A dark frame is the result of integrating the CCD for some amount of time with no light reaching the detector. Such a frame is non-zero due both to the Bias (as discussed in Section 2) and to the thermal dark current of the detector. That is, charge will accumulate on the detector, even in the absence of light, if the detector is run at any temperature above absolute zero. The charge on the detector is flushed before an exposure. Thus a Bias will have no dark current (the dark current has zero seconds to accumulate in a bias frame). In general, the higher the temperature of the detector, the larger the dark current. As the pixel-to-pixel temperature sensitivity is not uniform, there will generally be a 2-D pattern to the dark image. The details of this 2-D pattern will generally be a function of temperature. Most CCDs made since the 1980s have linear dark rates. That is, for most detectors the average dark-count at a given temperature will be an (increasing) linear function of exposure time.
More detailed technical background on dark frames, and methods for dark correction in general can be found in a number of sources. Among such sources available at our library are Berry & Burnell (2000), Howell (2000), Ratledge (1997), and Wodaski (2002).
Dark-Test Procedure Reduction of Dark Frames Analysis of the Dark Tests 2-D Dark Properties Strategy for Obtaining Dark Frames Guide Dark Properties
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