Lucky imaging

From Wikipedia, the free encyclopedia

Lucky image of M15 core

Lucky imaging is an astronomical photographic technique using a high-speed camera with exposure times short enough (100 ms or less) so that the changes in the atmosphere during the exposure are minimal. From these images, those least affected by the atmosphere (typically around 10%) are chosen and combined into a single image by shifting and adding the short exposures, yielding much higher resolution than would be possible with a single, longer exposure which includes all the frames.

1 Explanation
2 History
3 Popularity of technique
4 Alternative methods
5 References
6 External links

[edit] Explanation

Images taken with ground based telescopes are subject to the blurring effect of atmospheric turbulence (seen to the human eye as the stars twinkling). Many astronomical imaging programs require higher resolution than is possible without some correction of the images. Lucky imaging (also called lucky exposures) is one of several speckle imaging methods used to remove atmospheric blurring. Used at a 1% selection or less, lucky imaging can reach the diffraction limit of even 2.5 m aperture telescopes, a resolution improvement factor of at least five over standard imaging systems.

$The best 1% of exposures of the 0.8 arcsecond separation binary star zeta Boötis taken with the Nordic Optical Telescope on 13 May 2000. The Airy disc around each of the stars is caused by diffraction from the 2.56 m telescope aperture.$

The best 1% of exposures of the 0.8 arcsecond separation binary star zeta Boötis taken with the Nordic Optical Telescope on 13 May 2000. The Airy disc around each of the stars is caused by diffraction from the 2.56 m telescope aperture.

[edit] History

Lucky imaging was first used in the middle 20th century, and became popular for imaging planets in the 1950s and 1960s (using cine cameras or image intensifiers). The first numerical calculation of the probability of obtaining lucky exposures was an article by David L. Fried in 1978.^[1] In early applications of lucky imaging, it was generally assumed that the atmosphere "smeared-out" or "blurred" the astronomical images (see e.g. Nieto and Thouvenot 1991 in References below). In this work, the FWHM of the blurring was estimated, and used to select exposures. Later studies (e.g. Tubbs 2003, Law et al 2005 below) took advantage of the fact that the atmosphere does not "blur" astronomical images, but generally produces multiple sharp copies of the image (the point spread function has "speckles"). New methods were used which took advantage of this to produce much higher quality images than had been obtained assuming the image to be "smeared".

[edit] Popularity of technique

Both amateur and professional astronomers have begun to use this technique. Modern webcams and camcorders have the ability to capture rapid short exposures with sufficient sensitivity for astrophotography, and these devices are used with a telescope and the shift-and-add method from speckle imaging (also known as image stacking) to achieve previously unattainable resolution. If some of the images are discarded, then this type of video astronomy is called lucky imaging. Many methods exist for image selection, including the Strehl selection method from the Cambridge group^[2] and the image contrast selection used in the Selective Image Reconstruction method of Ron Dantowitz.^[3] The recent development of EMCCDs has allowed the first high quality lucky imaging of faint objects.

[edit] Alternative methods

Other approaches that can yield resolving power exceeding the limits of atmospheric seeing include adaptive optics, interferometry, other forms of speckle imaging and space-based telescopes such as NASA's Hubble Space Telescope.

[edit] References

^ Fried, David L. (December 1978). "Probability of getting a lucky short-exposure image through turbulence". Optical Society of America 68: 1651-1658.
^ http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/
^ Dantowitz, Ronald F.; Teare, Scott W.; Kozubal, Marek J. (May 2000). "Ground-based High-Resolution Imaging of Mercury". The Astronomical Journal 119 (5): 2455-2457. DOI:10.1086/301328.

Law et al 2005, Lucky Imaging: High Angular Resolution Imaging in the Visible from the Ground
Tubbs 2003, Lucky Exposures: Diffraction limited astronomical imaging through the atmosphere
Nieto and Thouvenot 1991, Recentring and selection of short-exposure images with photon-counting detectors. I - Reliability tests
C. L. Stong 1956 interviewing scientist Robert B. Leighton for Amateur Scientist, "Concerning the Problem of Making Sharper Photographs of the Planets", Scientific American, Vol 194, June 1956, p. 157. Early example of exposure selection with mechanical tip-tilt correction (using cine film and exposure times of 2 seconds or more).
William A. Baum 1956, "Electronic Photography of Stars", Scientific American, Vol 194?, March 1956. Discusses the selection of short exposures at moments when the image through a telescope is sharpest (using image intensifier and short exposures).