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

We present a concept dedicated to the detection of faint companions or othe r morphological components in the neighbouring of a star by means of corona graphy. Basically this method relies on destructive interferences so as to achieve extinction of the on-axis point-like source. In this :regard the re ported concept is a nulling interferometer designed for work with a single aperture. Two features are specific to our coronagraph, namely achromaticity and clos e-sensing, Achromaticity allows flexible choice of a working wavelength and of a large spectral bandwidth. Close-sensing provides the ability to explo re around the central source significantly closer than can be achieved by e xisting coronographs. Though our concept has been initially devised for use on a space-based inst rument 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 wi th adaptive optics. This especially regards close-sensing capabilities at a level (better than diffraction limit) unreachable by other coronagraphs. T he essential limitation regarding detection originates in residual wavefron t distortions whose effects are described here using the formalism of adapt ive optics. In this paper the principle, the generic set-up and the limitations are bri efly recalled. Algebraic derivations regarding the effect of atmospheric se eing and the use of adaptive optics are given. Theoretical expected detecti on capabilities for ground-based operation are derived, showing that detect ion of a companion fainter than the central star by 6 magnitudes with a Sig nal 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 mod est integration times. On-going progress in adaptive optics, make it concei vable to reach magnitude differences of 12 under similar conditions.