Post-calibration astrometric residuals along the RA and Dec axes for unsaturated Gaia-DR3 stars in the NGC 6397 field (Massari et al. 2025). Residuals were computed for each star as the difference between the calibrated coordinates on the individual VIS exposures and the corrected Gaia-DR3 coordinates (see text). Grey points represent a subsample of detections with a S/N > 100 on ERO exposures. The plot reveals two distinct groups of stars, represented as a large cloud slightly offset from the centre and a small compact cloud clearly separated from the centre, both consisting of stars lacking proper motion data in the DR3 catalogue. The position offset of the smaller ‘cloud’, located in the lower left, aligns well with previous estimates of the NGC 6397 absolute proper motion (μ_* = (3.6,−17.3) mas yr⁻¹, Kalirai et al. 2007), over a period of 7.7 years. A dashed square at the centre of the plot illustrates the size of the VIS pixel footprint for comparison. On this specific ERO field the internal astrometric precision is 2.7 mas RMS.