Top left: BLb residual flux of ζ Ori given in blue, from the first correction window of Fig. A.4, which was subtracted by a local linear regression of the flux acting as a model for the stellar signal, F_star(t), and for the long-term instrumental signal. Top right: same residual flux as a function of the CCD centroid position x_c indicating the correlation, which is approximated by the B-spline given in black. The correlation between flux and x_c is more dominant than with y_c, hence it is decorrelated first, as indicated by the AIC and BIC. Middle left: same BLb flux residuals, now corrected for the correlation with x_c. Middle right: correlation between the corrected flux residuals and y_c, together with the B-spline representation in black. Bottom left: final flux residuals, corrected for the correlation with both CCD centroid positions, ready to correct for remaining instrumental effects. The periodic variations in this final product is caused by the periodic T_CCD fluctuations (see Fig. A.4).