Positional residuals in RA on 24.11.2011 for stars in the DE0716−06 field computed with k = 10 and R_opt = 815 px. a) The original two-chip data showing the discontinuity at the chip edge with a full amplitude of g_x = 0.0376 px, which is approximated by the function G_x(y) (solid line); b) only data of chip 1; c) after offset of chip 2 photocentre positions in RA by ; d) an example of an epoch with good seeing and small discontinuity (g_x = 0.0044 px). The data corresponding to UCD are marked by an asterisk.