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Hubble Space Telescope
WFPC2 ISR 95-03

4. How Charge Transfer Traps Affect Photometric and Astrometric Results

The object just above trap 2-637 in Figure 5 is a good example of how traps can affect the observations of point sources. While all the tails from cosmic rays disappear during cosmic ray removal, the tails on real objects appear in both exposures and hence are retained in the combined image. The effect on photometry, position, and size measurements will depend on several factors, including the strength of the trap, what part of the object falls on the bad column, and the technique for measuring the object (e.g., the size of the aperture).

In the case of the object above trap 2-637, the photometric magnitude measured within a 6-pixel aperture (i.e., radius = 3 pixels) is about 0.1 mag fainter than it should be, due to the tail taking light out of the aperture. The FWHM is increased from 1.5 pixels to 2.2 pixels.

Using fixpix to interpolate across the bad column makes things even worse, with the magnitude being 0.4 mag too faint, a positional shift of 0.5 pixels in the X direction (i.e., the interpolation makes the measurement very sensitive to slight asymmetries in the two adjoining columns), and an increase in the FWHM to 2.9 pixels.

Using the reconstruction formula (described earlier) to correct for the trap provides quite accurate corrections for the photometry and FWHM determination, (i.e., to a few percent).

Another relevant question is what percentage of objects are affected by charge transfer traps. There are only about 10 traps that are bad enough to cause easily measured effects, compared to the total of 4 x 750 = 3000 columns. If we assume the average trap is at row 400, and the average width of a target of interest is 6 columns, we would predict that about (10 x 1/2 x 6) / 3000 = 1/100 objects would be affected. This appears to be a fairly good estimate, based on our experience of doing photometry on point sources.


Abstract
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