Multiple interferometer for real-time position control

Summary form only given. Real-time position control of a rigid body over long distances with accuracy at the nanometer level requires active feedback in six degrees of freedom. We are developing a system of multiple Michelson interferometers to meet the position-sensing requirements of X-ray interferometry over displacements up to 5 cm. Our goals are rather stringent: the system must be free of periodic systematic errors at the level of tens of picometers, work at bandwidths up to 10 kHz, and it must allow good control of optical wavefronts in order to correctly account for the effects of diffraction. We report on a quadruple Michelson interferometer to sense three degrees of freedom with independent verification of system accuracy from a Fabry-Perot interferometer.