  This paper presents the basic principle and theoretical relationships of an
original method allowing to retrieve the Wavefront Errors (WFE) of a ground or
space-borne telescope when combining its main pupil with a second, decentered
reference optical arm. The measurement accuracy of such a
"telescope-interferometer" is then estimated by means of various numerical
simulations, demonstrating a high performance excepted on limited areas near
the telescope pupil rim. In particular, it allows direct phase evaluation (thus
avoiding the use of first or second-order derivatives), which is of special
interest for the co-phasing of segmented mirrors in future giant telescopes
projects. We finally define the useful practical domain of the method, which
seems to be better suited for periodical diagnostics of space or ground based
telescopes, or to real-time scientific observations in some very specific cases
(e.g. the central star in extrasolar planets searching instruments).
