Co-phasing of segmented telescopes: A new approach to piston measurements - I. Optical concept

One of the main problems arising in the co-phasing of segmented telescopes is the problem of measurements of the relative pistons among the segments. These measurements become especially complicated when the pistons are in order of wavelength fractions. In this paper we present a new approach relative piston measurements which allows for a high-accuracy reconstruction of small pistons. Our approach is based on the shearing interferometry and compared to existed methods it has the following advantages: a high accuracy of piston reconstruction in presence of high levels of noise is combined with the ability to retrieve the pistons taking into account the segment's aberrations.