The output of an interferometer or correlator is different from the true astronomical visibility for a variety of reasons viz., instrumental effects and propagation effects in the earth's atmosphere and ionosphere.
There are disturbances in the ionosphere due to effects like, solar flare effects, geomagnetic storm effects etc. At low radio frequency, these effects of the ionosphere are most dominant. There are two other effects however which are more troublesome. The first is `scintillation', where because of diffractive effects the flux density of the source changes rapidly - the flux density modulation could approach 100%. The other is that slowly varying, large scale refractive index gradients cause the apparent source position to wander. At low frequencies, the source position could often wander by several arc minutes, i.e. considerably more than the synthesized beam, which need to be corrected.
Although the scintillation effects can't be corrected, to correct the other effects we need a source which can either be assumed to be a point source or have well determined structures. These sources can be termed as calibrator sources or simply calibrators.
For every observation, calibrator sources are observed at least once an hour and sometimes as frequently as every 10 mins. Calibrator observations are not only important for tracking instrumental phase and gain drifts, atmospheric and ionospheric gain and phase variations, but for monitoring the quality and sensitivity of the data and for spotting the occasional gain and phase jumps. If a calibrator source is within 10 deg. of the target source, atmospheric phase fluctuations will be better calibrated. So it is required to have a large set of calibrators for observations.