Raw unit weight residuals of the photometry vs. the centring error δ_μ. Left panel: G (all AF CCDs). Central panel: G_BP. Right panel: G_RP. The solid black line shows the median of the distribution, the dashed lines show the ± 1σ of the distribution. The centring error is defined as the difference between the source predicted AC position at the observing time and the nominal window centre. The residuals were produced only for the set of calibrators and only using data from the INIT period.