Dark
Dark frames are made at the same exposure time and ISO as the subject light frames but in the dark: use your lens/telescope cap or close the shutter for example. They contain the thermal noise associated with the sensor, the noise being proportional to temperature and exposure time. Hence, they should be made at approximately the same temperature as the light frames, this is the reason why we make dark frames at the end, or in the middle of the imaging session. Like with the BIAS frames, the more dark exposures are used for the calculation of the master dark, the less noise will be introduced into the corrected images. The master dark frame should be created by stacking dark frames with the median algorithm (or Winsorized by checking the rejection levels at the end of the process, they should be lower than 0.5 percent), but be sure to use No Normalisation.