Assessement of the relative error implied by the use of the true primary beam instead of the effective primary beam when analyzing interferometric on-the-fly data sets. Left: inverse Fourier transform of interferometer primary beam, (i.e. the autocorrelation of the antenna illumination). Right: relative error as a function of sampling rate of the primary beam. The curves of different colors show the results at different normalized uv distances (u/d_prim) from the center of .