Speckle interferometry is an image processing technique used in astronomy that can dramatically increase the resolution of ground-based telescopes. The principle of the technique is to take very short exposure images of astronomical targets, and then process the images so as to remove the effects of astronomical seeing. Use of the technique led to a number of discoveries, including the fact that Pluto had a moon (Charon), and the discovery of a number of binary stars that would otherwise look like a single larger star. The technique remaines widely used today, notably when imaging specific 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). Due to the nature of the airmass, the practical resolution limits are at mirror sizes well within existing mechanical limits, at about 4 m in diameter. 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 interferometry re-creates the original image through image processing techniques. The key to the technique, found by the French astronomer Antoine Labeyrie in 1970, is 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 shrink down to as small 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.
There are a number of different speckle interferometry methods. In one technique called shift-and-add , 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. Another speckle imaging approach is to calculate the bispectrum or closure phases from each short exposure. The "average bispectrum" can then be calculated and then inverted to obtain an image. This works particularly well using aperture masks.
The interesting part of this process is that the image can often contain a far higher resolution than would be given by a long exposure image. This is because the technique allows the software to remove the effects of astronomical seeing (turbulence in the atmosphere which makes the stars twinkle, and blurs long exposure images). In some cases 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 technique, called Aperture Masking Interferometry, was pioneered by the Cavendish Astrophysics Group.
Of course there is a downside: taking images at this short a time frame 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 plates capture 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 capture about 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).
Another limitation of the technique is that it requires extensive computer processing of the image, which was also hard to come by when it was first applied. Although the almost-universal Data General Nova served well in this role, it was slow enough to limit the application to only "important" targets. Again, this limitation has largely disappeared over the years, and nowadays desktop computers have more than enough power to make such processing a trivial task.
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 regain detailed images about flaws in the material.
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