Central-obscuration removal plates for focal-plane phase-mask coronagraphs with a centrally-obscured telescope

Focal-plane phase-mask coronagraphs, such as eight-octant phase-mask coronagraphs (8OPM), are one of the most promising tools for high contrast observations. However, coronagraphic performance would be degraded when combined with a centrally-obscured telescope. We propose pupil-remapping optics for removing the shade of a secondary mirror to generate a clear, circular pupil for the phase-mask coronagraph. First, we show the design of the pupil-remapping optics, called central-obscuration removal plates (CRPs). Next, we report laboratory experiments on the 8OPM coronagraph using manufactured CRPs. We also evaluate off-axis point-spread functions via both laboratory experiments and numerical simulations. Finally, we evaluate, via numerical simulations, limiting factors for coronagraphic performance, such as phase aberrations introduced by the CRPs, the effect of Fresnel diffraction, and chromatic behavior. The numerical simulations suggest that the phase aberrations could be a dominant limiting factor of the achievable contrast in the current laboratory experiments.