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Stacking (image stacking) is the combining of images, typically to improve your result's sensitivity and signal-to-noise ratio (SNR). A typical instance is stacking of a set of aligned images of the same astronomical object, but the technique is also used with images of similar objects as a means of discerning their common characteristics.
Lucky imaging is a kind of stacking selecting the best images of an object, but stacking may also be carried out by re-positioning each image to produce a kind of AO or may be used to "summarize" multiple images of a transient. Amateur astronomers find it useful as an affordable way to do more with their equipment and variants on the practice are also useful for leading-edge research.
When using it to analyze some type of object, images of the objects may be scaled and even stretched to compensate for the view angle, in an effort to align comparable parts of the objects in the stacked image. The practice is capable of averaging out noise and amplifying some kinds of features in the resulting image.
Such stacking is also used with spectral energy distributions, i.e., combining the spectra information of various observations, sometimes of more than one object of a class.