Speckle imaging
Encyclopedia
Speckle imaging describes a range of high-resolution astronomical imaging techniques based either on the shift-and-add
("image stacking") method or on speckle interferometry methods. These techniques can dramatically increase the resolution
of ground-based telescope
s.
. Use of these techniques led to a number of discoveries, including thousands of binary star
s that would otherwise look like a single star to a visual observer working with a similar-sized telescope, and the first images of sunspot
-like phenomena on other stars. Many of the techniques remain in wide use today, notably when imaging relatively bright targets.
In theory the resolution limit of a telescope is a function of the size of the main mirror, due to the effects of Fraunhofer diffraction
. This results in images of distant objects being spread out to a small spot known as the Airy disk. A group of objects spread out over a distance smaller than this limit looks like a single object. Thus larger telescopes can not only image dimmer objects because they collect more light on the larger mirror, but are also able to image smaller objects as well.
This breaks down due to the practical limits imposed by the atmosphere
, whose random nature disrupts the single spot of the Airy disk into a pattern of similarly-sized spots covering a much larger area (see image of binary on right). For typical seeing, the practical resolution limits are at mirror sizes well within existing mechanical limits, at a mirror diameter equal to the astronomical seeing
parameter r0 - about 20 cm in diameter for visible observations under good conditions. For many years telescope performance was limited by this effect, until the introduction of speckle interferometry and adaptive optics
provided paths to remove this limitation.
Speckle imaging recreates the original image through image processing
techniques. The key to the technique, found by the American astronomer David L. Fried
in 1966, was to take very fast images in which the atmosphere is effectively "frozen" in place. For infrared
images, exposure times are on the order of 100 ms, but for the visible region they drop to as little as 10 ms. In images at this time scale, or smaller, the movement of the atmosphere is too sluggish to have an effect; the speckles recorded in the image are a snapshot of the atmospheric seeing at that instant.
Of course there is a downside: taking images at this short an exposure is difficult, and if the object is too dim, not enough light will be captured to make the analysis possible. Early uses of the technique in the early 1970s were made on a limited scale using photographic techniques, but since photographic film captures only about 7% of the incoming light, only the brightest of objects could be processed in this way. The introduction of the CCD
into astronomy, which captures more than 70% of the light, lowered the bar on practical applications enormously, and today the technique is widely used on bright astronomical objects (e.g. stars and star systems).
The fact that many of the speckle imaging methods have multiple names results largely from amateur astronomers re-inventing existing speckle imaging techniques and giving them new names.
More recently, another use of the technique has developed for industrial applications. By shining a laser
(whose smooth wavefront is an excellent simulation of the light from a distant star) on a surface, the resulting speckle pattern can be processed to give detailed images of flaws in the material.
(also called image stacking), the short exposure images are lined up by the brightest speckle and averaged together to give a single output image. In the Lucky Imaging
approach, only the best few short exposures are selected. Early shift-and-add techniques aligned images according to the image centroid
, giving a lower overall Strehl ratio
.
astronomer Antoine Labeyrie
showed that information could be obtained about the high-resolution structure of the object from the speckle patterns using Fourier
analysis (speckle interferometry). In the 1980s methods were developed which allowed images to be reconstructed interferometrically from these speckle patterns.
One more recent type of speckle interferometry called speckle masking
involves calculation of the bispectrum
or closure phase
s from each of the short exposures. The "average bispectrum" can then be calculated and then inverted to obtain an image. This works particularly well using aperture masks
. In this arrangement the telescope aperture is blocked by astronomers apart from a few holes which allow light through, creating a small optical interferometer
with better resolving power than the telescope would otherwise have. This aperture masking
technique was pioneered by the Cavendish Astrophysics Group
.
One limitation of the technique is that it requires extensive computer processing of the image, which was hard to come by when the technique was first developed. Although the almost-universal Data General Nova
served well in this role, it was slow enough to limit the application to only "important" targets. This limitation has faded away over the years as computing power has increased, and nowadays desktop computers have more than enough power to make such processing a trivial task.
enables real-time monitoring of dynamical systems and video image analysis to understand biological processes.
:
Shift-and-add
The shift-and-add method is a form of speckle imaging commonly used for obtaining high quality images from a number of short exposures with varying image shifts. It has been used in astronomy for several decades, and is the basis for the image-stabilisation feature on some cameras...
("image stacking") method or on speckle interferometry methods. These techniques can dramatically increase the resolution
Optical resolution
Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...
of ground-based telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
s.
Explanation
The principle of all the techniques is to take very short exposure images of astronomical targets, and then process the images so as to remove the effects of astronomical seeingAstronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...
. Use of these techniques led to a number of discoveries, including thousands of binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
s that would otherwise look like a single star to a visual observer working with a similar-sized telescope, and the first images of sunspot
Sunspot
Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....
-like phenomena on other stars. Many of the techniques remain in wide use today, notably when imaging relatively bright targets.
