Lucky imaging

M15 core

Lucky imaging (also called lucky exposures) is one form of speckle imaging used for astronomical photography. Speckle imaging techniques use a high-speed camera with exposure times short enough (100 ms or less) so that the changes in the Earth's atmosphere during the exposure are minimal. With lucky imaging, those exposures 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.

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 is one of several 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.

Animated demonstration of the principle

The animation on the right shows how Lucky Imaging works. From a series of 50000 images taken at a speed of almost 40 images per second, 5 different long exposure images have been created. The first image is just the sum of all 50000 images, which is almost the same as the 21 minutes (50000/40 seconds) long exposure Seeing limited image. It looks like a typical star image, slightly elongated. The full width at half maximum (FWHM) of the Seeing disk is around 0.9 arcsec. The second image in the animation is again the sum of all 50000 single images but here with the center of gravity (centroid) of each image shifted to the same reference position. This is the tip-tilt corrected - or image stabilized - long exposure image. It already shows more details - two objects - than the Seeing limited image. The third image in the animation shows the 25000 (50% selection) best images added together with the brightest pixel in each image moved to the same reference position. In this image, we can almost see three objects. The forth image in the animation shows the 5000 (10% selection) best images added together with the brightest pixel in each image moved to the same reference position. The surrounding Seeing halo is further reduced, an Airy ring around the brightest object becomes clearly visible. The last image in the animation shows the 500 (1% selection) best images added together with the brightest pixel in each image moved to the same reference position. The Seeing halo is further reduced. The signal to noise ratio of the brightest object is the highest in this image. The difference between the Seeing limited image and the best 1% images selected result is quite remarkable: a triple system can be detected. The brightest component in the West is a V=14.9 magnitude M4V star. This component is the Lucky Imaging reference source. The weakest - tertiary - component a M7-M8 spectral type star. The distance of the system is about 45 pc. Airy rings can be seen, which indicate that the diffraction limit of the Calar Alto 2.2m telescope was reached. The signal to noise ratio of the point sources increases with stronger selection. The Seeing halo on the other side is more suppressed. The separation between the two brightest objects is around 0.55 arcsec and between the two faintest objects less than 0.15 arcsec. At a distance of 45 pc this corresponds to 6.75 times the distance between Earth and Sun, around 1.4 quadrillion kilometers (1.4x10¹⁵ km).

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. [cite journal| url=http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1978OSAJ...68.1651F&db_key=INST| year=1978| first=David L.| last=Fried| title=Probability of getting a lucky short-exposure image through turbulence| journal=Optical Society of America| volume=68| month= December| pages= 1651–1658] In early applications of lucky imaging, it was generally assumed that the atmosphere "smeared-out" or "blurred" the astronomical images. [Nieto and Thouvenot 1991, [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1991A%26A...241..663N&db_key=AST Recentring and selection of short-exposure images with photon-counting detectors. I - Reliability tests] ] In this work, the FWHM of the blurring was estimated, and used to select exposures. Later studies [Law et al 2006, [http://esoads.eso.org/cgi-bin/nph-bib_query?bibcode=2005astro.ph..7299L&db_key=PRE Lucky Imaging: High Angular Resolution Imaging in the Visible from the Ground] ] [Tubbs 2003, [http://isbin.com/isbn/3836497697/Lucky-Exposures-Robert-Tubbs&q=978-3836497695&p=1 Lucky Exposures: Diffraction limited astronomical imaging through the atmosphere] , Published by VDM (Saarbrucken), ISBN : 3836497697.] 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".

Lucky Imaging and Adaptive Optics Hybrid Systems

In 2007 astronomers at Caltech and the University of Cambridge announced the first results from a new hybrid Lucky Imaging and Adaptive Optics (AO) system. The new camera gave the first diffraction-limited resolutions on 5m-class telescopes in visible light. The research was performed on the 5m-diameter Palomar Hale telescope; the resolution achieved was twice that of the 2.5m diameter Hubble Space Telescope.

When combined with an AO system Lucky Imaging selects the periods when the turbulence that the adaptive optics system must correct is reduced. In these periods, lasting a small fraction of a second, the correction given by the AO system is sufficient to give excellent resolution with visible light. The Lucky Imaging system sums the images taken during the excellent periods to produce a final image with much higher resolution than is possible with a conventional long-exposure AO camera.

This technique is applicable to getting very high resolution images of only relatively small astronomical objects, up to 10 arcseconds in diameter, as it is limited by the precision of the atmospheric turbulence correction. It also requires a relatively bright 14th-magnitude star in the field of view on which to guide. Being above the atmosphere, the much more expensive Hubble Space Telescope is not limited by these concerns and so is capable of much wider-field high-resolution imaging.

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 first suggested [cite journal| url=http://ukads.nottingham.ac.uk/abs/2001A%26A...368L...1B| title=Diffraction-limited 800 nm imaging with the 2.56 m Nordic Optical Telescope| last=Baldwin| first=John E.| coauthors= Tubbs, Robert N.; Cox, Graham C.; Mackay, Craig D.; Wilson, Richard W.; Andersen, Michael I.| journal=Astronomy and Astrophysics| volume=368| pages= L1–L4| doi=10.1051/0004-6361:20010118| month=March| year=2001] by John E. Baldwin from the Cambridge group [ [http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/ Lucky Imaging Web Site Home ] ] and the image contrast selection used in the Selective Image Reconstruction method of Ron Dantowitz. [cite journal| url=http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=2000AJ....119.2455D&db_key=AST&amp| title=Ground-based High-Resolution Imaging of Mercury| last=Dantowitz| first=Ronald F.| coauthors= Teare, Scott W.; Kozubal, Marek J.| journal=The Astronomical Journal| volume=119| issue= 5| pages= 2455–2457| doi=10.1086/301328| month=May| year=2000] The recent development of EMCCDs has allowed the first high quality lucky imaging of faint objects.

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.

References

* 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).

External links

* [http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/LI_Amateur.htm Amateur lucky imaging]
* [http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/ The Cambridge University Institute of Astronomy lucky imaging website]
* [http://www.astro.caltech.edu/~nlaw/lucky_palomar/ Results from Lucky Imaging combined with Adaptive Optics, from Caltech]
* [http://www.astro.caltech.edu/~nlaw/thesis/thesis.html Detailed PhD thesis about astronomy with Lucky Imaging, from 2006]
* [http://www.caha.es/astralux-hubbles-sharp-resolution-from-calar-alto.html Lucky Imaging with Astralux at the 2.2m Calar Alto telescope]
* [http://www.mpia.de/ASTRALUX Details of the Calar Alto Lucky imaging instrument]
* [http://www.not.iac.es/instruments/luckycam/ Details of the LuckyCam instrument at the Nordic Optical Telescope]
* [http://news.bbc.co.uk/2/hi/science/nature/4456988.stm BBC News article: UK stargazers enjoy 'Lucky' break]
* [http://news.bbc.co.uk/2/hi/science/nature/6975961.stm BBC News article: 'Clearest' images taken of space]