A method of reducing 'fixed pattern noise' and the influence of 'hot pixels' by taking a frame with the shutter remaining closed and subtracting the results from a frame previously taken with the shutter open.
Dark frame subtraction is a method of reducing noise generated by a digital camera's electronics, often called 'fixed pattern noise', by following an exposure with another exposure with the shutter curtains remaining closed. The signal from the second 'dark' frame is subtracted from the signal obtained from the first frame on a pixel by pixel basis. The second frame should be taken under the same conditions as the exposed frame. Such conditions include ambient temperature, internal camera temperature, sensor amplification/ISO setting, and length of exposure.