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A metrology concept for multiple telescope astrometry

Medium to large angle observations, e.g. for global astrometry, can be implemented in space by means of either a common telescope, fed by a Beam Combiner (as in Hipparcos), or by individual telescopes set in a rigid geometry (as in Gaia). We investigate the applicability of auto-collimation and cophasing techniques for implementation of a monitoring system alternative to more conventional point-to-point metrology. Apart different implementation constraints, the most relevant difference consists in the auto-collimation approach characteristics of monitoring simultaneously comparably large sections of the optical system, thus evaluating collective properties closer to those experienced by the stellar beams.