Analysis of stellar interferometers as wave-front sensors

The basic principle and theoretical relationships of an original method are presented that allow the wave-front errors of a ground or spaceborne telescope to be retrieved when its main pupil is combined with a second, decentered reference optical arm. The measurement accuracy of such a telescope-interferometer is then estimated by means of various numerical simulations, and good performance is demonstrated, except in limited areas near the telescope pupil/s rim. In particular, it permits direct phase evaluation (thus avoiding the use of first- or second-order derivatives), which will be of special interest for the cophasing of segmented mirrors in future giant-telescope projects. Finally, the useful practical domain of the method is defined, which seems to be better suited for periodic diagnostics of space- or ground-based telescopes or to real-time scientific observations in some specific cases (e.g., the central star in instruments that search for extrasolar planets).