Dark or thermal count frames
During long integrations at moderate cooling a significant amount of charge can accumulate in the CCD pixels due to thermal excitation (always present, even if sensor temperature is low). The purpose of producing dark or thermal count images is to subtract the contribution from thermally generated charge in the image. The dark count will begin appearing in certain pixels on the CCD which have particularly high dark currents; these are referred to as “hot-pixels”. It is the hot-pixels that cause the greatest problem in the image degradation because they are often as bright as some of the dim stars in the image, so that simple thresholding to remove the hot-pixels often results in a significant loss of image information. Median filtering is helpful in removing the hot-pixels because these are often isolated, however the median filter can significantly “soften” the image, especially in cases when the PSF is less than 2-3 pixels wide.
The normal procedure is to take many dark images at the same exposition time of the session and then averaged, however the noise will be higher when multiple images are averaged due to the higher read noise compared to reading a single image with an equivalent total integration time. The integration should not be so long that the hot pixels approach saturation. Management of the dark count frames is somewhat easier than for the flat field frames because the same dark frame can be applied to all images, regardless of what filters or optics were used to produce the image.