Lucky imaging

From Academic Kids

Missing image
Zboo_lucky_image_1pc.png
The best 1% of exposures of the 0.8 arcsecond separation binary star Zeta Bootis taken with the Nordic Optical Telescope on 13 May 2000. The Airy disc around each of the stars is caused by diffraction from the 2.56m telescope aperture.
Missing image
M15_core_lucky_10pc.gif
Lucky image of M15 core


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 programmes 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.

Lucky imaging uses a high-speed camera with exposure times short enough (100ms or less) so that the changes in the atmosphere during the exposure are minimal. From these images, those (typically around 10%) least affected by the atmosphere 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.

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 (http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/) and the image contrast selection (http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=2000AJ....119.2455D&db_key=AST&amp) used in the Selective Image Reconstruction method of Ron Dantowitz. The recent development of EMCCDs has allowed the first high quality lucky imaging of faint objects.

Used at a 1% selection or less lucky imaging can reach the diffraction limit of even 2.5m aperture telescopes, a resolution improvement factor of at least five over standard imaging systems.

Lucky imaging was first used in the middle 20th Century, and became popular for imaging planets in the 1960s and 1970s (using cine cameras). The first theoretical description of the probability of obtaining lucky exposures was an article (http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1978OSAJ...68.1651F&db_key=INST) by David L. Fried in 1978.

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

External links

Amateur lucky imaging (http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/LI_Amateur.htm)

The Cambridge University Institute of Astronomy lucky imaging website (http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/)

Lucky imaging with the 2.56m Nordic Optical Telescope (http://www.strw.leidenuniv.nl/~tubbs/lucky/)

Lucky imaging of the international space station (ISS) (http://www.worldpicturenews.com/web/ImageLoader.aspx?id=70692&context=custom&type=portfolio)

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