Difference between the reference and the measured position, in arcseconds, of Gaia DR2 catalogue sources with 16 < [3.6] < 19 total magnitudes extracted from the EDF-N channel 1 mosaic. The blue dashed lines indicate the size of one mosaic pixel. The blue dotted lines go through the origin. The shaded regions are ellipses containing 68% and 99% of all sources, respectively. For clarity, only one in ten sources is plotted.