Because of the variable amount of decorrelation introduced by phase noise, an interferometer cannot easily measure absolute fluxes. All measurements are relative to some source of known flux. In practice, planets are used because they are among the few astronomical objects sufficiently strong at millimeter wavelength for which flux density predictions are possible and with
Planets are too big to be measured directly in interferometry, except for the smallest at short baselines. Thus, single-dish measurements are used to bootstrap the flux of strong quasars (point sources) from the planets. Weaker sources are then referred to stronger ones in interferometry because single-dish measurements are not sensitive enough.
Since the quasars are highly variable, we need to observe at least the flux from the strongest of them (single-dish measurements) each month. Because of this variability, an error in the flux scale during one configuration does not result in a simple scale factor in the final image, but introduces severe artefacts.
Accordingly, flux measurements have to be considered as a HIGH priority project on the interferometer, since they may ultimately limit the image quality. Special observing dates are selected from the best possibilities offered by the planets visibilities. In order to interpolate the flux for all the frequencies available at the Plateau de Bure Interferometer, we need one frequency at each edge of the frequency band to derive the spectral index of each source.