Gaia contains two identical telescopes, pointing in two directions separated by a 106.5° basic angle and merged into a common path at the exit pupil. The optical path of both telescopes is composed of six reflectors (M1-M6), the last two of which are common (M5-M6). Both telescopes have an aperture of 1.45m × 0.5m and a focal length of 35m. The telescope elements are built around the hexagonal optical bench with a ~3m diameter, which provides the structural support. 

Diagram of the hexagonal optical bench and the mirror system, together with the focal plane. Credit: EADS Astrium

Although the optical design is fully reflective, based on mirrors only, diffraction effects with residual aberrations induce systematic chromatic shifts of the diffraction images and thus of the measured star positions. This effect, usually neglected in optical systems, is also critical for Gaia. These systematic chromatic displacements will be calibrated as part of the on-ground data analysis using the colour information provided by the photometry of each observed object.

	Summary
	Astrometry