Calibration is the first step in CCD image processing and is the technique of mathmatically manipulating raw CCD data file(s) into to a workable image-- one that can be later turned into a beautiful image (via post-processing) or used for scientific research. In various forms it's the technique used by all astronomers (both amateur and professional) to strip (like peeling an onion) the digital "noise" from an image. The goal is to reveal as much as possible the true photons that travelled through space and time to strike the CCD detector. The process is similar for all CCD imaging rigs-- like the one I use all the way up to the Hubble Space Telescope.

There are a number of different types of imperfections in a raw CCD image which can be eliminated or reduced via the image calibration proces: 1) basline noise and pixel values introduced by the actual CCD chip and camera electronics (handled with a master bias frame), 2) random noise introduced by the camera electronics (handled with a master dark frame), 3) imperfections in the telescope optical train (handled with a master flat frame), and 4) the effects of the earth's atmosphere. The last element can only be dealt with by the use of expensive adaptive optics systems (mostly at professional observatories) or by placing the telescope in space!

In addition to removing imperfections in raw CCD files, most amateur astronomers combine (or "stack") multiple sub exposures of the same object in the calibration process. This serves to increase the signal to noise ratio of the image, bringing out subtle details and removing noise in the images not caused by the instrumentation. Because of the mathematics involved, stacking multiple images produces a result that is virtually the same as taking a very long single exposure.

The application I use for image calibration is called DeepSkyStacker and is a free software package developed by an amateur astronomer in Europe. Despite the fact that DSS is distributed for no cost, it is extremely powerful, and is widely used by amateur astronomers.