Motion of chip 2 relative to the reference frame of the upper chip: unreduced displacements V_z, i, e along the X- and Y-axes of 15 bright lower chip stars in each sky field at each epoch (small dots). The same after correction for the residual parameters (filled dots). The model motion H(t) (dashed line), which at this scale looks the same for both the unreduced and the corrected displacements.