Clock drive
In astronomy, a clock drive (also known as a sidereal drive or field rotator) is a motor-controlled mechanism used to move an equatorial mounted telescope along one axis to keep the aim in exact sync with the apparent motion of the fixed stars on the celestial sphere. == Overview == Clock drives work by rotating a telescope mount's polar axis, the axis parallel to the Earth's polar axis (also called the right ascension axis) in the opposite direction to the Earth's rotation one revolution every 23 hours and 56 minutes (called sidereal day), thereby canceling that motion.