Extremely high-resolution tip-tilt-piston mirror mechanism for the VLT-NAOS field selector

NAOS (Nasmyth Adaptive Optics System) is the adaptive optics system presently developed for the ESO VLT. The field selectors are to feed the NAOS wavefront sensor with the light coming from an appropriate reference source which can be up to 1 arcmin (on the sky) distant from the center of the field of view. A large input tip-tilt-piston mirror selects the required part of the field of view. A second active mirror redirects the selected field to the wavefront sensor. The displacement of both mirrors are synchronized. The NAOS Field Selector consists of two extremely accurate tip-tilt-piston mirror mechanisms controlled in closed loop. Each mechanism provides a mechanical angle amplitude of ±6° with a resolution and mechanical stability of 0.42 arcsec rms over 20 minutes. This implies a dynamic range of 100'000 which requires an extremely accurate, very high resolution closed loop control. Both mirrors are made in SiC for low mass and inertia. The design configuration of the mechanism in based on three electromagnetic actuators 120° apart with the mobile magnets mounted on flexure guides. The mirror is supported by a combination of flex pivots and a membrane for flexibility in tilt and high radial stiffness. All kinematic joints consist of flexure elements so that the mechanism is essentially frictionless. The control system is implemented on a VME bus operated with the VxWorks OS with high electrical resolution (= 18-bit) A/D and D/A interface boards. The controller has been carefully designed to achieve the best overall performances, i.e., a very good noise rejection, and a relatively low settling time.