10.3.1 Introduction

Because of the focus and pointing errors, and possible drifts in receiver gains, amplitude calibration has always been difficult at mm wavelengths. In addition to these basic single-dish effects, the variable amount of decorrelation introduced by phase noise (atmospheric and/or instrumental) make it difficult, if not impossible, for an interferometer to measure absolute flux densities.

All measurements need to be relative to some source of known flux. In practice, planets are used because they are among the few astronomical objects sufficiently strong at millimeter wavelengths for which flux density predictions are possible and sufficiently accurate. They are then used as primary calibrators to bootstrap the flux of the stronger quasars which are point sources. Since the quasars are highly variable, a regular monitoring (each month) is needed. These observations require a very good weather with a small amount of precipitable water vapor (< 4 mm) and a stable atmospheric phase. If not properly taken into account, the quasar variability can produce an error in the flux scale during one configuration which does not result in a simple scale factor in the final image, but introduces artifacts.