The flux density scale is usually set by observing an unresolved reference source whose flux density is believed to be stable and whose intensity has previously been measured on an absolute scale. As there are few such sources that remain unresolved to long-baseline arrays, it is rarely possible to use a primary flux density calibrator also as the primary phase calibrator. Good phase calibrators are strong and unresolved, so are likely to vary with time. The flux densities of phase calibrators must usually be determined at the time of the observations by measuring their amplitude ratios to standard flux density calibrators, often using only a short-baseline subset of all the antennas in the array. Note that self-calibration has no absolute amplitude scale and therefore produces images whose flux density scale is whatever was specified in the input model.