The last prompt offered by PSS in expert mode is SSUB. If a response is given then PSS creates a copy of the input dataset with the sources found in either SEARCH or PARAM modes subtracted. This is performed by simply subtracting the psf scaled by the flux from the input data. It is therefore possible, especially for weak backgrounds, that negative data values will be produced. It would then be necessary to ignore these points using one of the QUALITY applications before source searching was performed on this dataset.
If sources are required only to be ignored, a better technique is to use the SSZAP procedure which is described in Section 6.6. The most suitable use of PSS source subtraction is in removing sources to improve backgrounds. The psf model is unlikely to be sufficiently good that no artifacts remain in the image after subtraction, especially of this source is extended in some non-trvial way.