Earthbound telescope beats even Hubble, thanks to “adaptive optics”
Lab testing of the Large Binocular Telescopes adaptive secondary mirror system © 2008 Large Binocular Telescope Corporation
The Large Binocular Telescope (LBT) atop Mount Graham, Arizona is providing images with sharpness never seen before, by combining two giant 28-foot mirrors with two flexible secondary mirrors fitted with the latest “adaptive optics.”
Until recently, astronomers using ground-based telescopes had to live with the distortion caused by the Earth’s atmosphere. The atmospheric distortion that makes stars twinkle also significantly blurs images of distant objects. In recent tests using adaptive optics that cancel this atmospheric distortion, the LBT delivered images over three times sharper than the Hubble Space Telescope. The test used just one of the telescope’s primary mirrors. When adaptive optics are in place for both mirrors, it is expected that the LBT will achieve images ten times sharper than Hubble’s.
Results “so extraordinary that we thought it might be a fluke”
The Strehl ratio - a measure of image quality - is 100 percent for a perfect image. Without adaptive optics, ground-based telescopes have Strehl Ratios of less than 1 percent. Adaptive optics on other telescopes have improved image quality to about 30 to 50 percent. The LBT’s adaptive optics have provided images with Strehl rations as high as 84 percent.
“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. 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.” said Simone Esposito, leader of the testing team.
The Large Binocular Telescope atop Mount Graham, Arizona © 2008 Large Binocular Telescope Corporation
Three-foot-wide mirror is 1/16-inch thick
The remarkable results were achieved through the combination of several innovative technologies. The first is the telescope’s smaller 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 secondary mirror is three feet in diameter and only 1/16-inch thick. The mirror is so thin and pliable that it can easily be manipulated by actuators pushing on the 672 tiny magnets glued to its back, a configuration which offers far greater flexibility and accuracy than previous systems on other telescopes. An innovative sensor detects atmospheric distortions and manipulates the mirror in real time to cancel out the blurring, allowing the telescope to literally see as clearly 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 ten millionths of a millimeter.