Speckle imaging

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.

This improvement of resolution 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 scattered over a much larger area (see the adjacent image of a binary).

The key to the technique, found by the American astronomer David L. Fried in 1966, was to take very fast images in which case the atmosphere is effectively "frozen" in place.

The downside of the technique is that taking images at this short an exposure is difficult, and if the object is too dim, not enough light will be captured to make analysis possible.

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.

In fact, when using an average, the signal-to-noise ratio should be increased by a factor of the square root of the number of images.

A number of software packages exist for performing this, including IRAF, RegiStax, Autostakkert, Keiths Image Stacker, Hugin, and Iris.

[8] This technique was first implemented in 1971 at Palomar Observatory (200-inch telescope) by Daniel Y. Gezari, Antoine Labeyrie and Robert V.

[9] Methods developed in the 1980s allowed simple images to be reconstructed from this power spectrum information.

Typical short-exposure image of a binary star ( ζ Boötis ) as seen through atmospheric turbulence. 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 . The speckles move around rapidly, so that each star appears as a single fuzzy blob in long exposure images.
Slow-motion speckle imaging movie, showing how a high-magnification (negative) image of a star breaks up into multiple blobs (speckles), entirely an atmospheric effect.
Lucky imaging images of Jupiter at 5 μm, using stacks of individual Gemini Observatory frames each with a relatively long 309-msec exposure time, illustrate the principle that coherence time τ 0 increases with wavelength. [ 3 ] [ 4 ]