Cosmic rays have been removed through comparison of successive images. Nonetheless, individual bright pixels are clearly visible throughout the field. Showing Warm Pixels These bright pixels are warm pixels, or pixels with an elevated dark current. Some of these pixels are permanently warm, but most become warm during the course of the month, probably as a consequence of the on-orbit bombardment by heavy nuclei. STIS, the other instrument currently aboard HST that uses CCDs, exhibits a similar behavior.
Most warm pixels return to normal after the CCDs are brought to room temperature for a few hours.
To repair warm pixels we run monthly decontaminations, in which the instrument warms up to about 22 C for a period of six hours. Decontaminations are also needed to let UV-blocking contaminants that build up on the CCD windows evaporate, thus temporarily restoring the UV throughput of the camera. For more detailed information, see the WFPC2 Instrument Handboo k . Because of the time variability of warm pixels, standard dark correction does not deal with them adequately. Even dark frames taken within a day of the observation will contain some warm pixels that vary significantly from those in the observation.