When searching for companions of bright targets, at what M(target)-M(QSO) does it become necessary to use PSF subtraction?
The table below gives the brightness of “object-like” features in the PSF, expressed as a delta-magnitude from the bright target. The third column gives the delta-magnitude detection limit, if one assumes “detection” means features which are three times brighter than the PSF features. At some radii, the OTA diffraction spikes are important. In those cases we give a range where fainter limits can be reached if the OTA diffraction spikes are avoided; this can be done by observing at several different spacecraft roll angles. These results are derived from TinyTim models of the WFPC2 PC PSF at F555W. Observers with M(target)-M(QSO) larger than the limiting values below should be prepared to use PSF subtraction.
