A note on the pileup model

It is easy to misinterpret the "PSF fraction" parameter in the pileup model. A natural reading is to suppose that this means the fraction of the PSF which is included in the source extraction region. This is not correct. For the pileup model to work well the extraction region should be large enough to include essentially all the PSF. The "PSF fraction" parameter is actually the fraction of counts detected which should not be included in the pileup calculation. The manual will be modified to make this clearer. My thanks to Pat Broos for pointing out this potential confusion.