Chromaticity in all-reflective telescopes for astrometry

Aims.Chromatic effects are usually associated with refractive optics, so reflective telescopes are assumed to be free from them. We show that all-reflective optics still bears significant levels of such perturbations, which are especially critical to modern micro-arcsecond astrometric experiments.Methods.We analyze the image formation and measurement process to derive a precise definition of the chromatic variation of the image position, and we evaluate the key aspects of optical design with respect to chromaticity.Results. The fundamental requirement related to chromaticity is the symmetry of the optical design and of the wavefront errors. Finally, we address some optical engineering issues, such as manufacturing and alignment, providing recommendations to minimize the degradation that chromaticity introduces into astrometry.