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If one needs to use the guide CCD images, one should correct them for dark current just as for the main CCD. The guide CCD will have the same sort of dark current-temperature relation as the main CCD, so one must obtain guider dark frames at the same detector temperature as the guider images. The guide CCD is small, so it takes very little time to obtain a set of 5 to 9 guide dark frames. One then bias-corrects the darks and median-combines them to make a master guide dark just as discussed for the main CCD frames. Figure 16 gives an example of a single raw guide dark frame, the same frame after bias-correction, and the median of 5 bias-correction guide darks. All three images are shown at the same contrast.
Inspection of Fig. 16 shows that median-combining the dark frames produces a less noisy master dark. There are many hot pixels, as well as the hot upper-right corner. Figure 17 shows a raw guide image, the same image after bias-correction, and again after dark correction. The result of dark-correction is to remove most of the hot pixels remaining after bias-correction. Note that the hot corner is also mostly corrected by dark subtraction.
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