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
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