Obtaining Dark Images

Dark images are a measure of the dark current in the CCD at a specific temperature for a specific exposure length. Due to the bias level and read noise, it is difficult to scale one dark exposure to another exposure length. Dark images are therefore taken at each of the configured exposure lengths described in Section 4.3.2. Dark images are subtracted from image frames to remove the bias level, the dark level, and to subtract hot pixels to improve photometry. We also plan to use dark frames to create a hot pixel map to flag false identifications from hot pixels.

Dark images are obtained by the automated scheduler astrod. The ndarks configuration parameter (Section 4.3.3) describes the number of darks taken at each exposure length. The default is 6. In addition, a dark run must be scheduled, as described in Section 4.4.2. One dark run per night should be fine for normal operation.