Speckle imaging

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).

Slow-motion speckle imaging movie, showing what you see through a telescope when you look at a star at high magnification (negative images). The telescope used had a diameter of about 7r₀. Notice how the star breaks up into multiple blobs (speckles) -- entirely an atmospheric effect. Speckle imaging techniques attempt to recreate the image of the object as it was before being corrupted by the atmosphere. Some telescope vibration is also noticeable in this movie.

Speckle imaging describes a range of high-resolution astronomical imaging techniques based on the analysis of large numbers of short exposures that freeze the variation of atmospheric turbulence. They can be divided into the shift-and-add ("image stacking") method and the speckle interferometry methods. These techniques can dramatically increase the resolution of ground-based telescopes, but are limited to bright targets.

The principle of all the techniques is to take very short exposure images of astronomical targets, and then process those so as to remove the effects of astronomical seeing. Use of these techniques led to a number of discoveries, including thousands of binary stars that would otherwise appear as 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.

The resolution of a telescope is limited by 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 whose images are closer together than this limit appear as a single object. Thus larger telescopes can image not only dimmer objects (because they collect more light), but smaller objects as well.

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Wikipedia