The achromatic phase knife coronagraph

We present the concept of a nulling coronagraph which theoretically attains the brightness ratio of 108suitable to directly detect exo-Earths in the vicinity of their parent stars. The concept is derived from the classical Foucault's optics test and Zernike's self-destructive phase contrast principle. We propose an achromatic set-up which should dramatically improve its wide band efficiency. The whole system is simulated against a numerical model which validates a formal analytical approach. The coronagraph is then compared to other concepts and its advantages and shortcomings are outlined, suggesting future laboratory developments and tests on the sky.