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 t o each other and scanning the same great circle in the sky. Since the inten ded performances impose very strict stability requirements, a sophisticated metrological system is needed, whose actual implementation requires an int egrated design of telescopes and laser interferometers. This paper examines the response of the instrument to small perturbations, special emphasis be ing placed on the location of star images. In order to exploit laser interf erometry as much as possible, the original design was reconsidered, the bas e-angle implementation was improved, the number of degrees of freedom was r educed and the critical parameters were factorized. Although the present pa per considers only the original proposal, in which each telescope was desig ned as a Fizeau interferometer, the results can be readily extended to othe r configurations.