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2 Principle of AIC

The optical sketch of Achromatic Interfero Coronagraph (AIC) is shown in Fig. 1. This coronagraph, described with more details in Paper I (Baudoz et al. 1999), is a modified Michelson interferometer. The main modification is the addition of a focus in only one arm of the interferometer. The focus is inserted so as to achieve a rotation of the pupil by 180 degres and so as to dephase the light wave by 180 degrees whatever the wavelength (achromatic dephasing). With such a set-up and when the Optical Path Difference (OPD) is kept at zero, the light from an unresolved on-axis source is utterly and achromatically nulled (Paper I). Any off-axis source escapes the interference process and gives two twin-images, symmetrically displayed with respect to the axis. The actual set-up of work is schematically described in Fig. 1. It performs the same nulling process than the basic device but rather looks like a double pass Michelson Fourier Interferometer (Connes & Michel 1975). Splitting of the incoming parallel beam and recombination of the separate beams are achieved by two separate halves of a unique coated plate, which are placed in appropriate position so as to avoid the use of a compensation plate. This configuration provides two output beams, one is the "destructive port'' for imaging in coronographic mode, the other is the "constructive port'' in which are found the photons removed from the coronagraphed output. The constructive port is used to control OPD by a servo-loop (Sect. 3.1).


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