http://www.nctimes.com/articles/2006/12/03/science/16_20_5412_2_06.txt

Artificial star to shine over, guide Palomar Observatory

By: QUINN EASTMAN - Staff Writer

PALOMAR MOUNTAIN ---- In February, astronomers at Palomar Observatory expect to create an artificial star in the Earth's upper atmosphere.

The star, really more like a lamp powered from the ground by a laser beam, is part of a system called adaptive optics. The system allows scientists to correct for the fluctuations in the atmosphere that make stars twinkle to the naked eye ---- and blur in their telescopes.

Although Palomar has been testing the lamp for a few years, February is the first time it will be used for astronomy, said Antonin Bouchez, head of the adaptive optics team at the California Institute of Technology, which runs Palomar.

"We're sort of handing it over to the astronomers for regular use,"
Bouchez said recently.

Adaptive optics works by measuring the changing light from a bright reference star ---- either natural or artificial ---- and having a computer calculate how to re-distort the incoming light to make that reference point stop wavering.

The computer is connected to crystals on the back side of a flexible 6-inch mirror. When electric current is applied, the crystals push or pull the mirror several hundred times per second, bringing the atmosphere-distorted image back into focus.

Research telescopes all over the world use adaptive optics, and Palomar Observatory's 200-inch Hale has been using it since 1999. It allows Earth-based telescopes to compete effectively with satellite-mounted instruments such as the Hubble Space Telescope.

Adaptive optics have helped astronomers get a close look at asteroids, Jupiter's Great Red Spot and Saturn's moon Titan, for example.

It allowed Caltech planet hunter Mike Brown to find a 150-mile-wide moon orbiting Eris, the 9-billion-mile-distant "dwarf planet" whose discovery drove planetary scientists to create a new classification for it ---- and Pluto ---- this fall.

However, most of the times adaptive optics have been used, astronomers pick out natural stars that are close to what they want to look at. So far, that approach has limited the available targets.
Adaptive optics has been used mostly to study planets rather than distant galaxies.

"We're trying to use adaptive optics on fainter objects," said Jenny Roberts, a scientist at NASA's Jet Propulsion Laboratory who manages Palomar's system. "The laser gives you an automatic guide star that is bright enough to put wherever you want."

Astronomers had been dreaming for years about being able to correct for the atmosphere's effects without leaving it.

Horace Babcock, an astronomer at the Mt. Wilson Observatory near Los Angeles, came up with the idea of having a deformable mirror adjust to a reference light in the 1950s.

Interested in keeping track of Soviet satellites, the U.S. Air Force played a major role in developing modern adaptive optics. Astronomers working with the Air Force convinced the military to declassify the technology in 1991.

Only a couple research telescopes ---- the University of California's Lick telescope east of San Jose and the Keck II on Hawaii's Mauna Kea
---- have been using an artificial laser-created guide star.
Palomar's laser system makes three, and more are coming.

Near the top of the Hale's dome, the laser gets spread out into a beam a couple feet wide.

In the upper atmosphere, the yellow laser light excites sodium atoms, which come from the debris of burned-up meteorites. Immediately after a meteor shower strikes the Earth, more sodium is available for the laser to light up, Bouchez said.

Every time the laser guide star gets used, Palomar has to work with the Federal Aviation Administration and the military to make sure the beam doesn't come near any airplanes or satellites.


Although the laser's 4 watts of power aren't enough to damage airplanes, they could interfere with pilots' vision. Radio-wielding spotters have to stand outside to catch any planes that might stray near the observatory.

Bouchez said Caltech engineers are working on making a deformable mirror with 10 times as many crystals, which will increase the system's resolution. Their work is aimed at solving engineering problems for an instrument that hasn't been built yet: the Thirty Meter Telescope.

A consortium of American and Canadian research institutions are deciding where a telescope with a mirror six times the size of Palomar's 200-inch can be built.

The most likely locations are Chile's Atacama Desert or Hawaii's Mauna Kea, according to project manager Gary Sanders, who gave a presentation at Palomar Observatory in October.

The huge telescope will be the first designed with adaptive optics built in, Sanders said.

To be effective, the power and number of lasers will have to increase along with the size of the mirror.

"From one telescope, we may eventually have several lasers shining into different parts of the sky," Bouchez said.

Contact staff writer Quinn Eastman at (760) 740-5412 or qeastman@nctimes.com.