In general, imaging observations are preceded by a delay calibration on the subarray build, done by the telescope operators. This is applied at the correlator and does not need to be solved for again, though it can be a useful sanity check to solve for delays on the bandpass calibrator during reductions.
A typical duration and cadence for bandpass calibrator visits is for 10 minutes every 3 hours.
It is recommended to visit a phase calibrator every 30 minutes for a duration of 2 minutes.
An absolute polarisation calibrator such 3C286 or 3C138 should be visited at least once during the observation if the user wishes to have polarisation angle information. J1939-6342 is effectively unpolarised and can be used to solve for leakage. The gain calibrator can generally be used to obtain more extensive parallactic angle coverage. Please find further details here: Polarisation calibration.
Some a priori corrections are generally applied to the correlator before imaging observations are done: key among them is a delay calibration. While the cable delays of the array are stable, small timing offsets can be introduced during synchronisation of the digitizers. After initialising the array the standard operating procedure is to run a short delay calibration observation. During the observation, the noise diode, as well as a bright, well-known source, are used to calculate and apply time-variable solutions for the antenna-based delays.