  Non Common Path Aberrations (NCPA) are often considered as a critical issue
in Adaptive Optics (AO) systems, since they introduce bias errors between real
wavefronts propagating to the science detectors and those measured by the
Wavefront Sensor (WFS). This is especially true when the AO system is coupled
to a coronagraph instrument intended for the discovery and characterization of
extra-solar planets, because useful planet signals could be mistaken with
residual speckles generated by NCPA. Therefore, compensating for those errors
is of prime importance and is already the scope of a few theoretical studies
and experimental validations on-sky. This communication presents the conceptual
optical design of a pseudo-interferometer arrangement suitable to accurate NCPA
calibration, based on two WFS cooperating in real-time. The concept is
applicable to both classical imaging and spectroscopy assisted by AO, and to
high-contrast coronagraphs searching for habitable extra-solar planets.
Practical aspects are discussed, such as the choice of WFS and coronagraph
types, or specific requirements on additional hardware components, e.g.
dichroic beamsplitters
