June 16, 2010
The Universe, Crisp And Clear
The next generation of adaptive optics has arrived at the Large Binocular Telescope in Arizona, providing astronomers with a new level of image sharpness never before seen.
Developed in a collaboration between Italy's Arcetri Observatory of the Istituto Nazionale di Astrofisica, or INAF, and the University of Arizona's Steward Observatory, this technology represents a remarkable step forward for astronomy.
Until relatively recently, ground-based telescopes had to live with wavefront distortion caused by the Earth's atmosphere that significantly blurred the images of distant objects (this is why stars appear to twinkle to the human eye). While there have been advancements in adaptive optics technology to correct atmospheric blurring, the LBT's innovative system takes this concept to a new level.
This success was achieved through the combination of several innovative technologies. The first is the secondary mirror, which was designed from the start to be a main component of the LBT rather than an additional element as on other telescopes. The concave secondary mirror is .91 meters in diameter (3 feet) and only 1.6 millimeters thick.
The mirror is so thin and pliable that it can easily be manipulated by actuators pushing on 672 tiny magnets glued to the back of the mirror, which offers far greater flexibility and accuracy than previous systems on other telescopes. An innovative "pyramid" sensor detects atmospheric distortions and manipulates the mirror in real time to cancel out the blurring, allowing the telescope to literally see as clear as if there were no atmosphere.
Incredibly, the mirror is capable of making adjustments every one thousandth of a second, with accuracy to better than 10 nanometers (a nanometer is one millionth the size of a millimeter).
In closed-dome tests beginning May 12 and sky tests every night since May 25, astronomer Simone Esposito and his INAF team tested the new device, achieving exceptional results.
The LBT's adaptive optics system, called the First Light Adaptive Optics system, or FLAO, immediately outperformed all other comparable systems, delivering an image quality greater than three times sharper than the Hubble Space Telescope using just one of the LBT's two 8.4 meter mirrors. When the adaptive optics are in place for both mirrors and their light is combined appropriately, it is expected that the LBT will achieve image sharpness 10 times that of the Hubble.
Setting a New Standard for Optical Astronomy
The index of the perfection of image quality is known as the Strehl Ratio, with a ratio of 100 percent equivalent to an absolutely perfect image. Without adaptive optics, the ratio for ground-based telescopes is less than 1 percent. The adaptive optics systems on other major telescopes today improve image quality up to about 30 percent to 50 percent in the near-infrared wavelengths where the testing was conducted.
In the initial testing phase, the LBT's adaptive optics system has been able to achieve unprecedented Strehl Ratio of 60 to 80 percent, a nearly two-thirds improvement in image sharpness over other existing systems.
The results exceeded all expectations and were so precise the testing team had difficulty believing its findings. However, testing has continued since the system was first put on the sky on May 25, and the LBT's adaptive optics have functioned flawlessly and achieved peak Strehl Ratios of 82 to 84 percent.
"The results on the first night were so extraordinary that we thought it might be a fluke, but every night since the adaptive optics have continued to exceed all expectations. 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," Esposito said.
More images from the adaptive optics system are available at the LBT Observatory website.
A Decade of Effort Delivers Technological Triumph
Development of the LBT's adaptive optics system took longer than a decade through an international collaboration. INAF, in particular the Arcetri Observatory, conceived the instrument design and developed the electro-mechanical system, while the University of Arizona Mirror Lab created the optical elements, and the Italian companies Microgate and ADS International engineered several components.
A prototype system was previously installed on the Multiple Mirror Telescope, or MMT, at Mt. Hopkins, Ariz. The MMT system uses roughly half the number of actuators as the LBT's final version, but it demonstrated the viability of the design. The LBT's infrared test camera, which produced the accompanying images, was a joint development of INAF in Bologna and the MPIA in Heidelberg.
"This has been a tremendous success for INAF and all of the partners in the LBT," said Piero Salinari, research director at the Arcetri Observatory, INAF. "After more than a decade and with so much care and effort having gone into this project, it is really rewarding to see it succeed so astoundingly."
More on LBT
The $120 million LBT on Mount Graham utilizes two giant 8.4 meter mirrors and with the new adaptive optics the telescope will have the resolution of a 22.8-meter, or approximately 75-foot telescope. The new adaptive optics will enable versatile instruments such as the near-infrared camera spectrometer, which allows astronomers to penetrate interstellar dust clouds and reveal the secrets of the youngest and most distant galaxies, to achieve their full potential on the LBT.
The LBT is an international collaboration among institutions in the U.S., Italy and Germany. The LBT Corporation partners are:
* The University of Arizona on behalf of the Arizona university system
* Istituto Nazionale di Astrofisica, Italy
* LBT Beteiligungsgesellschaft, Germany, representing the Max Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University
* The Ohio State University
* The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota and University of Virginia
Image 1: The movable secondary mirror during its installation in the Arcetri lab. The image shows the 672 tiny magnets spread on the back of the mirror. The reflecting face of the mirror is facedown. The upper instrument contains the electro-mechanical devices that control the magnets.
Image 2: A central region of the globular cluster M92 at 1.6Ã¼m as observed with the Hubble Space Telescope (left) and the LBT in adaptive mode (right). It is clear that the resolution and depth achieved with LBT surpass even those of the Hubble image.
Image 3: A double star as observed with the LBT in standard mode (left), and with the adaptive correction activated (right). Because of the atmospheric blurring, the fainter companion of the star cannot be identified in the images taken in standard mode, while it is easily visible when the adaptive module is activated. A third faint star becomes also visible in the upper right part of the frame, thanks to the increased efficiency of the telescope in adaptive mode.
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