A long exposure allows a view of the five-laser array pointing toward the sky above Mount Hopkins. The newly developed system will help astronomers see more clearly. COURTESY OF THOMAS STALCUP

The UA's coolest lasers aren't the ones set to rock music at Flandrau Planetarium.

A pentagram of green lasers, developed by the University of Arizona's Center for Astronomical Adaptive Optics, occasionally pierces the dark, silent sky above Mount Hopkins.

A similar laser array, the latest breakthrough in taking the twinkle out of starlight, will soon be installed on Mount Graham's Large Binocular Telescope and may one day accompany the world's largest next-generation telescopes to mountain tops in Chile and Hawaii.

The lasers are a key component of the latest advance in adaptive optics, which help astronomers see more clearly by measuring the turbulence above a ground-based telescope and then correcting for it with deformable mirrors.

Using a laser as a guide star is nothing new, and using multiple lasers was proposed as early as the late '80s, said Michael Hart, an astronomer at the UA's Steward Observatory.

It was always considered feasible, but it's a "wicked" mathematical problem requiring very fast computers, Hart said.

The simple answer to why it hasn't been done before now? "Because it's really hard," said Hart.

Hart was recently awarded nearly $2 million by the National Science Foundation to prepare a ground-level laser adaptive optics system for the Large Binocular Telescope after successfully demonstrating its use on the UA/Smithsonian MMT Observatory atop Mount Hopkins in the Santa Rita Mountains south of Tucson.

Hart said the MMT test proved the system works. Hart and his fellow researchers from the UA's astronomy and optical sciences departments published the results of that successful test in the journal Nature in August.

Current systems of adaptive optics calculate turbulence by using an existing "guide star" or by projecting a single, very high frequency yellow laser light to reflect off sodium atoms that congregate in a 6-mile thick band about 55 miles above the surface of the Earth.

Instruments called wavefront sensors measure the degree of turbulence within a cone that extends from that beacon to the telescope mirror and a computer sends messages to thin, pliable mirrors to correct the aberrations caused by wind and weather, 1,000 times a second.

That single guide star or laser beacon does a "very good job for a telescope mirror up to 10 meters in diameter. It gets worse as the telescope mirror gets bigger," Hart said.

At a wider diameter, that triangular cone leaves out more and more of the column of atmosphere that extends from the telescope mirror into space.

Since most of the aberrations are caused by the ground level of the atmosphere, Hart figured that he could get rid of most of the disturbance by using a green laser to project five guide stars. Signals from them overlap within the cylinder that extends from the telescope mirror to where the lasers reflect off nitrogen and oxygen atoms about 12 to 18 miles above the Earth.

"You stitch these five signals together to get a unified view of what is happening in the atmosphere," Hart said.

For telescopes smaller than 10 meters, the advantage of the system is that it clears a wide-angle path.

The pentagram beam starts as two lasers that get combined at the base of the telescope and are directed through a hologram - a computer-etched lens that splits the beam into five equally spaced points of light.

His research should lead to a system that can be used on the giant telescopes now being developed, such as the 23-meter Giant Magellan Telescope and the Thirty Meter Telescope.

"If you are going to spend a billion dollars on a telescope," he said, "make damn sure you milk it for all the science you can get out of it."

He started with the 6.4-meter MMT, he said, because "you don't want to do this for the very first time on a very large telescope."

Prior to installation of his ground-layer adaptive optics system, the 6.5-meter UA/Smithsonian MMT adapted its images the "old-fashioned" way - using a guide star.

The pentagram of beacons should have plenty of uses when the mirror is used in wide-angle fashion, allowing astronomers to image a panorama rather than a particular point of light.

Hart sees a long future for the MMT, whose prominence as the 14th largest telescope on Earth is dimmed only by the worsening quality of the night sky around it. With this new laser capability, it could do wide-field studies in infrared wavelengths where atmospheric blurring, not light pollution, is the major problem.

"Our future may well lie in the direction Michael is talking about," said MMTO director Faith Vilas. "As much as we have fought to control light pollution, we still have a lot of ambient light concerns."

For the Large Binocular Telescope on Mount Graham near Safford, the ground-layer adaptive optics systems will be especially useful when its two 8.4-meter mirrors are combined to create a very wide field of view.

Director Richard Green said the LBT's international partners are working with Hart's U.S. team to develop the system. The Germans "have these wonderful big lasers in hand," Green said. The Italian partners are developing the camera that will analyze the distortions. The system should be in place by 2012.

Ultimately, Green said, the LBT will also employ a higher level yellow sodium laser. Right now, it must rely on guide stars, which aren't available in every direction an astronomer might want to look.

Hart is also part of the development team for the Giant Magellan Telescope, a 23-meter telescope proposed for a mountaintop in Chile. That next-generation telescope will use an array of laser systems, coupled with multiple deformable mirrors.

 

DID YOU KNOW

The MMT Observatory, operated by the University of Arizona and the Smithsonian Institution atop Mount Hopkins in the Santa Rita Mountains, does not use multiple mirrors to collect light, even though its initials once stood for "Multiple Mirror Telescope."

The partners simply decided not to change the historic name in May 2000 when they replaced the six mirrors that added up to a 4.5-meter telescope with a single 6.5-meter telescope mirror that was cast and polished at the UA's Steward Observatory Mirror Lab.

A meter is slightly smaller than 3.3 feet, so the 6.5-meter telescope has a diameter slightly larger than 21.3 feet.

Contact reporter Tom Beal at tbeal@azstarnet.com or 573-4158.