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Very high dynamical range coronagraphs targeting direct exo-planet detection(10^9 - 10^10 contrast) at small angular separation (few lambda/D units)usually require an input wavefront quality on the order of ten thousandths ofwavelength RMS. We propose a novel method based on a pre-optics setup thatbehaves partly as a low-efficiency coronagraph, and partly as ahigh-sensitivity wavefront aberration compensator (phase and amplitude). Thecombination of the two effects results in a highly accurate correctedwavefront. First, an (intensity-) unbalanced nulling interferometer (UNI)performs a rejection of part of the wavefront electric field. Then therecombined output wavefront has its input aberrations magnified. Because of theunbalanced recombination scheme, aberrations can be free of phase singularpoints (zeros) and can therefore be compensated by a downstream phase andamplitude correction (PAC) adaptive optics system, using two deformablemirrors. In the image plane, the central star's peak intensity and the noiselevel of its speckled halo are reduced by the UNI-PAC combination: theoutput-corrected wavefront aberrations can be interpreted as an improvedcompensation of the initial (eventually already corrected) incident wavefrontaberrations. The important conclusion is that not all the elements in theoptical setup using UNI-PAC need to reach the lambda/10000 rms surface errorquality.

Precise Wavefront Correction with an Unbalanced Nulling Interferometer for Exo-Planet Imaging Coronagraphs