Image drift

One of the observers noted that when following a source for an extended period of time the source drifts in the order of 10 pixels ($\sim$1.5 arcsec) per hour. Examination of the data showed that the drift is mostly in the vertical direction with respect to the telescope. The fact that the drift is similar in V and I excludes differential diffraction affects, while the straight tracks for the drifts makes it unlikely to be a problem in the tracking or guiding. The most likely cause is flexure of the camera with respect to the 45 degree pick-off mirror. Checking the mirror fixation itself does not show any slack, but it might be that the whole probe moves.