  In real telescopes, the optical parameters evolve with time, and the
degradation is often not uniform. This introduces variations in the image
profile and therefore photo-centre displacements which, unless corrected, may
result in astrometric errors. The effects induced on individual telescopes and
interferometric arrays are derived by numerical implementation of a range of
cases. The results are evaluated with respect to the potential impact on the
most relevant experiments for high precision astrometry in the near future,
i.e. Gaia, PRIMA and SIM, and to mitigation techniques applicable from design
stage to calibrations.
