  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 of
wavelength RMS. We propose a novel method based on a pre-optics setup that
behaves partly as a low-efficiency coronagraph, and partly as a
high-sensitivity wavefront aberration compensator (phase and amplitude). The
combination of the two effects results in a highly accurate corrected
wavefront. First, an (intensity-) unbalanced nulling interferometer (UNI)
performs a rejection of part of the wavefront electric field. Then the
recombined output wavefront has its input aberrations magnified. Because of the
unbalanced recombination scheme, aberrations can be free of phase singular
points (zeros) and can therefore be compensated by a downstream phase and
amplitude correction (PAC) adaptive optics system, using two deformable
mirrors. In the image plane, the central star's peak intensity and the noise
level of its speckled halo are reduced by the UNI-PAC combination: the
output-corrected wavefront aberrations can be interpreted as an improved
compensation of the initial (eventually already corrected) incident wavefront
aberrations. The important conclusion is that not all the elements in the
optical setup using UNI-PAC need to reach the lambda/10000 rms surface error
quality.
