CCD Temperature Variability and Dark Current Correction
An important step in CCD image calibration is the removal of the thermal signature commonly known as "dark count". An accurate measurement of the dark count in a CCD image cannot be made from the image itself. To determine the dark count, a time exposure is taken with the camera shutter closed to produce an image containing the thermal signature of the CCD chip. This "dark frame" is then subtracted from the "normal" data image of same exposure to remove the thermal signature present in the data image.
Dark count is the product of a "dark current", or rate per second of electrons created by the CCD, and the exposure time in seconds. Thus the amount of dark count present in a CCD image depends on the temperature of the CCD chip. In fact, the dark current is slightly different for each pixel of the CCD. The dark current varies with temperature by a complex mathematical relationship. To remove the thermal signature, a dark frame must accurately estimate the dark count unique to each pixel of the image.