Charge Transfer Traps in the WFPC2

The characteristics of charge transfer traps in the WFPC2 are examined and the effect they may have on observations are discussed. The dark tail created just above the location of the trap, as well as any bright tail created in the column above the trap when a bright source is clocked out during the readout, can be modeled as simple exponential decays. Three previously unidentified traps on WF2 are added to the list of “bad pixels”. There is no evidence of new traps forming. Figure 1: A portion of a WF2 image (before cosmic ray removal) showing the location of five of the seven traps on WF2. The strongest trap is 2-337, which shows a dark tail just above the trap, and three bright tails where cosmic rays are clocked across the trap during the readout. Trap 2-637 shows both a bright tail from a cosmic ray, and a tail from a point source. The other traps are either too weak to show noticeable effects on this particular image, or did not happen to have strong cosmic rays crossing their columns. Instrument Science Report WFPC2 95-03