the completion of the Keck laser guide star system.
The artificial star results when light from a 15-watt dye laser causes a
naturally occurring layer of sodium atoms to glow about 90 km (56 miles)
above the earth’s surface. It would take another two years of sophisticated
research and design before the laser system could be integrated into the
Keck II adaptive optics system.

In the early morning hours of September 20th, all subsystems finally came
together to reveal the unique capability of the Keck LGS AO system and its
potential to resolve extremely faint objects. The system locked on a 15th
magnitude star, a member of a well-known T Tauri binary called HK Tau and
revealed details of the circumstellar disk of the companion star. It was the
first time an adaptive optics system on a very large telescope had ever used
an artificial guide star to resolve a faint object.

A key challenge the LGS AO team faced was how successful the efforts would
be to integrate and achieve good performance measurements for each required
sub-system. Concerns about the power of the laser and its spot quality,
operation of the laser traffic control system, the ability of the new
sensors to lock on fainter guide stars, and being able to optimize the image
quality through an accurate understanding of the aberrations that could not
be measured by using the laser guide star, were all factored into the
evening’s observing.

“First light was a superb team effort,” said Dr. Peter Wizinowich, team
leader for the adaptive optics team at W.M. Keck Observatory. “It was very
satisfying to have each of the many subsystems perform so well on our first
attempt. To quote Virgil, ‘Audentes Fortuna Juvat,’ fortune favors the
bold.”

The quality of the LGS AO first light images was extremely high. While
locked on a 14th magnitude star, the Keck LGS AO system recorded “Strehl
ratios” of 36 percent (at 2.1 micron wavelength, 30-second exposure time),
compared to four percent for uncorrected images. Strehl ratios measure the
degree to which an optical system approaches “diffraction-limited”
perfection, or the theoretical performance limit, of the telescope.

Another performance metric, the “full width at half maximum” (FWHM), for
this 14th magnitude star was 50 milli-arcseconds, compared to 183
milli-arcseconds for the uncorrected image. FWHM measurements help
astronomers determine the actual edges of an object, where the detection may
be imprecise or difficult to determine. The measurement of 50
milli-arcseconds is about equivalent to being able to distinguish a pair of
car headlights in New York while standing in Los Angeles.

Throughout the evening, the laser guide star held steady and bright, shining
at an approximate magnitude of 9.5, about 25 times fainter than what the
human eye can see, but ideal for the Keck adaptive optics system to measure
and correct for atmospheric distortions.

Additional work is underway before the Keck LGS AO system can be considered
fully operational. The Keck LGS AO system will be available for limited
shared risk science next year, with full deployment to the Keck user
community in 2005.

“Even with just this first test, astronomers are already clamoring to use
the laser guide star system to study distant galaxies with an unprecedented
resolution and power,” said Dr. David Le Mignant, adaptive optics instrument
scientist at the W.M. Keck Observatory, California Association for Research
in Astronomy. “By next year, adaptive optics will be used to study the rich
formation history of early galaxies.”

The importance of this breakthrough to worldwide astronomy was summed up by
Dr. Matt Mountain, the director of the Gemini Observatory, which operates
twin 8-meter telescopes, one on Mauna Kea and one on Cerro Pachon in Chile:
“This is a critical milestone for all ground-based astronomy, not just for
our current generation of eight to 10-meter class telescopes, but also for
our dreams of 30-meter telescopes.”

Team members responsible for the Keck LGS AO system are Antonin Bouchez,
Jason Chin, Adam Contos, Scott Hartman, Erik Johansson, Robert Lafon, David
Le Mignant, Chris Neyman, Paul Stomski, Doug Summers, Marcos van Dam, and
Peter Wizinowich, all from the W.M. Keck Observatory, California Association
for Research in Astronomy. The team gave special thanks to their
collaborators at LLNL: Dee Pennington, Curtis Brown and Pam Danforth.

The laser guide star adaptive optics system was funded by the W.M. Keck
Foundation.

The W.M. Keck Observatory is operated by the California Association for
Research in Astronomy, a scientific partnership of the California Institute
of Technology.