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Mount Graham telescope now offers clearest universe images ever

UA, Italian project clears up starlight distortions with flexible mirrors

Fulfilling decades of promise, the international Large Binocular Telescope on Mount Graham is now capable of delivering the sharpest images of the universe ever taken.

Astronomers from the University of Arizona and Italy have recently completed the initial testing of the world’s most ambitious system to compensate for the natural distortions of starlight caused by Earth’s ever-changing atmosphere, and have declared great success with this “adaptive optics” system on the LBT.

The gargantuan LBT consists of two giant glass mirrors, each with a diameter of 8.4 meters (27 feet), mounted side by side on the same massive steel structure. Incoming light from faint stars and galaxies bounces off each of these primary mirrors toward separate secondary mirrors, which re-direct the light toward a variety of sensitive cameras and light-splitting instruments called spectrographs.

Thanks to a collaboration between the University of Arizona’s Steward Observatory and Italy’s Arcetri Observatory of the Instituto Nazionale di Astrofisica, one of these normally rigid secondary mirrors has been replaced with a highly complex flexible mirror backed by 672 tiny magnets. These tightly packed magnets, driven by a computer, push and pull on the thin surface of the mirror up to 1,000 times per second so that its shape constantly and precisely cancels out the unavoidable twinkling in incoming starlight caused by waves and eddies in the atmosphere.

The pliable secondary mirror is 0.9 meters (3 feet) in diameter, but only 1.6 millimeters (0.063 inches) thick, and can be adjusted with an accuracy of better than ten-millionths of a millimeter. In tests that began on May 12, the system achieved 84 percent of the maximum possible correction of the incoming light.

This is nearly twice as good as any existing adaptive optics system in civilian astronomy - which tend to have small flexible mirrors inside an instrument, rather than the large surface area of the LBT’s secondary mirror. It is three times sharper than the Hubble Space Telescope can deliver, and about 80 times better than a ground-based research telescope can achieve without such a system.

Astronomers are eager to use the new adaptive optics system, and are even more thrilled that it worked as well in its first tests on the LBT as it had in the laboratory in Florence, Italy.

“This is an incredibly exciting time as this new adaptive optics system allows us to achieve our potential as the world’s most powerful optical telescope,” said LBT Director Richard Green. “The successful results show that the next generation of astronomy has arrived, while providing a glimpse of the awesome potential the LBT will be capable of for years to come.”

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The science observations that will benefit the most from the new system will be those taken in near-infrared light; these wavelengths are slightly longer than those in the visible part of the spectrum that human eyes can see, and somewhat like trying to match a puzzle piece with fewer notches, they are therefore a bit easier for the adaptive optics to match and correct.

One area of research will study the properties of individual stars in very crowded fields such as inside a globular cluster, or in the central regions of nearby galaxies when there is a good foreground star to use as the reference point for the adaptive optics system, Green said in response to questions from the Tucson Sentinel.

In crowded fields, the power of the LBT will allow astronomers to establish whether a very faint star is part of the cluster or not, thus including its properties in the “census” of the cluster when it most likely would have been missed before. In nearby galaxies, the LBT will enable astronomers to resolve individual stars whose brightnesses and colors would have been blended together before, thereby promoting much more accurate “stellar archaeology” of the evolutionary history of the galaxy, Green explained.

Green’s personal favorite subject area that will be aided by the new system is spectroscopically measuring the mass of black holes in the nuclei of nearby galaxies.

“Small black holes have a very limited region of influence where their gravity dominates over that of the other material in the core of the galaxy,” Green said. “By sharpening up the galaxy image, we don't get as much blending of light from regions farther out, so the dynamical probe is much cleaner.”

Each of the primary mirrors on the LBT will be gaining its own flexible secondary mirror, whose thin concave shape has proven to be quite fragile in the lab and in shipping, where two already have been lost to damage. A new second unit is assembled in Italy and scheduled to be tested “on the sky” at the LBT in the spring.

When working together at maximum capability (known as the diffraction limit), the two 8.4-meter mirrors of the LBT can yield the resolving power of a telescope with a single mirror over 22 meters in diameter, or more than 10 times better than the Hubble telescope.

When both adaptive optics secondary mirrors are installed and working properly, University of Arizona astronomer Phil Hinz will be able to use a new NASA-funded instrument on the LBT in a powerful campaign to take direct images of planets around other stars.

Nearly all of the more than 460 “exoplanets” found to date have been detected indirectly from their tug on their parent star, or by the faint dip in light that they cause in the star as they pass in front of it as viewed from Earth. The technique to be exploited by Hinz, known as nulling interferometry, will combine the light from both LBT primary mirrors in a carefully controlled way that blacks out the central star and gives three times higher resolution than any current system, Green said.

The LBT is an international collaboration among institutions in the United States, Italy and Germany, who have invested more that $120 million in the project.

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The roots of the telescope trace back to 1989, although land-use issues with the San Carlos Apache Tribe – and environmentalists concerned about the impact of the observatory’s construction on the population of rare Mount Graham red squirrels – delayed the first scientific observations by the giant telescope until October 2005.

In one sign that the red squirrels continue to thrive despite the presence of the LBT and two other major telescopes on the 10,200-foot peak east of Tucson, KGUN reported on Friday that the Arizona Department of Transportation has declined to spend $1.25 million to create crossing paths for the squirrels on Mount Graham, and will instead return the funds to the federal government.

Arizona Game and Fish estimated last fall that approximately 250 red squirrels live on Mount Graham, which is very near the estimated population at the start of LBT construction.

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Click image to enlarge

R. Cerisola

A picture of the movable secondary mirror during its installation in the Arcetri lab. The image shows the 672 tiny magnets spread over the back of the mirror. The reflecting face of the mirror is face down. The upper portion contains the electro-mechanical devices that control the magnets.

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