Low Noise Dark Frame Procedures

From Ron Zissell (ZRE):

    Dark Frame Calculation:
   After my camera has reached -44.6 C , I take a series of exposures
   in the order B,D,B,D,B,D,B . Three 900 second darks with biases
   bracketed by bias frames.  I wrote an IRAF script to median filter
   the biases and the darks to give two frames BB and DD individual
   dark+bias frames are them constructed by the expression
   (DD-BB)/N + BB where N is a number that gives  10, 20, 30, 60, 90,
   120, 180, 300, 600, 900 second dark+bias frames. These can be
   subtracted directly from the star exposure before the image is saved.
   For the case of U Ori I images i have to build 01 and 02 second
   darks.
     Dividing the long exposure dark gives short darks with much
   reduced noise without having to make multiple dark exposures for
   each exposue time.