1 Introduction

The amazing growth of exo-planetary detection since the discovery of 51 Peg B (Mayor & Queloz 1995) has triggered considerable efforts for finding new methods to record the direct signal of exo-Earths shrouded by the scattered light of their parent star due to telescope optics (Marcy et al. 2000). Indeed the huge contrast ratio of 10⁹ to 10⁶ (in V and N bands respectively) between a G-star and its orbiting exo-Earth, demands enormous dynamic range imaging possibilites that only nulling techniques, whether interferometric or coronagraphic, could attain. Unlike Lyot's mask, nulling coronagraphs offer both the angular resolution, a few tens of milli-arc-seconds (mas), and the required dynamic range to hunt exo-Earths around a statistically meaningful sample of nearby G-type stars. Pending Bracewell interferometers, e.g. Darwin (Leger et al. 1996) or TPF (Beichman et al. 1999) a large optical telescope, e.g. NGST (Mather 1997), with an embarked coronagraph seems the most likely instrument to collect photons of an exo-planet within 2010-2015 horizon. Following the original concept Gay's Achromatic Interferometric Coronagraph, AIC hereafter (Gay & Rabbia 1996), other designs have been proposed (Roddier & Roddier 1997) and (Rouan et al. 2000) which, unlike AIC, present inherent chromaticity limiting their net nulling efficiency over a wide spectral band.

Following our earlier work on the NGST Exo-planet Finder (Boccaletti et al. 2000), we propose hereafter a coronagraphic design which overcomes the chromatism problem of common coronagraphs, i.e. both the retardation and size of the phase mask. In the next section we outline the general theory of our coronagraph. Section 3 gives a generic optical set-up to overcome the chromatism problem. In Sect. 4 the conceptual design is validated by a number of numerical experiments. Finally we compare the theoretical efficiency of our coronagraph and discuss its limitations and sensitivity to various optical and operational parameters. A mathematical description of the concept is also given in the Appendix.

Figure 1: Pupil intensity with perfect wavefront and its corresponding Airy pattern ( top left and right). Intensity distribution after the Phase Knife Coronagraph has been applied ( bottom-left): the two thin crescents encircle the pupil area perpendicular to the Knife-Edge direction. "Butterfly shape" of the point spread function of a system where half the amplitude is $\pi$-shifted in the image plane ( bottom-right), and where a Lyot stop has been applied in the conjugate pupil plane of (c).