In theory the resolution limit of a telescope is a function of the size of the main mirror, due to the effects of Fraunhofer diffraction
Fraunhofer diffraction
In optics, the Fraunhofer diffraction equation is used to model the diffraction of waves when the diffraction pattern is viewed at a long distance from the diffracting object, and also when it is viewed at the focal plane of an imaging lens....
. This results in images of distant objects being spread out to a small spot known as the Airy disk. A group of objects spread out over a distance smaller than this limit looks like a single object. Thus larger telescopes can not only image dimmer objects because they collect more light on the larger mirror, but are also able to image smaller objects as well.
This breaks down due to the practical limits imposed by the atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
, whose random nature disrupts the single spot of the Airy disk into a pattern of similarly-sized spots covering a much larger area (see image of binary on right). For typical seeing, the practical resolution limits are at mirror sizes well within existing mechanical limits, at a mirror diameter equal to the astronomical seeing
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...
parameter r0 - about 20 cm in diameter for visible observations under good conditions. For many years telescope performance was limited by this effect, until the introduction of speckle interferometry and adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
provided paths to remove this limitation.
Speckle imaging recreates the original image through image processing
Image processing
In electrical engineering and computer science, image processing is any form of signal processing for which the input is an image, such as a photograph or video frame; the output of image processing may be either an image or, a set of characteristics or parameters related to the image...
techniques. The key to the technique, found by the American astronomer David L. Fried
David L. Fried
David L. Fried is a scientist, best known for his contributions to optics. Fried described what has come to be known as Fried's seeing diameter, or r0 . The seeing diameter is effectively a limiting aperture due to atmospheric turbulence, and is found either empirically or statistically. The...
in 1966, was to take very fast images in which the atmosphere is effectively "frozen" in place. For infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
images, exposure times are on the order of 100 ms, but for the visible region they drop to as little as 10 ms. In images at this time scale, or smaller, the movement of the atmosphere is too sluggish to have an effect; the speckles recorded in the image are a snapshot of the atmospheric seeing at that instant.
Of course there is a downside: taking images at this short an exposure is difficult, and if the object is too dim, not enough light will be captured to make the analysis possible. Early uses of the technique in the early 1970s were made on a limited scale using photographic techniques, but since photographic film captures only about 7% of the incoming light, only the brightest of objects could be processed in this way. The introduction of the CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
into astronomy, which captures more than 70% of the light, lowered the bar on practical applications enormously, and today the technique is widely used on bright astronomical objects (e.g. stars and star systems).
The fact that many of the speckle imaging methods have multiple names results largely from amateur astronomers re-inventing existing speckle imaging techniques and giving them new names.
More recently, another use of the technique has developed for industrial applications. By shining a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
(whose smooth wavefront is an excellent simulation of the light from a distant star) on a surface, the resulting speckle pattern can be processed to give detailed images of flaws in the material.
Techniques based on the shift-and-add method
In one technique called shift-and-addShift-and-add
The shift-and-add method is a form of speckle imaging commonly used for obtaining high quality images from a number of short exposures with varying image shifts. It has been used in astronomy for several decades, and is the basis for the image-stabilisation feature on some cameras...
(also called image stacking), the short exposure images are lined up by the brightest speckle and averaged together to give a single output image. In the Lucky Imaging
Lucky imaging
Lucky imaging is one form of speckle imaging used for astronomical photography. Speckle imaging techniques use a high-speed camera with exposure times short enough so that the changes in the Earth's atmosphere during the exposure are minimal.With lucky imaging, those optimum exposures least...
approach, only the best few short exposures are selected. Early shift-and-add techniques aligned images according to the image centroid
Centroid
In geometry, the centroid, geometric center, or barycenter of a plane figure or two-dimensional shape X is the intersection of all straight lines that divide X into two parts of equal moment about the line. Informally, it is the "average" of all points of X...
, giving a lower overall Strehl ratio
Strehl ratio
The Strehl ratio, named after the German physicist and mathematician Karl Strehl , is a measure for the optical quality of telescopes and other imaging instruments...
.
Techniques based on speckle interferometry
In 1970 the FrenchFrance
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
astronomer Antoine Labeyrie
Antoine Émile Henry Labeyrie
Antoine Émile Henry Labeyrie is a French astronomer and holds since 1991 the "Observational Astrophysics" chair at the Collège de France....
showed that information could be obtained about the high-resolution structure of the object from the speckle patterns using Fourier
Fourier
Fourier most commonly refers to Joseph Fourier , French mathematician and physicist, or the mathematics, physics, and engineering terms named in his honor for his work on the concepts underlying them:In mathematics:...
analysis (speckle interferometry). In the 1980s methods were developed which allowed images to be reconstructed interferometrically from these speckle patterns.
