Lucky imaging (also called lucky exposures) is one form of speckle imaging used for astrophotography. Speckle imaging techniques use a

high-speed camera A high-speed camera is a device capable of capturing moving images with exposures of less than 1/1,000 second or frame rates in excess of 250 fps. It is used for recording fast-moving objects as photographic images onto a storage medium. After r ...

with exposure times short enough (100 ms or less) so that the changes in the

Earth's atmosphere The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...

during the exposure are minimal. With lucky imaging, those optimum exposures least affected by the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...

(typically around 10%) are chosen and combined into a single image by shifting and adding the short exposures, yielding much higher

angular resolution Angular resolution describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object, thereby making it a major determinant of image resolut ...

than would be possible with a single, longer exposure, which includes all the frames.

Explanation

Images taken with ground-based

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obse ...

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 The resolution of an optical imaging system a microscope, telescope, or camera can be limited by factors such as imperfections in the lenses or misalignment. However, there is a principal limit to the resolution of any optical system, due to t ...

of even 2.5 m aperture telescopes, a resolution improvement factor of at least five over standard imaging systems. File:zboo lucky image 1pc.png,

Zeta Bootis Zeta (, ; uppercase Ζ, lowercase ζ; grc, ζῆτα, el, ζήτα, label=Demotic Greek, classical or ''zē̂ta''; ''zíta'') is the sixth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 7. It was derived fr ...

imaged with the

Nordic Optical Telescope The Nordic Optical Telescope (NOT) is an astronomical telescope located at Roque de los Muchachos Observatory, La Palma in the Canary Islands. The telescope saw first light in 1988, and was officially inaugurated during September 1989. Regular ...

on 13 May 2000 using the lucky imaging method. (The Airy discs around the stars are

diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...

from the 2.56 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 processing. The effect of the Earth's atmosphere is to break the image of each star up into speckles.

Demonstration of the principle

The sequence of images below shows how lucky imaging works. 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 The Two Micron All-Sky Survey, or 2MASS, was an astronomical survey of the whole sky in infrared light. It took place between 1997 and 2001, in two different locations: at the U.S. Fred Lawrence Whipple Observatory on Mount Hopkins, Arizona, and ...

ID J03323578+2843554. North is up and East on the left. The difference between the 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. The distance of the system is about 45

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, an ...

s (pc). Airy rings can be seen, which indicates that the diffraction limit of the

Calar Alto Observatory The Calar Alto Observatory (Centro Astronómico Hispano en Andalucía or "Spanish Astronomical Centre in Andalusia") is an astronomical observatory located in Almería province in Spain on Calar Alto, a mountain in the Sierra de Los Filabres ra ...

's 2.2 m 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 0.53 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 (10⁹ km).

History

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 An image intensifier or image intensifier tube is a vacuum tube device for increasing the intensity of available light in an optical system to allow use under low-light conditions, such as at night, to facilitate visual imaging of low-light proce ...

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. In early applications of lucky imaging, it was generally assumed that the atmosphere ''smeared-out'' or ''blurred'' the astronomical images. In that work, the full width at half maximum (FWHM) of the blurring was estimated, and used to select exposures. Later studies 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) could substantially increase the probability of obtaining a "lucky exposure" for given average astronomical seeing conditions.

Lucky imaging and adaptive optics hybrid systems

In 2007 astronomers at

Caltech The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...

and the

University of Cambridge , mottoeng = Literal: From here, light and sacred draughts. Non literal: From this place, we gain enlightenment and precious knowledge. , established = , other_name = The Chancellor, Masters and Schola ...

announced the first results from a new hybrid lucky imaging and

adaptive optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...

(AO) system. The new camera gave the first diffraction-limited resolutions on 5 m-class telescopes in visible light. The research was performed on the Mt. Palomar

Hale telescope The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in San Diego County, California, US, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation in 1928, he orchestrated the planning, de ...

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. Compared to space telescopes like the 2.4 m Hubble, the system still has some drawbacks including a narrow

field of view The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation. Human ...

for crisp images (typically 10" to 20"),

airglow Airglow (also called nightglow) is a faint emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky never to be completely dark, even after the effects of starlight and diffu ...

, and electromagnetic frequencies 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 The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...

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 An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either ...

s have begun to use this technique. Modern

webcam A webcam is a video camera which is designed to record or stream to a computer or computer network. They are primarily used in videotelephony, livestreaming and social media, and security. Webcams can be built-in computer hardware or peripher ...

s and

camcorder A camcorder is a self-contained portable electronic device with video and recording as its primary function. It is typically equipped with an articulating screen mounted on the left side, a belt to facilitate holding on the right side, hot-sw ...

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 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 by John E. Baldwin from the Cambridge group and the image contrast selection used in the Selective Image Reconstruction method of Ron Dantowitz. The development and availability of electron-multiplying CCDs (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 Google LLC () is an American Multinational corporation, multinational technology company focusing on Search Engine, search engine technology, online advertising, cloud computing, software, computer software, quantum computing, e-commerce, ar ...

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 Computational photography refers to digital image capture and processing techniques that use digital computation instead of optical processes. Computational photography can improve the capabilities of a camera, or introduce features that were no ...

techniques. Short exposures avoid blurry images or blowing out highlights, and averaging multiple shots reduces noise. HDR+ is processed on hardware accelerators including the Qualcomm Hexagon DSPs and

Pixel Visual Core The Pixel Visual Core (PVC) is a series of ARM-based system in package (SiP) image processors designed by Google. The PVC is a fully programmable image, vision and AI multi-core domain-specific architecture (DSA) for mobile devices and in future ...

Alternative methods

Other approaches that can yield resolving power exceeding the limits of atmospheric seeing include

interferometry Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber o ...

, other forms of speckle imaging and space-based telescopes such as NASA's

References

External links

Amateur lucky imagingLucky imaging with Astralux at the 2.2 m Calar Alto telescopeDetails of the Calar Alto and La Silla lucky imaging instrumentsDetails of the LuckyCam instrument at the Nordic Optical TelescopeBBC News article: 'Clearest' images taken of space
{{DEFAULTSORT:Lucky Imaging Astronomical imaging Photographic techniques Speckle imaging