1. Introduction
What makes CCDs possible is the fact that the charge deposited in a given pixel during the exposure can be efficiently transferred to adjoining pixels along the same column when the CCD is read out at the end of the exposure. The ideal CCD would have pixels with 100% charge transfer efficiency. While this is very nearly true for the vast majority of the 2,560,000 pixels on the WFPC2, there are about 30 pixels which do not transfer charge as efficiently. These "traps", or "defects", may transfer as little as 20% of the electrons during each time step of the readout. Only about 0.001% of the pixels are actually defective but about 0.4% of the image is affected since all the pixels in rows above the trap (i.e., higher value of Y) are clocked out across the bad pixel during the readout. The worst traps leave long tails, and are often known as "bad columns".
Figure 1 shows a portion of an image from WF2 with the location of five of the seven known charge transfer traps on this chip. The strongest trap is at (337,201) which transfers only 19% of the charge during each time step. The rows below Y = 201 are unaffected since they are transferred down the column during the readout and hence never pass over the bad pixel. Figure 2 shows an idealized model of how the bright tail is created, assuming a uniform background of 8 counts in every pixel during the exposure.
Idealized Model of a Charge Transfer Trap Compared to Real Data
Parameters:
Trap Location (WF2) = (637,389)
Cosmic Ray Location = (637,645)
Transfer Efficiency = 34 %
Background = 8 counts
Charge Leftover Charge from Total Charge Background C(J-1) Observed C(J-1)
Pixel Deposited Previous Pixel Charge Transferred Subtracted / C(J) Value / C(J)
----- --------- ------------------------ ------ ------------ ---------- ------ ------- -------
(equilibrium producing a background of 8 counts)
(637,644) 8 15.53 23.53 * 0.34 = 8.00
(hit with a 989 count cosmic ray)
(637,645) 989 23.53 - 8.00 = 15.53 1004.53 * 0.34 = 341.53 333.53 333.48
(637,646) 8 1004.53 - 341.53 = 662.97 670.97 228.13 220.13 0.66 217.91 0.65
(637,647) 8 442.84 450.84 153.29 145.29 0.66 146.06 0.67
(637,648) 8 297.55 305.55 103.89 95.89 0.66 100.33 0.69