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4. How Charge Transfer Traps Affect Photometric and Astrometric Results

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.

Instrument Science Report WFPC 95-03

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