Herve Le Coroller

Extrasolar planet imaging

The coronagraphic techniques serving to reject most light from a star, when trying to image a nearby planet, can be pushed with an adaptive holographic element. Located after the coronagraph, it can in principle remove most of the residual star light by adding a phase-shifted holographic reconstruction of it . The scheme is also usable within each sub-aperture of a diluted hypertelescope array, sufficiently large to resolve details of an exo-Earth. A possible panoramic version of the previously mentioned Exo-Earth Imager is shaped as a virtual bubble of 400 km diameter , consisting of thousands of 3-meter mirrors, free-flying and arranged co-spherically. The half-size focal sphere is explored by beam combiners, one for each exo-Earth observed within tens of parsecs. Each beam-combiner includes a kilometer-sized corrector of spherical aberration at F/2, which is also diluted and consisting of small free-flyers. The instrument is expected to provide direct coronagraphic images of exo-Earths, resolved in 50x50 resels, with enough dynamic range obtained in 30mn exposures to search colored features and their seasonal variations, indicative of photosynthetic life .

Simulations of coronagraphy with a dynamic hologram for the direct detection of exo-planets

In a previous paper,1 we discussed an original solution to improve the performances of coronagraphs by adding, in the optical scheme, an adaptive hologram removing most of the residual speckle starlight. In our simulations, the detection limit in the flux ratio between a host star and a very near planet (5λ/D) improves over a factor 1000 (resp. 10000) when equipped with a hologram for cases of wavefront bumpiness imperfections of λ/20 (resp. λ/100). We derive, in this paper, the transmission accuracy required on the hologram pixels to achieve such goals. We show that preliminary tests could be performed on the basis of existing technologies.