12 July 2008 Laboratory experiments on the 8-octant phase-mask coronagraph
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A four-quadrant phase-mask (FQPM) coronagraph can suppress perfectly stellar light when a star can be regarded as a point-like source. However, the FQPM coronagraph is highly sensitive to partially resolved stars, and shows second-order sensitivity to tip-tilt error leakage. Higher-order sensitivity is required for extremely high-contrast imaging of nearby stars. We propose an eight-octant phase-mask (EOPM) for achieving fourth-order sensitivity to tip-tilt errors. We manufactured the phase-mask utilizing a nematic liquid crystal (LC) device, which is composed of eight segments. A phase retardation of the LC can be adjustable by an applied voltage to the device. The LC phase-mask can be switched between FQPM-mode and EOPM-mode by applying appropriate voltages to the segments. We carry out experiments on the phase-mask coronagraph with various tip-tilt errors. The experimental results show the higher-order behavior of the EOPM compared to the FQPM. We present a current status of the laboratory experiments on the EOPM coronagraph, and also show coronagraphic performance of the EOPM derived from numerical simulations.
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Naoshi Murakami, Ryosuke Uemura, Naoshi Baba, Hiroshi Shibuya, Jun Nishikawa, Lyu Abe, Motohide Tamura, Nobuyuki Hashimoto, "Laboratory experiments on the 8-octant phase-mask coronagraph", Proc. SPIE 7010, Space Telescopes and Instrumentation 2008: Optical, Infrared, and Millimeter, 70101J (12 July 2008); doi: 10.1117/12.788145; https://doi.org/10.1117/12.788145

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