Dark frames are long exposures that are taken with no light incident on the CCDs. They are used to detect CCD counts (the dark current) caused by thermal generation (at the interfaces between the silicon and oxide layers), and the rate of charged particle and secondary radiation events. Estimated dark current and cosmic ray event rates are given in section 4.8, "Dark Backgrounds", on page 71 and section 4.9, "Cosmic Rays", on page 75, respectively. Observers are cautioned that the calibration provided by the pipeline may not use the most up-to-date dark frames (partly because constructing these may involve data taken after the observation). This matters because new hot pixels are being generated all the time. Data to construct improved super-darks is available in the archive with at least 5 long dark exposures being taken weekly.