Cheese runs source detection on full-field images and creates “Swiss cheese” masks from the output. cheese produces the event, exposure, and mask images that are required in a user-selected energy band. Running cheese is not required if only the spectral files with all counts including point sources are required, or if excluding point sources is not of interest. Several issues in particular should be noted by the user.
First, the dist parameter set the minimum accepted distance between sources. If two sources are closer together than this limit, they will be combined. If this is problematic, decrease the value of dist.
Second, sources are detected on an instrument-by-instrument basis, and a detection in one instrument does not guarantee removal from another instrument. For example, if a source were detected in MOS1 and MOS2, but fell in a chip gap on the pn, even if the source is very bright and obviously impacts the surrounding pixels, it will not be removed. This is the default behavior. The output emllist.fits can be modified by the user, and region and makemask re-run in order to fix this issue.
Third, setting a single minimum maximum likelihood mlmin value may produce a mask that misses “obvious” sources, or may exclude regions where there is no “obvious” source. Since the mlmin value is determined over all the instruments, there may be complex reasons for a mismatch between the value and the user's expectation of the value. Careful examination (and perhaps by-hand modification of the emllist.fits file) is required to assure that the source identification/removal is appropriate for your science.
