by Mark.Wieringa » Tue Aug 20, 2019 10:08 am
Hi Brodie,
A phase calibrated ATCA observation at 5 GHz or above should have better than 0.5 arcsec position accuracy (usually <0.2"). This is assuming a reasonable beam shape and a config with km baselines.
Your beam sizes seem to be in the 1-2" range, which should give sub arcsecond position accuracy for 5 sigma detections.
For observations where you need to know the position errors you can use two phase calibrators. That way you can calibrate one against the other and check the accuracy.
For imaging you don't really need to centre the target unless it has moved outside the half power beam radius and in that case you really should have pointed the telescope at the actual source position.
If you're finding large offsets, there must be something wrong with either the phase calibration (check secondary) or the computed position. If the phase stability is bad enough to move your source by a few arcseconds it generally also gives a very poor, decorrelated image - i.e., flux is missing and distributed over several beam widths.
Cheers,
Mark