Lucky imaging

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Image:Zboo lucky image 1pc.png

Image:M15 core lucky 10pc.gif

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.

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 and the image contrast selection 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 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. 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".

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.

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