Speckle imaging

From Academic Kids

Missing image
Typical short-exposure image of a binary star (Zeta Bootis in this case) as seen through atmospheric seeing. Each star should appear as a single point, but the atmosphere causes the images of the two stars to break up into two patterns of speckles (one pattern above left, the other below right). The speckles are a little difficult to make out in this image due to the coarse pixel size on the camera used. The speckles move around rapidly, so that each star appears as a single fuzzy blob in long exposure images. The telescope used had a diameter of about 7r0 (see definition of r0 under astronomical seeing, and the example simulated image through a 7r0 telescope).

Speckle Imaging (also known as video astronomy) describes a range of high-resolution astronomical imaging techniques based either on the shift-and-add (or image stacking) method or on speckle interferometry methods. These techniques can dramatically increase the resolution of ground-based telescopes. 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 seeing. Use of these techniques led to a number of discoveries, including a number of binary stars that would otherwise look like a single star to a visual observer working with a similar-sized telescope. 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 re-creates the original image through image processing techniques. The key to the technique was found by the American astronomer David L. Fried in 1966 which 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 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. In 1970 the French 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.

There are a number of different speckle imaging methods. In one technique called shift-and-add (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. Another type of speckle imaging is speckle interferometry. One type of speckle interferometry involves calculation 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. 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 film 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 captures 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.

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 regain detailed images about flaws in the material.

See also:


fr:Interférométrie des tavelures


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