The purpose of the offline calibration (carried out when the acquisition process is ended at the observatory) is to estimate as precisely as possible all the various terms of Equation 4. The basic principle is to measure the visibilities of sources with known (spatial and frequency) structure, in practice continuum point-like sources (called calibrators), which flux (so ) is also known or can be deduced. By simple division, the product is determined at the time intervals in which these calibrators are observed, and interpolated for the intermediate times in which the project source is. The signal to noise ratio of the calibrator measurements is the main limiting factor in this process.
Another important limitation concerns the validity of the interpolations used to predict the values of the calibration factors on the studied sources. It is desirable to estimate the quality of these predictions, and to ignore data which for any reason may deviate substantially from the predictions (for example due to instrumental problems, strong atmospheric variations, etc).