7.3.2 Phase referencing by a point source in the primary beam

We now consider the simple case where the field contains a strong point source: it can be a continuum source (quasar) or a line source (maser). In that case all phase fluctuations with period longer than $\sim 2 \ensuremath{T_\mathrm{\scriptscriptstyle I}}$ are removed, where is the integration time. However statistical errors may be mistaken for true atmospheric phase fluctuations, causing additional decorrelation.

This method gives good results, but for very specific projects which can be observed in very poor atmospheric conditions (e.g. observation of radio emitting quasars, of stars with strong maser lines).