MEASUREMENT OF POSITION AND ORIENTATION OF OPTICAL ELEMENTS IN INTERFEROMETRIC GRAVITY WAVE DETECTORS

Interferometric gravitational wave detectorslike VIRGO or LIGOwill 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 devicecamera to determine all six degreesof freedomby 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 segmenton an industrial standard bus system (VME) cratefor 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 rateof 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 bett...