Confusion

The presence of field sources in the primary beam can limit image quality, especially for continuum observations. These sources produce unwanted sidelobes in the images and can lead to dynamic range and aliasing problems. The brightest sources expected (on average) in the half power primary beam for positions away from the galactic plane are listed in an earlier table. Note that confusion gets much worse at lower frequencies and that sidelobes are much larger in the case of snap-shot observations. If a strong confusing source happens to be close to your object, you may have to image it too.