Achromatic multi-four quadrant phase mask coronagraph: laboratory demonstration


Raphaël Galicher, Pierre Baudoz, Jacques Baudrand, François Assémat, Anthony Boccaletti


Paris Observatory


Direct imaging provides unique informations on both orbit and atmosphere of exoplanets orbiting in the outer part of their stellar systems. Orbital parameters of such objects are expected to put strong constraints on the models of planetary formation. Spectroscopy and polarimetry will study the physical characteristics of planetary atmospheres. However, the two main limitations of direct imaging appear when pursuing such characterizations: a very small angular separation (fractions of arcsec) and a very large flux ratio (10 thousands to 10 billion) between the star and its planet. The first problem can be tackled using an extreme adaptive optics or going to space. To overcome the high contrast limitation, coronagraphs are proposed to reduce the stellar flux without affecting the planetary flux. Several instrumental concepts have already been proposed using amplitude or phase focal masks or pupil apodization. Their performances are always quite sensitive to chromatism except for a very few cases and one challenge is the designing of an achromatic coronagraph. Our team have worked on the multi-four quadrant phase mask (MFQPM) coronagraph for a few years. The device uses several monochromatic four quadrant phase mask (FQPM) coronagraphs put in cascade so that the star flux is reduced over a large spectral band. As the FQPM technology is well under control, MFQPM is easy to built. In this paper, we recall the principle of the technique. We then present experimental results with a detection of a laboratory Neptune-like planet in a visible wide bandpass (about 500 to 700 nm). We finally compare the experimental results to numerical predictions.

Attached documents

Lyot2010proc s8 talk GalicherR.pdf
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