Measurement of position and orientation of optical elements in interferometric gravity wave detectors

Interferometric gravitational wave detectors like VIRGO or LIGO will use mirrors suspended as pendulums, forming systems of optical cavities; this requires precise control of alignment and of the mirror pendular motions. We describe a system for the measurement of position and orientation of a mass with respect to a local reference frame. We use a single computer read out charge coupled device camera to determine all six degrees of freedom by observing reference marks on the mass, and, in the case of small mass misalignment, reflections of two auxiliary laser beams. The derived information is available in a shared memory segment on an industrial standard bus system (VME) crate for further digital processing, which will be necessary for alignment and damping of the pendular movements. The mass coordinates can be calculated at the camera frame rate of 60 Hz. In a table top experiment with fixed test mass, the short term rms measurement errors for the translational degrees of freedom were measured to be better than 0.5 μm; for two rotational degrees of freedom, the errors were better than 0.5 μrad, and for the third one about 15 μrad. The long term stability over several days is about an order of magnitude less; it is limited by drifts, to a large extent of thermal origin.