The four-quadrant phase-mask coronagraph. I. Principle

We describe a new type of coronagraph, based on the principle of a phase ma sk as proposed by Roddier and Roddier a few years ago but using an original mask design found by one of us (D. R.), a four-quadrant binary phase mask (0, pi) covering the full field of view at the focal plane. The mutually de structive interferences of the coherent light from the main source produce a very efficient nulling. The computed rejection rate of this coronagraph a ppears to be very high since, when perfectly aligned and phase-error free, it could in principle reduce the total amount of light from the bright sour ce by a factor of 10(8), corresponding to a gain of 20 mag in brightness at the location of the first Airy ring, relative to the Airy peak. In the rea l world the gain is of course reduced by a strong factor, but nulling is st ill performing quite well, provided that the perturbation of the phase, for instance, due to the Earth's atmosphere, is efficiently corrected by adapt ive optics. We show from simulations that a detection at a contrast of 10 m ag between a star and a faint companion is achievable in excellent conditio ns, while 8 mag appears routinely feasible. This coronagraph appears less s ensitive to atmospheric turbulence and has a larger dynamic range than othe r recently proposed nulling techniques : the phase-mask coronagraph (by Rod dier and Roddier) or the Achromatic Interfero-Coronagraph (by Gay and Rabbi a). We present the principle of the four-quadrant coronagraph and results o f a first series of simulations. We compare those results with theoretical performances of other devices. We briefly analyze the different limitations in space or ground-based observations, as well as the issue of manufacturi ng the device. We also discuss several ways to improve the detection of a f aint companion around a bright object. We conclude that, with respect to pr evious techniques, an instrument equipped with this coronagraph should have better performance and even enable the imaging of extrasolar giant planets at a young stage, when coupled with additional cleaning techniques.