Tracking corrections are derived directly from the pointing model in a way which completely avoids cumulative errors. Even after hours of tracking, errors in position are due only to the discrepancy between the pointing model and physical reality. Differential tracking Differential rates can be applied to the normal corrected telescope tracking by entering data through the display sequence. These differential rates are not required for correcting normal sidereal tracking. They are used only on the rare occasions when a controlled drift in the telescope apparent place is needed, for example when tracking a comet or planet.
Source coordinates The control system accepts coordinates as either mean or apparent.
Apparent place is a geocentric position referred to the true equinox and equator of the epoch of observation. In almost all cases, mean place is the correct form. Apparent place is only used for objects within the solar system whose ephemerides are published in apparent coordinates. All AAT coordinates are pre-1976 IAU recommendations, i.e. they are in the FK4 system, not FK5. Because the AAT points so well, it is important that observers have accurate coordinates for their target objects.