Three processes known as calibration, alignment and image stacking are used in astronomical imaging.
Calibration is broadly the process of correcting raw images for effects such as vignetting so that they represent as accurately as possible the intensity of light falling on the sensor. This calibration process is normally undertaken using custom software such as Images Plus, DeepSkyStacker or MaxIm DL. Unwanted image elements, such as fixed signals arising from thermal currents, are also removed where possible.
Alignment is the process of overlaying the raw images in a sequence to check that all the stars are perfectly aligned. The packages typically feature automated star recognition and registration, de-rotation, and sub-pixel alignment.
Stacking involves using various mathematical processes to combine a number of aligned images to produce a master image. The processes typically used include averaging, auto adaptive weighted averaging, median, kappa-sigma clipping, entropy weighted averaging, maximum etc.