Taking and Processing of CCD's Dark Field Diagram

Taking and processing of the dark field diagram of a CCD in astronomy are discussed in this paper. From several dark field diagrams taken from Bejing Observatory, it is found that the electric dark current at some pixels has a variation of direct proportion with accumulation time. But some other pixels may have rather high electric dark current randomly and accidentally. No matter what reason in the second part comes from inside or outside the CCD divice, those high dark current values should not be recorded as “dark field”. Generally we can get a satisfactory “dark field” diagram with the “median point” method from several source diagrams, which may be taken in just one or several nights in the same observing run.