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46.4 Hot Pixels

A hot pixel has been defined as a pixel whose dark current exceeds 0.01 DN/sec (usually few hundred per chip). Time variability has been seen in the hot pixels (Instrument Science Report 93-01) with the following characteristics:

The number of hot pixels gradually increases over time (about 30 new pixels per month per CCD).
The number of hot pixels decreases (by 10-30%) after decontamination procedures.
Individual pixels evolve: some undergo large variations in count rate, others remain constant in time, a very few decrease at times other than decons. The pipeline calibrates the data with time averaged dark files: these files have improved signal-to-noise, but don't correct for time variations in the hot pixels. If hot pixels are more of a concern than signal-to-noise, an custom dark file could be generated from dark frames taken close in time to the observation, and that alternate dark used to recalibrate the data. Some alternate dark reference files have been generated and are listed in the WWW WF/PC-1 Reference File Memo. It is also possible to generate a more appropriate reference file from a set of individual dark frames available through the STScI Archive; this can be done using crrej.
Through April 1993, two dark frames were usually obtained each month. As of May 1993, additional darks were taken, for a total of about 12 per month for each camera. By calibrating with a reference file generated from dark frames taken within one month of the science observation, it is possible to reduce the number of residual hot pixels by nearly an order of magnitude. Suspect pixels can be compared with new lists of known hot pixels which are kept on WWW.

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