One more recent type of speckle interferometry called speckle masking
Speckle masking
Speckle masking is a speckle imaging method which involves estimation of the bispectrum or closure phases from each of the short exposures. The "average bispectrum" can then be calculated and then inverted to obtain an image. With a normal telescope aperture, a large number of short exposures must...
involves calculation of the bispectrum
Speckle masking
Speckle masking is a speckle imaging method which involves estimation of the bispectrum or closure phases from each of the short exposures. The "average bispectrum" can then be calculated and then inverted to obtain an image. With a normal telescope aperture, a large number of short exposures must...
or closure phase
Closure phase
The closure phase is an observable quantity in imaging astronomical interferometry, which allowed the use of interferometry with very long baselines. It forms the basis of the self-calibration approach to interferometric imaging...
s from each of the short exposures. The "average bispectrum" can then be calculated and then inverted to obtain an image. This works particularly well using aperture masks
Aperture masking interferometry
Aperture Masking Interferometry is a form of speckle interferometry, allowing diffraction limited imaging from ground-based telescopes. This technique allows ground based telescopes to reach the maximum possible resolution, allowing ground-based telescopes with large diameters to produce far...
. In this arrangement the telescope aperture is blocked by astronomers apart from a few holes which allow light through, creating a small optical interferometer
Optical interferometry
Optical interferometry combines two or more light waves in an opticalinstrument in such a way that interference occurs between them.Early interferometers used white light sources and also monochromatic light from atomic sources...
with better resolving power than the telescope would otherwise have. This aperture masking
Aperture masking interferometry
Aperture Masking Interferometry is a form of speckle interferometry, allowing diffraction limited imaging from ground-based telescopes. This technique allows ground based telescopes to reach the maximum possible resolution, allowing ground-based telescopes with large diameters to produce far...
technique was pioneered by the Cavendish Astrophysics Group
Cavendish Astrophysics Group
The Cavendish Astrophysics Group is based at the Cavendish Laboratory at the University of Cambridge. The group operates all of the telescopes at the Mullard Radio Astronomy Observatory except for the 32m MERLIN telescope, which is operated by Jodrell Bank.The group is the second largest of three...
.
One limitation of the technique is that it requires extensive computer processing of the image, which was hard to come by when the technique was first developed. Although the almost-universal Data General Nova
Data General Nova
The Data General Nova was a popular 16-bit minicomputer built by the American company Data General starting in 1969. The Nova was packaged into a single rack mount case and had enough power to do most simple computing tasks. The Nova became popular in science laboratories around the world, and...
served well in this role, it was slow enough to limit the application to only "important" targets. This limitation has faded away over the years as computing power has increased, and nowadays desktop computers have more than enough power to make such processing a trivial task.
Biology
Speckle imaging in biology refers to the underlabeling of periodic cellular components (such as filaments and fibers) so that instead of appearing as a continuous and uniform structure, it appears as a discrete set of speckles. This is due to statistical distribution of the labeled component within unlabeled components. The technique, also known as dynamic speckleDynamic speckle
In physics, dynamic speckle is a result of the temporal evolution of a speckle pattern where variations in the scattering elements responsible for the formation of the interference pattern in the static situation produce the changes that are seen in the speckle pattern, where its grains change...
enables real-time monitoring of dynamical systems and video image analysis to understand biological processes.
See also
- Astronomical interferometerAstronomical interferometerAn astronomical interferometer is an array of telescopes or mirror segments acting together to probe structures with higher resolution by means of interferometry....
- Holographic interferometryHolographic interferometryHolographic interferometry is a technique which enables static and dynamic displacements of objects with optically rough surfaces to be measured to optical interferometric precision . These measurements can be applied to stress, strain and vibration analysis, as well as to non-destructive testing...
- Electronic Speckle Pattern InterferometryElectronic speckle pattern interferometryElectronic Speckle Pattern Interferometry , also known as TV Holography, is a technique which uses laser light, together with video detection, recording and processing to visualise static and dynamic displacements of components with optically rough surfaces...
- Bispectral analysisSpeckle maskingSpeckle masking is a speckle imaging method which involves estimation of the bispectrum or closure phases from each of the short exposures. The "average bispectrum" can then be calculated and then inverted to obtain an image. With a normal telescope aperture, a large number of short exposures must...
- Optical interferometryOptical interferometryOptical interferometry combines two or more light waves in an opticalinstrument in such a way that interference occurs between them.Early interferometers used white light sources and also monochromatic light from atomic sources...
- Aperture synthesisAperture synthesisAperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection...
- Aperture Masking InterferometryAperture masking interferometryAperture Masking Interferometry is a form of speckle interferometry, allowing diffraction limited imaging from ground-based telescopes. This technique allows ground based telescopes to reach the maximum possible resolution, allowing ground-based telescopes with large diameters to produce far...
- Diffraction-limited system
- Lucky ImagingLucky imagingLucky imaging is one form of speckle imaging used for astronomical photography. Speckle imaging techniques use a high-speed camera with exposure times short enough so that the changes in the Earth's atmosphere during the exposure are minimal.With lucky imaging, those optimum exposures least...
- Super-resolutionSuper-resolutionSuper-resolution are techniques that enhance the resolution of an imaging system. Some SR techniques break the diffraction-limit of systems, while other SR techniques improve over the resolution of digital imaging sensor....
Example images
All of these were obtained using speckle imaging and have higher resolution than can be obtained with e.g. the Hubble Space TelescopeHubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
: