VST telescope azimuth and altitude axes control hardware

In a modern telescope the Azimuth (AZ) and Altitude (ALT) Axes have to perform a very accurate pointing and tracking. The tracking of celestial objects is a critical operation, during which the telescope axes have to compensate the earth rotation, allowing a stable image integration on the scientific instrument CCD. To accomplish this goal, the two axes control system is provided with state of the art encoders and tachometers, for position and speed feedback, together with motors controlled in torque preload, to avoid backlash effect. The closed loop control system is based on an axes Local Control Unit (LCU), based on VME Computer. In this paper the AZ and ALT Axes Control System of the VST (VLT Survey Telescope) is described. The VST is a 2.6 m class Alt-Azimuthal Survey Telescope in installation phase at Cerro Paranal in Northern Chile, at the European Southern Observatory (ESO) observation site. The VST Telescope was designed and implemented by I.N.A.F., Osservatorio Astronomico di Capodimonte. Nowadays the AZ and ALT axes mechanics and the related control system have already been accepted by ESO, shipped to Chile and integrated in Paranal.