A Fizeau interferometer for astrometry in space: the metrology point of view

In space astrometry one seeks to determine positions, parallaxes and proper motions of stars to within 50 prad. The proposed GAIA mission is based on a rotating instrument made up of two telescopes set at a large base angle to each other and scanning the same great circle in the sky. Since the intended performances impose very strict stability requirements, a sophisticated metrological system is needed, whose actual implementation requires an integrated design of telescopes and laser interferometers. This paper examines the response of the instrument to small perturbations, special emphasis being placed on the location of star images. In order to exploit laser interferometry as much as possible, the original design was reconsidered, the base-angle implementation was improved, the number of degrees of freedom was reduced and the critical parameters were factorized. Although the present paper considers only the original proposal, in which each telescope was designed as a Fizeau interferometer, the results can be readily extended to other configurations.