WFPC2 Instrument Handbook for Cycle 10

PSF Anomaly in F1042M Filter

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We note that the F1042M filter has an anomalous PSF containing additional light in a broad halo component. This is due to the CCD detector becoming transparent at these wavelengths, so that light is reflected and scattered by the back of the CCD producing a defocused halo. Figure 5.11 compares the F1042M PSF with the more normal PSF seen slightly blueward in F953N. This scattering will impact photometry in the F1042M filter relative to other filters, since a greater fraction of the counts will lie outside the 1 arcsecond diameter aperture used herein for photometry on standard stars.

Figure 5.11: Comparison of azimuthal averages for observed F1042M and F953N PSFs. Courtesy John Krist.