A central region of the globular cluster M92 at 1.6μm as observed with the Hubble Space Telescope (left) and the LBT in adaptive optics mode (right). Credit: LBT/Hubble ' />

A telescope in Arizona has taken the sharpest pictures yet of deep space from Earth with a new system that provides a level of clarity never seen before.

The technology relies on adaptive optics – a mechanism that allows a telescope’s mirror to bend in order to compensate for the blurring of light as it passes through the Earth’s atmosphere.


A central region of the globular cluster M92 at 1.6μm as observed with the Hubble Space Telescope (left) and the LBT in adaptive optics mode (right). Credit: LBT/Hubble

The new system, installed on the $120 million Large Binocular Telescope (LBT) on Mount Graham in Arizona, has delivered images three times sharper than the Hubble Space Telescope, LBT scientists said. And that’s with only one of its twin 27.6-foot (8.4-meter) mirrors working.

“The results on the first night were so extraordinary that we thought it might be a fluke, but every night since then the adaptive optics have continued to exceed all expectations,” said astronomer Simone Esposito, leader of a team from Italy’s Arcetri Observatory of the Istituto Nazionale di Astrofisica (INAF). “These results were achieved using only one of LBT’s mirrors. Imagine the potential when we have adaptive optics on both of LBT’s giant eyes.”

INAF collaborated with the University of Arizona’s Steward Observatory to build the adaptive optics device, called the First Light Adaptive Optics system (FLAO).

The Hubble Space Telescope has achieved groundbreaking images thanks to its perch above Earth’s atmosphere. It and other space telescopes also have another advantage over ground-based observatories: They can focus on one celestial object for hours or even days, steadily collecting light to form a brighter image. Imagine putting such a system on the moon!

Bad weather can often interrupt observing sessions from Earth, and even under perfectly clear conditions, a ground-based optical telescope scanning the night sky must stop observing when the sun rises.

Yet the Hubble telescope itself – with a main mirror 7.9 feet (2.4 meters) wide – is somewhat small compared to the largest ground-based observatories. However, those Earth-bound telescopes are hampered by the atmosphere, which blurs light passing through much the way water blurs the view from the bottom of a swimming pool.

That’s where adaptive optics come in.

The LBT has a 3-foot (0.91-meter) wide secondary mirror built in. This mirror, only 0.06 inches (1.6 millimeters) thick, is so pliable that it can easily be bent by devices pushing on the 672 tiny magnets glued to its back.

A special sensor detects atmospheric distortions in real time and controls the mirror to adjust its position to compensate, effectively canceling out the blurring. The mirror can make adjustments every one-thousandth of a second, with accuracy to better than ten nanometers (a nanometer is one millionth the size of a millimeter).

Astronomers measure image sharpness with a term called the Strehl ratio, where 100 percent would be a perfect image. Without adaptive optics, most ground-based observatories have a Strehl ratio of less than 1 percent.

With the new system, the LBT has achieved peak Strehl ratios of 82 to 84 percent.

“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 Richard Green, director of the LBT. “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.”

The idea of putting observatories on the Moon has been around for quite some time. The first, and so far only, lunar astronomical observatory was deployed by the Apollo 16 crew in 1972. The Far Ultraviolet Camera/Spectrograph used a 3-inch diameter Schmidt telescope to photograph the Earth, nebulae, star clusters, and the Large Magellanic Cloud. The tripod mounted astronomical equipment was placed in the shadow of the Lunar Module so it would not overheat. The Far Ultraviolet Camera took pictures in ultraviolet light which would normally be blocked by the Earth’s atmosphere. It had a field of view of twenty degrees, and could detect stars having visual magnitude brighter than eleven. 178 images were recorded in a film cartridge returned to Earth. The observatory still stands on the Moon today.

Why is the moon such a good place for astronomy? First of all, the moon has no atmosphere. The sky is perfectly black and the stars do not twinkle. Stars and galaxies can be observed at all wavelengths including x-ray, ultraviolet, visible, infrared, and radio. In addition, night time on the Moon lasts about 350 hours. This would permit scientists to watch deep space objects for very long periods, or to accumulate signals on very faint sources such as dim stars, galaxies, or planets around other stars.

In contrast, the Hubble Space Telescope, NASA’s current premier telescope for space research, is in a low earth orbit some 350 km high (the moon is 450,000 km away). Sunrise and sunset are only 90 minutes apart on the HST, meaning that the dark time (the time HST is in Earth shadow) is only 45 minutes long which is a major constraint for astronomers.

Unlike orbiting spacecraft, the moon is a very large and ultra-stable platform for telescopes of any kind and has no seismic activity unless there is meteoric impact. Average ground motion on the surface is estimated to be less than 1 micron (one millionth of a meter, or about the thickness of a hair).

This stability is crucial for ‘optical interferometers’, instruments needed to carry out a systematic search of planets around other stars within our own galaxy. An interferometer is an array of several telescopes that work together to increase magnification ability. The new technique of using adaptive optics will likely be incorporated in future interferometer designs.

The Moon is very near to the Earth relative to other planets. Round trip light travel time is about 2.5 seconds. This means a telescope on the Moon can be controlled from ground station with a nearly instantaneous response. (This goes for all kinds of remotely controlled operations, not just telescopes). Except for rare meteoric hits, a lunar telescope could last almost indefinitely as there is no weather on the moon. The retro-reflectors left on the moon by the Apollo astronauts, for example, are still in operation after more than thirty years.

A telescope on the moon will remain productive for many decades at low cost. The purpose of the NASA Lunar Telescope Deployment task is to develop and demonstrate telerobotic technologies that enable an unmanned lunar observatory that is constructed and operated from Earth. Specifically, the task is to study an optical interferometric telescope for the moon.

Posted by: Soderman/NLSI Staff
Source: http://www.space.com/scienceastronomy/telescope-space-pictures-adaptive-optics-100616.html

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