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Hubble Space Telescope
WFPC2 ISR 95-03

2. Characteristics of Traps

Figure 3 shows several tails resulting from trap 2-637. These tails were taken from a variety of observations, both before and after the April 23, 1994 cooldown of the WFPC2. The solid line shows a simple exponential decay, as described in our idealized model. It appears that the behavior of the bright traps has been relatively constant with time, and the simple exponential decay provides a fairly good fit to the data.

Tails created by trap 2-637, including the two tails shown in Figure 1 (filled circles and triangles), as well as several other tails from observations both before and after the April 23, 1994 cooldown of WFPC2.

The trap can therefore be characterized by the exponential scale length, a, defined by:

where C is the number of counts above the background, N0 is the deviation from the background at the start of the tail, and DY is the distance from the start of the tail. For example, trap 2-637 has a value of a = -0.1803 for bright tails. The decay rate is simply 10a, or 66%, and the transfer efficiency is 1 - 10a, or 34%.

What about the dark tails seen just above some of the traps? Although the decay rate for the dark tails are different than for the bright tail, an exponential decline again fits the data quite well. For example, the dark tail decay rate for trap 2-337 is only about 3% while the bright tail decay rate is 19%. This difference can easily be seen in Figure 1, where the dark tail has a deviation from the background of only 13 counts and extends roughly 60 pixels, while the bright tails have deviations of about 50 counts but only extend about 20 pixels.

Table 1 lists the currently known traps, including three traps on WF2 which were previously unidentified. Examinations of observations taken shortly after the WFPC2 was installed (i.e., the observations of R136 taken February 2, 1994), as well as during thermal vacuum testing before launch, both indicate that these traps were already present. There is no evidence of new traps forming in the WFPC2. Figure 4 shows the locations of all known traps. Note that only the strongest traps can be measured reliably. The unmeasured traps generally have transfer efficiencies in the range of 50-90%.

(Tentative) Location and Characteristics of Traps

Location of all known charge transfer traps on the WF2, PC, WF3, and WF4 CCDs, respectively.


1. - Introduction
2. - Characteristics of Traps
3. - Removing the Effects of the Charge Transfer Traps
4. - How Charge Transfer Traps Affect Photometric and Astrometric Results