Mechanical features of the OzPoz fiber positioner for the VLT

OzPoz is a multi-fiber positioner which will feed Nasmyth spectrographs on one of ESO's VLT unit telescopes. Its concept follows that of the positioner for the two degree field facility on the Anglo-Australian Telescope. Thus its fibers will be fed from prisms housed in buttons which attach magnetically to steel focal plates; a robotic system will position the buttons; and the plates will be interchanged so one can be re-configured while the other is gathering starlight. However, OzPoz has a number of novel features, most notably the use of a pneumatically operated gripper which relies for its accuracy and friction free rotation on air bearings. The robot motions also employ air bearings, with vacuum preloading. The mechanism which exchanges focal plates has been carefully designed to ensure it will survive the maximum likely earthquake on Paranal without significant damage.