The purpose of the offline calibration (carried out when the
acquisition process at PdB is ended) 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).