Editing Lucky imaging

{{Short description|Technique for astrophotography}}
[[File:M15 core lucky 10pc.gif|thumb|Lucky image of [[Globular Cluster M15|M15]] core]]

'''Lucky imaging''' (also called '''lucky exposures''') is one form of [[speckle imaging]] used for [[astrophotography]]. Speckle imaging techniques use a [[high-speed camera]] with [[shutter speed|exposure times]] short enough (100&nbsp;ms or less) so that the changes in the [[Earth's atmosphere]] during the exposure are minimal.

With lucky imaging, those optimum exposures least affected by the [[atmosphere]] (typically around 10%) are chosen and combined into a single image by [[shift-and-add|shifting and adding]] the short exposures, yielding much higher [[angular resolution]] than would be possible with a single, [[long-exposure photography|longer exposure]], which includes all the frames.

== Explanation ==
Images taken with ground-based [[telescope]]s are subject to the blurring effect of atmospheric turbulence (seen to the 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&nbsp;m aperture telescopes, a resolution improvement factor of at least five over standard imaging systems.

<gallery>
File:zboo lucky image 1pc.png|[[Zeta Bootis]] imaged with the [[Nordic Optical Telescope]] on 13 May 2000 using the lucky imaging method. (The [[Airy disc]]s around the stars are [[diffraction]] from the 2.56&nbsp;m telescope aperture.)
File:Zeta bootis short exposure.png|Typical short-exposure image of this binary star from the same dataset, but without using any [[speckle imaging|speckle]] processing. The effect of the Earth's atmosphere is to break the image of each star up into speckles.
</gallery>

==Demonstration of the principle==
The sequence of images below shows how lucky imaging works.<ref>{{cite journal |last=Hippler |first=Stefan |display-authors=etal |year=2009 |title=The AstraLux Sur Lucky Imaging Instrument at the NTT |url=http://www.eso.org/sci/publications/messenger/archive/no.137-sep09/messenger-no137.pdf |journal=The Messenger |volume=137 |pages=14–17 |bibcode=2009Msngr.137...14H}}</ref> From a series of 50,000 images taken at a speed of almost 40 images per second, five different long exposure images have been created. Additionally, a single exposure with very low image quality and another single exposure with very high image quality are shown at the beginning of the demo sequence. The astronomical target shown has the [[2MASS]] ID J03323578+2843554. North is up and East on the left.
{| class=wikitable
| valign="top" | [[File:LuckySingleExposureStrehl_3.5Percent.png|LuckySingleExposureStrehl 3.5Percent|150px]]
| valign="top" | Single exposure with low image quality, not selected for lucky imaging.
| valign="top" | [[File:Lucky Single Exposure Strehl 16Percent.png|Lucky Single Exposure Strehl 16Percent|150px]]
| valign="top" | Single exposure with very high image quality, selected for lucky imaging.
|-
| valign="top" | [[File:LuckyImagingDemonstration1.png|150px]] 
| valign="top" | This image shows the average of all 50,000 images, which is almost the same as the 21 minutes (50,000/40 seconds) long exposure [[Astronomical seeing|seeing]] limited image. It looks like a typical star image, slightly elongated. The full width at half maximum (FWHM) of the [[Astronomical seeing|seeing]] disk is around 0.9 arcsec.
| valign="top" | [[File:LuckyImagingDemonstration2.png|150px]] 
| valign="top" | This image shows the average of all 50,000 single images but here with the center of gravity (centroid) of each image shifted to the same reference position. This is the [[adaptive optics|tip-tilt]]-corrected, or image-stabilized, long-exposure image. It already shows more details — two objects — than the [[Astronomical seeing|seeing]]-limited image.
|-
| valign="top" | [[File:LuckyImagingDemonstration3.png|150px]] 
| valign="top" | This image shows the 25,000 (50% selection) best images averaged, after the brightest pixel in each image was moved to the same reference position. In this image, we can almost see three objects.
| valign="top" | [[File:LuckyImagingDemonstration4.png|150px]] 
| valign="top" | This image shows the 5,000 (10% selection) best images averaged, after the brightest pixel in each image was moved to the same reference position. The surrounding [[Astronomical seeing|seeing]] halo is further reduced, an [[Airy_disk|Airy ring]] around the brightest object becomes clearly visible.
|-
| valign="top" | [[File:LuckyImagingDemonstration5.png|150px]] 
| valign="top" | This image shows the 500 (1% selection) best images averaged, after the brightest pixel in each image was moved to the same reference position. The [[Astronomical seeing|seeing]] halo is further reduced. The [[signal-to-noise ratio]] of the brightest object is the highest in this image.
|}

