speckle suppression

Speckle suppression is the reduction of speckle noise in an image produced by a telescope. Speckle noise is a general term that includes signal noise inherent in the instrument.

Adaptive optics can be classified as speckle suppression, as do attempts to realize better angular resolution than indicated by the Rayleigh criterion. For the latter, the following practices are of use:

• imaging using more than one aperture size and/or shape. A single telescope may be modified with masks, or different telescopes can be used (apodization). This results in a different point-spread function and Airy disk.
• recording and using the polarization of the light.
• using more than one aim of the telescope (angular differential imaging or ADI).
• observing different parts of the spectra (spectral differential imaging or SDI).
• PSF fitting - untangling overlapping Airy disks.

Combining information from multiple such images can be used to reveal detail otherwise hidden (stacking and/or lucky imaging).

Speckle suppression has value for many types of observation, and is of particular interest when used in conjunction with coronagraphs for imaging objects very near stars such as extra-solar planets or protoplanetary disks. The term also applies to additional techniques for producing a good signal-to-noise ratio in direct imaging exoplanets. Nulling is using interferometry, specifically light interference patterns to place a dark band over the star. If multiple such images are taken, varying factors, processing can be devised produce an image taking advantage of the information that each individual image captures.