The Four-Quadrant Phase Mask Coronagraph. II. Simulations

In the rst article (Rouan et al. 2000) we have described the principle of a coronagraph utilizing a four-quadrant phase-mask and the results of numerical simulations obtained in the perfect case. In this second article, we have performed additional numerical simulations to assess in more detail the performances and limitations of this coronagraph under real conditions. The eeect of geometrical parameters such as shape and size of both the phase-mask and the Lyot stop is studied. We also analyze the eeect of low and high-order aberrations generated for instance by the atmospheric turbulence. An important issue is the wavelength dependence of the phase-mask. We show that the performance decreases rapidly as the spectral bandwidth is increased and, as a consequence we discuss the manufacturing of achromatized masks using multiple thin lms. An optical concept is proposed.