Achromatic interfere coronagraphy - I. Theoretical capabilities for ground-based observations

We present a concept dedicated to the detection of faint companions or other morphological components in the neighbouring of a star by means of coronagraphy. Basically this method relies on destructive interferences so as to achieve extinction of the on-axis point-like source. In this :regard the reported concept is a nulling interferometer designed for work with a single aperture. Two features are specific to our coronagraph, namely achromaticity and close-sensing, Achromaticity allows flexible choice of a working wavelength and of a large spectral bandwidth. Close-sensing provides the ability to explore around the central source significantly closer than can be achieved by existing coronographs. Though our concept has been initially devised for use on a space-based instrument it is the goal of this paper to show bs theoretical analysis that it can be efficient and powerful on ground-based large telescopes equipped with adaptive optics. This especially regards close-sensing capabilities at a level (better than diffraction limit) unreachable by other coronagraphs. The essential limitation regarding detection originates in residual wavefront distortions whose effects are described here using the formalism of adaptive optics. In this paper the principle, the generic set-up and the limitations are briefly recalled. Algebraic derivations regarding the effect of atmospheric seeing and the use of adaptive optics are given. Theoretical expected detection capabilities for ground-based operation are derived, showing that detection of a companion fainter than the central star by 6 magnitudes with a Signal to Noise Ratio (SNR) of 5 appears to be a reasonable goal from raw data, with a 4-meter class telescope in rather ordinary conditions and with modest integration times. On-going progress in adaptive optics, make it conceivable to reach magnitude differences of 12 under similar conditions.