When I subtract a Tiny Tim model from an observed WFPC2 PSF, there are large residuals in the diffraction spikes. What can be done?
The diffraction spikes consist of alternating bright and dark bands extending out from the star. The positions and intensities of these bands are extremely sensitive to a number of factors, including the position of the star, the focus of the telescope, the object color, etc. The diffraction spikes in Tiny Tim models are poor matches to the observed ones. Observed PSFs have the same problem. Thus, there is no way to get around this problem. You should never use the spiders to normalize a PSF when subtracting, whether it is an observed or model PSF.
