Please note that for grism data omdetect is only used to locate the positions of possible zero and first-order spectra- the SAS task omgrism is used to refine the positions and compute the spectra. For Fast-mode data the photometric parameters computed by omdetect are not used- the photometry is recomputed using the SAS task omlcbuild.
If you think that the photometry of one or more sources could be improved (eg in crowded fields), then it is worth considering using either omsource or omphotom to recompute the photometry of one or more sources in the source-list produced by omdetect.
extended Sources using an exposure image
