Speckle Imaging - Explanation

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 seeing. Use of these techniques led to a number of discoveries, including thousands of binary stars 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.