I am aware that for a CCD image to be produced, various corrections have to be applied due to the limitations of CCDs in astronomy. One such correction is the fact that CCDs are not perfectly linear since each detector pixel has a maximum size - it can only collect so many electrons. So sources brighter than a certain number of counts/pixel will have unreliable fluxes (overfilled pixels will bloom - overflowing). What is of interest to me is: there are various additional processing stages for a CCD image: calibration, subexposures, etc. Could someone explain these stages, other instrumental effects that need to be corrected before a final CCD can be produced? Why the need for numerous subexposures? Thanks.