Cascade six-level phase-mask achromatic coronagraph.

A cascade six-level phase-mask achromatic coronagraph is proposed. We show that, for the ideal aberration-free case, the average suppression of the central starlight in a three-stage six-level phase-mask achromatic coronagraph (SLPMC) can reach about 10⁻8 over a 40% bandwidth. We also do the optimization analysis of the working wavelength of each phase mask in the three-stage SLPMC. With optimization, the starlight can be stably suppressed to less than 10⁻8 (average about 10⁻¹°) over the whole 40% bandwidth.