Study and test of a hybrid coronagraph

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Abstract
We report on variations around the technique of Achromatic Interfero Coronagraph (AIC). The AIC is an imaging device providing nulling of a star without nulling the environment of this star. This instrument already shows its capabilities with observations at Observatoire de Haute Provence and Canada France Hawaii Telescope. The main drawback of the AIC is the two symmetrical images it gives on the detector for a given feature. This characteristic is not annoying for compact features such as faint companions, blobs, ... but can be confusing for diluted features. We propose a method to suppress the symmetry of the output image of the AIC. As the AIC, the hybrid coronagraph we proposed is based on a Michelson interferometer with an image plane in one arm. A mask is placed in this image plane to cover the whole plane except the central part of the Airy disk. The hybrid coronagraph is then very close to the phase mask in the concept but since the interferences of the 2 arms are still destructive, it is less wavelength dependent. The only remaining dependency due to the mask diameter versus the wavelength can be largely decreased by an appropriate width for the mask. We eventually outlined first laboratory results with a hybrid coronagraph prototype.

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