The difference between the [[Astronomical seeing|seeing]] limited image (third image from top) and the best 1% images selected result is quite remarkable: a triple system has been detected. The brightest component in the West is a V=14.9 magnitude M4V star. This component is the lucky imaging reference source. The weaker component consists of two stars of spectral classes M4.5 and M5.5.<ref>{{Cite journal|doi = 10.1088/0004-637X/754/1/44|title = The Astralux Large M-Dwarf Multiplicity Survey|year = 2012|last1 = Janson|first1 = Markus|last2 = Hormuth|first2 = Felix|last3 = Bergfors|first3 = Carolina|last4 = Brandner|first4 = Wolfgang|last5 = Hippler|first5 = Stefan|last6 = Daemgen|first6 = Sebastian|last7 = Kudryavtseva|first7 = Natalia|last8 = Schmalzl|first8 = Eva|last9 = Schnupp|first9 = Carolin|last10 = Henning|first10 = Thomas|journal = The Astrophysical Journal|volume = 754|issue = 1|page = 44|arxiv = 1205.4718|bibcode = 2012ApJ...754...44J|s2cid = 118475425}}</ref> The distance of the system is about 45 [[parsec]]s (pc). Airy rings can be seen, which indicates that the diffraction limit of the [[Calar Alto Observatory]]'s 2.2 m telescope was reached. The signal to noise ratio of the point sources increases with stronger selection. The [[Astronomical seeing|seeing]] halo on the other side is more suppressed. The separation between the two brightest objects is around 0.53 arcsec and between the two faintest objects less than 0.16 arcsec. At a distance of 45 pc this corresponds to 7.2 times the distance between Earth and Sun, around 1 billion kilometers (10<sup>9</sup> km).

==History==

[[File:Gemini North Infrared View of Jupiter.jpg|thumb|upright=1.4|Lucky imaging of Jupiter at 5 μm, using stacks of individual Gemini Observatory frames each with a relatively long 309-msec exposure time]]

Lucky imaging methods were first used in the middle 20th century, and became popular for imaging planets in the 1950s and 1960s (using cine cameras, often with [[image intensifier]]s). For the most part it took 30 years for the separate imaging technologies to be perfected for this counter-intuitive imaging technology to become practical. The first numerical calculation of the probability of obtaining ''lucky exposures'' was an article by [[David L. Fried]] in 1978.<ref>{{Cite journal|doi=10.1364/JOSA.68.001651|title=Probability of getting a lucky short-exposure image through turbulence|year=1978|last1=Fried|first1=David L.|journal=Journal of the Optical Society of America|volume=68|issue=12|page=1651}}</ref>

In early applications of lucky imaging, it was generally assumed that the atmosphere ''smeared-out'' or ''blurred'' the astronomical images.<ref>{{Cite journal|bibcode = 1991A&A...241..663N|title = Recentring and selection of short-exposure images with photon-counting detectors|last1 = Nieto|first1 = J. -L|last2 = Thouvenot|first2 = E.|journal = Astronomy and Astrophysics|year = 1991|volume = 241|page = 663}}</ref> In that work, the [[full width at half maximum]] (FWHM) of the blurring was estimated, and used to select exposures. Later studies<ref>{{Cite journal|doi = 10.1051/0004-6361:20053695|title = Lucky imaging: High angular resolution imaging in the visible from the ground|year = 2006|last1 = Law|first1 = N. M.|last2 = MacKay|first2 = C. D.|last3 = Baldwin|first3 = J. E.|journal = Astronomy & Astrophysics|volume = 446|issue = 2|pages = 739–745|arxiv = astro-ph/0507299|bibcode = 2006A&A...446..739L|s2cid = 17844734}}</ref><ref>{{Cite thesis |last1 = Tubbs |first1 = Robert Nigel |year = 2003 |title = Lucky exposures: Diffraction limited astronomical imaging through the atmosphere |type=PhD thesis |publisher=University of Cambridge |hdl=1810/224517 |hdl-access=free |doi = 10.17863/CAM.15991 |doi-access=free}}</ref> 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''.

In the early years of the 21st century, it was realised that turbulent intermittency (and the fluctuations in [[astronomical seeing]] conditions it produced)<ref>{{Cite journal|doi=10.1098/rspa.1949.0136|title=The nature of turbulent motion at large wave-numbers|journal=Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences|year=1949|volume=199|issue=1057|pages=238–255|bibcode=1949RSPSA.199..238B|last1=Batchelor|first1=G. K.|last2=Townsend|first2=A. A.|s2cid=122967707}}</ref> could substantially increase the probability of obtaining a "lucky exposure" for given average astronomical seeing conditions.<ref>{{Cite journal|doi = 10.1051/0004-6361:20079214|title = The point spread function in Lucky Imaging and variations in seeing on short timescales|year = 2008|last1 = Baldwin|first1 = J. E.|last2 = Warner|first2 = P. J.|last3 = MacKay|first3 = C. D.|journal = Astronomy & Astrophysics|volume = 480|issue = 2|pages = 589–597|bibcode = 2008A&A...480..589B|doi-access = free}}</ref><ref>{{Cite book|doi = 10.1117/12.671170|chapter = The effect of temporal fluctuations in r<sub>0</sub> on high-resolution observations|title = Advances in Adaptive Optics II|year = 2006|last1 = Tubbs|first1 = Robert N.|series=Proceedings of SPIE |volume = 6272|pages = 62722Y|s2cid = 119391503}}</ref>

==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 5&nbsp;m-class telescopes in visible light. The research was performed on the Mt. Palomar [[Hale Telescope]] of 200-inch-diameter aperture.  The telescope, with lucky cam and adaptive optics, pushed it near its theoretical angular resolution, achieving up to 0.025 arc seconds for certain types of viewing.<ref>{{cite journal |first=Richard Tresch |last=Fienberg |url=http://www.skyandtelescope.com/astronomy-news/sharpening-the-200-inch |title=Sharpening the 200 Inch |journal=Sky and Telescope |date=14 September 2007}}</ref>
Compared to space telescopes like the 2.4 m Hubble, the system still has some drawbacks including a narrow [[field of view]] for crisp images (typically 10" to 20"), [[airglow]], and electromagnetic frequencies [[Extinction (astronomy)|blocked by the atmosphere]].

