Multi-spectral piston sensor for co-phasing giant segmented mirrors and multi-aperture interferometric arrays

This paper presents the optical design of a multi-spectral piston sensor suitable to co-phasing giant segmented mirrors equipping the Future Extremely Large Telescopes (ELTs). The general theory of the sensor is described in detail and numerical simulations have been carried out, demonstrating that direct piston and tip-tilt measurements are feasible within accuracies respectively close to 20 nm and 10 nano-radians. Those values are compatible with the co-phasing requirements, although the method seems to be perturbed by uncorrected atmospheric seeing