'Lucky Camera' Takes Sharpest Ever Images Of Stars

Until now, images from ground-based telescopes have been invariably blurred by Earth's atmosphere. Astronomers have developed a technique, known as adaptive optics (AO), to correct the blurring, but so far it has only worked successfully in the infrared, where the smearing is greatly reduced.  However, a new noise-free, high-speed camera has been developed at the Institute of Astronomy in Cambridge that, when used behind the infrared Palomar Adaptive Optics System, at last makes very high resolution imaging possible in ordinary visible light. 

The camera works by recording partially corrected adaptive optics images at high speed (20 frames per second or more).  Software then checks each image to sort out which are the sharpest.  Many are still significantly smeared by the atmosphere, but a small percentage of them are unaffected.  These are combined to produce the final high-resolution image that astronomers want.  The technique is called "Lucky Imaging" because it depends on the chance fluctuations in the atmosphere sorting themselves out and providing a set of images that is easier for the adaptive optics system to correct. 

This work was carried out on the 200-inch (5.1 meter) Hale Telescope on Palomar Mountain.  Like all other ground-based telescopes, the images it normally produces are typically 10 times less detailed than those of the Hubble Space Telescope.  Palomar’s adaptive-optics system produces superb images in the infrared, but until now, its images in visible light have remained markedly poorer than Hubble images.  With the new Lucky Camera, astronomers were able to obtain images that are twice as sharp as those produced by the Hubble Space Telescope—a remarkable achievement. 

The images produced in the study are the sharpest direct images ever taken in visible light either from the ground or from space.  "The system performed even better than we were expecting.  It was fantastic to watch the first images come in and see that we were easily doing better than Hubble," says Nicholas Law, a postdoctoral scholar at Caltech and principal investigator for the instrument. 

Most astronomical objects are so far away that astronomers are desperate to see more and more detail within them.  The new pictures of the globular star cluster M13, located 25,000 light years away, are sharp enough that astronomers are able to find stars as little as one light-day apart.  A light-year is the distance light travels in one year (almost 6 trillion miles).  A light-day is the distance light travels in just one day. Stars in the vicinity of the solar system are much farther apart –the nearest star to our solar system is over four light-years away. 

The astronomers also observed very fine detail in objects such as the Cat's Eye Nebula (NGC 6543).  It is eight times closer to earth than M13, allowing filaments that are only a few light-hours across to be resolved.

The use of the camera at Palomar was a demonstration of the potential of visible-light adaptive optics and offers a glimpse of the detailed imagery to come. Astronomers at Caltech and the Jet Propulsion Laboratory are currently developing the first-ever astronomical adaptive-optics system fully capable of capturing visible-light images.  It will routinely allow the 200-inch telescope at Palomar to outperform the Hubble Space Telescope at even blue wavelengths.  Using state-of-the-art deformable mirrors, sensors, and a powerful laser, the upgraded Palomar adaptive-optics system will provide finer correction of the atmospheric blurring than any present adaptive optics system, allowing long-exposure images with the same fine detail as the "lucky" images taken recently.

Caltech's Richard Dekany, principal investigator for the new system, says that the upgraded instrument could be available as early as 2010. "These Lucky Imaging results underscore the science potential of diffraction-limited visible-light observations on large ground-based telescopes," he explains.

To get even sharper pictures, astronomers will need to use bigger telescopes.

The results open up the possibility of further improvements on even larger telescopes, such as the 10-meter Keck telescopes on the top of Mauna Kea in Hawaii or in the future even larger telescopes, such as the Thirty Meter Telescope (TMT).

Working on the Lucky Imaging project were Law, Dekany, Mike Ireland, and Anna Moore from Caltech and the Palomar 200-inch crew. Other team members included Craig Mackay from Cambridge, James Lloyd from Cornell University, and Peter Tuthill, Henry Woodruff, and Gordon Robertson from the University of Sydney.