During observations of solar system objects some problems where noted with differential tracking. The system works properly when the autoguider is used, including for rather high ($\sim$400 arcsec per hour!) differential tracking rates. However, data taken with the telescope tracking without using the autoguider show very significant distortions and is much poorer than one might expect. The data points to some problem in the way non-sidereal tracking rates are applied. This is currently being investigated.

Thomas Augusteijn 2008-11-14