Separation between pairs of stars in the NGC 6388 data set plotted against frame number before any distortion correction. The left panel shows the separation in the x-direction, the middle panel in the y-direction, and the right panel the full separation . The left y-axes are given in pixels while the right ones give the measured distances in seconds of arc. The small straight lines mark the frames after which a new five point sequence of jitter movements was started. The x-axes can also be seen as a time sequence as the individual frames were obtained subsequently, the first ten with an exposure time of 240 s and the last 20 with an exposure time of 120 s.