Astrometric precision as a function of the flux density of SgrA*. Each point represents the 2D standard deviation of six individual 30 s position fits within 3 min, as in Figs. 1 and 2, divided by . Here both polarizations were averaged. The contiguous line indicates a photon-noise-limited phase noise (e.g. Wyant 1975) , where Nph is the number of photons.