When combined with an AO system, lucky imaging selects the periods when the turbulence 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 averages 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 [[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 [[astronomer]]s have begun to use this technique. Modern [[webcam]]s and [[camcorder]]s 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 [[shift-and-add|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 ratio|Strehl]]-selection method first suggested<ref>{{Cite journal|doi = 10.1051/0004-6361:20010118|title = Diffraction-limited 800 nm imaging with the 2.56 m Nordic Optical Telescope|year = 2001|last1 = Baldwin|first1 = J. E.|last2 = Tubbs|first2 = R. N.|last3 = Cox|first3 = G. C.|last4 = MacKay|first4 = C. D.|last5 = Wilson|first5 = R. W.|last6 = Andersen|first6 = M. I.|journal = Astronomy & Astrophysics|volume = 368|pages = L1–L4|arxiv = astro-ph/0101408|bibcode = 2001A&A...368L...1B|s2cid = 18152452}}</ref> by [[John E. Baldwin]] from the Cambridge group<ref>{{cite web |date=27 January 2020 |url=http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/ |title=Lucky Imaging |publisher=Institute of Astronomy, University of Cambridge |access-date=2021-02-11}}</ref> and the image contrast selection used in the Selective Image Reconstruction method of Ron Dantowitz.<ref>{{Cite journal|doi = 10.1086/301328|title = Ground-based High-Resolution Imaging of Mercury|year = 2000|last1 = Dantowitz|first1 = Ronald F.|last2 = Teare|first2 = Scott W.|last3 = Kozubal|first3 = Marek J.|journal = The Astronomical Journal|volume = 119|issue = 5|pages = 2455–2457|bibcode = 2000AJ....119.2455D|doi-access = free}}</ref>

The development and availability of [[charge-coupled device#Electron-multiplying CCD|electron-multiplying CCD]]s (EMCCD, also known as LLLCCD, L3CCD, or low-light-level CCD) has allowed the first high-quality lucky imaging of faint objects.

On October 27, 2014, [[Google]] introduced a similar technique called HDR+. HDR+ takes a burst of shots with short exposures, selectively aligning the sharpest shots and averaging them using [[computational photography]] techniques. Short exposures avoid blurry images or blowing out highlights, and averaging multiple shots reduces noise.<ref>{{Cite web|url=http://ai.googleblog.com/2014/10/hdr-low-light-and-high-dynamic-range.html|title=HDR+: Low Light and High Dynamic Range photography in the Google Camera App|website=Google AI Blog|language=en|access-date=2019-08-02}}</ref> HDR+ is processed on [[Hardware acceleration|hardware accelerators]] including the [[Qualcomm Hexagon|Qualcomm Hexagon DSPs]] and [[Pixel Visual Core]].<ref>{{Cite web|url=http://ai.googleblog.com/2018/02/introducing-hdr-burst-photography.html|title=Introducing the HDR+ Burst Photography Dataset|website=Google AI Blog|language=en|access-date=2019-08-02}}</ref>

==Alternative methods==
Other approaches that can yield resolving power exceeding the limits of atmospheric [[astronomical seeing|seeing]] include [[adaptive optics]], [[optical interferometry#Astronomical optical interferometry|interferometry]], other forms of [[speckle imaging]] and [[space-based telescope]]s such as NASA's [[Hubble Space Telescope]].

==References==
{{reflist}}

==Further reading==
{{refbegin}}
*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.&nbsp;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).
{{refend}}

==External links==
{{commons category}}
*[http://www.ast.cam.ac.uk/research/instrumentation.surveys.and.projects/lucky.imaging/latest.results/amateur.lucky.imaging Amateur lucky imaging]
*[http://www.caha.es/news/releases-mainmenu-163/11295-astralux-hubbles-sharp-resolution-from-calar-alto Lucky imaging with Astralux at the 2.2&nbsp;m Calar Alto telescope]
*[http://www.mpia.de/ASTRALUX Details of the Calar Alto and La Silla lucky imaging instruments]
*[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/6975961.stm BBC News article: 'Clearest' images taken of space]
*[http://www.stanmooreastro.com/IntensifiedAstronomicalImaging.htm Lucky imaging using gen 3 intensifier tubes]

{{DEFAULTSORT:Lucky Imaging}}
[[Category:Astronomical imaging]]
[[Category:Photographic techniques]]
[[Category:Speckle imaging]]