Left panel: plot of the combined astrometric uncertainty from the two epochs of K_s-band data plotted vs. the magnitude of the second epoch (for details of the error estimation see Sect. 3.4). The median and standard deviation of the astrometric uncertainty above the linearity limit (at 14.3   mag) of the dataset in 2003 with 20.0   s DIT were fitted by polynomials. This fit of the median (lower line) and the sum the median and standard deviation (upper line) are drawn in all three plots. Middle panel: astrometric uncertainty of stars residing within a circle with around the centre of the observed field. Right panel: astrometric uncertainty of stars residing outside this radius, which are excluded from further analysis.