The presence of field sources in the primary antenna beam can limit continuum image quality. These sources produce unwanted sidelobes in the images and can lead to dynamic range and aliasing problems. The brightest source expected on average in the half-power primary antenna beam (away from the Galactic plane) is listed in Table 1.1. Note that confusion becomes much worse as you go to lower frequencies, and that this can be particularly serious for snapshot observations (See Section 1.7.1). The SUMSS catalogue and database, and the Molonglo Galactic Plane Survey (MGPS-2) catalogue and database are useful references to search for potential confusing sources in the 16cm band, where the effects of confusion are greatest.
At short wavelengths and/or long baselines, atmospheric refraction can cause serious phase errors. The problem becomes progressively less serious at longer wavelengths and shorter baselines. Atmospheric conditions are most favourable during winter and at night. A seeing monitor at Narrabri continuously monitors the phase stability at a frequency of 21 GHz and over a baseline of 200 m. See The ATCA Seeing Monitor for details (though note this paper describes the initial system; in 2008 a new satellite beacon was adopted, resulting in a change in the frequency from 30GHz to 21GHz).
As with other synthesis instruments, system errors such as DC offsets and sampler harmonics can lead to artefacts at the centre of the field. These artefacts have not yet been observed to affect the data from the CABB correlator, but if such an error would severely degrade your science goals (e.g. in a detection experiment), it is preferable to displace the source positions a few (synthesized) beamwidths from the field centre.