Standard self-cal and source removal methods assume that phase and amplitude changes are strictly due to source structure and distance from the field centre. This assumption is not valid in the case of the errors mentioned above. These corrupted model visibilities are then subtracted from the data. So, in a sense the method is exactly the opposite of self-calibration. Interestingly, we can use exactly the same procedure to get rid of fake polarized sources.
Instrumental polarization increases toward the edge of the field.
So we can easily get a significant stokes Q and U signal from a strong intrinsically unpolarized source, such as the HII region W3, when it is detected at the edge of a field. If we asume that the fake Q and U signals are proportional to the actual total intensity signal, we can use a good total intensity image as a model for the polarized emission. An example of the technique is illustrated in the two images of Stokes U emission above. The effects of the strong sources have been almost completely removed so that we can more easily study the real U emission from the central part of the field.