Like those generated by ground-based telescopes, the images normally produced by the Hale Telescope have been less detailed than those taken by Hubble, primarily because of atmospheric turbulence which limits the resolution or "sharpness" of the images. In the past, astronomers have used a technique called adaptive optics (AO) to correct for such blurring, but so far the technique has been applied successfully only in the infrared spectrum.
However, a recent low-noise, high-speed camera used in conjunction with the infrared Palomar Adaptive Optics System allowed for high-resolution imaging in the visible spectrum. The technique has been dubbed "Lucky imaging" as it relies on selecting images at those fortunate moments when chance fluctuations in the atmosphere are the smallest. Such a technique allowed astronomers to obtain images twice as sharp as those produced by Hubble. Nicholas Law, principal investigator for the instrument and postdoctoral scholar at Caltech, states that "the system performed even better than [they] were expecting." "It was fantastic to watch the first images come in and see that we were easily doing better than Hubble," he added. Fellow researcher Craig Mackay from the University of Cambridge said, "To produce images sharper than Hubble from the ground is a remarkable achievement by anyone's standards."
In order to appreciate how Lucky imaging is better than adaptive optics, one needs to first understand how adaptive optics works. Adaptive optics here refers to optical systems that can adapt to compensate for atmospheric distortions. (Incidentally, it is these distortions that also cause stars to appear to twinkle when viewed by the unaided eye.)
Adaptive optic systems work by measuring these distortions using a sensor and correcting for them, usually through the use of deformable mirrors; a computer rapidly calculates the changes in mirror shape required to correct for these distortions, and then reshapes the mirror surface as needed. Such changes must be carried out on millisecond (one-thousandth of a second) timescales, as fluctuations in the atmosphere occur quite rapidly. Today, in addition to its use in improving astronomical imaging, adaptive optics is also used in high-resolution imaging of the retina.
Lucky imaging builds on adaptive optics in that it records the corrected images at high speed, at 20 frames per second or more. While a majority of the images are smeared due to atmospheric effects, a small percentage of them are unaffected. Now instead of simply carrying out adaptive optics, in which all the images get added together irrespective of their quality, the Lucky imaging technique uses software to preferentially select the sharpest, least distorted images, which are then combined to form the final high-resolution image.
Lucky imaging has been used to study, among other subjects, the globular cluster M13 that is located 25,000 light years away and the Cat's Eye Nebula 3,000 light years away. (A light year is the distance covered by light in a year, and is almost six trillion miles.) The images obtained are detailed enough to resolve structures that are as small as a light-day apart in M13 and few light-hours apart in the Cat's Eye Nebula. The images obtained have resolutions of about 0.1 arc seconds. For comparison, if a two-kilometer wide crater on the surface of the moon was to be seen from the earth it would roughly measure an arc second.
According to scientists at Caltech, the Lucky imaging is a prelude to upcoming advances in adaptive optics that will allow for even sharper images. To obtain such high-resolution images astronomers will need to use bigger telescopes, like the 10-meter Keck telescopes at Mauna Kea in Hawaii and the Thirty Meter Telescope (TMT), currently scheduled for completion in 2014. The Hubble Space Telescope, in contrast, has a diameter of about 2.4 meters.
Mackay added, "the images space telescopes produce are of extremely high quality but they are limited to the size of the telescope. Our techniques can do very well when the telescope is bigger than Hubble and has intrinsically better resolution." However, it is relevant to note that Lucky camera reaches its maximum resolution only when looking at fairly bright objects in very narrow fields of view. The brightness necessitates fewer pictures to be taken for a good view, while the small field of view allows the camera to take pictures rapidly. For larger, less-bright objects, Hubble still produces the sharpest images, and as Mackay asserts, the Lucky imaging technique is not meant to replace Hubble.
Professor of Astronomy at Amherst College George Greenstein sums up the current state of astronomical imaging as follows: "There has always been a tension between doing astronomy using ground-based telescopes and doing it from space. Orbiting telescopes are free of the blurring effects of our atmosphere, and so have a wonderfully clear view of the heavens.
"They are, however, enormously expensive as compared to ground-based telescopes," Greenstein said. "For the price of the Hubble, we could have built 16 state-of-the-art ground-based telescopes, for instance … The technique of Adaptive Optics is revolutionizing astronomy from the ground. The newest images from Caltech and the University of Cambridge already are rivaling those from Hubble. I'm willing to bet that in the near future they will exceed it. Stay tuned!"