These proposals monitor the instrument's health throughout the cycle, including the stability of the cameras, their signal chain electronics, and the internal optical alignment of the WFPC2. The internal flat fields are intended as delta-flats, in order to monitor the stability for the main photometric filter set as well as provide data for the generation of high S/N flat fields. The darks are required not only for the generation of darks for the pipeline but also for tracking the evolution of hot pixels and mapping low-level CCD defects such as traps.
The internal visflats obtained in most of the visible WFPC2 filters will be used with the thermal vacuum test flat fields and the Earth flats to generate "superflats"; the intflats are obtained as a backup for the visflats (for example, if the cal-channel should fail).
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