Astrophotography by Dennis Friedman

Basic calibration requires one or more (preferably five) dark frames combined together to create a master dark frame. 

The resulting dark frame is subtracted from the raw. 

This removes dark current, hot pixels and bias level. 

Basic calibration will not correct for CCD sensitivity variation or vignetting. Dark Frames must be the same exposure length as your raw image and should be taken during the same session as the raws, using the same optical settings (focus, temp, position, eyepiece, etc)