A Host-star Calibration Based Polarimeter for Earth-like Exoplanet Imaging

We propose a polarimeter, which is dedicated to Earth-like exoplanet imaging for future space missions. We adopt a minimum-polarization-component design philosophy, which makes a compact and robust system as well as high-performance achievable in the real world. Our polarimeter consists of two polarization components of a liquid crystal variable retarders (LCVR) and a Wollaston prism. The polarimeter can deliver an extra contrast better than 10−4.5. Combined with one of the currently available coronagraphs that are delivering a contrast on the order of 10−6.5, the coronagraph and polarimeter system can deliver a contrast better than 10−11 at a small inner working angle in the visible over the entire imaging plane. We discuss the polarimeter design concept and dedicated data-reduction technique. Our unique host-star calibration algorithm allows the starlight to be totally removed, regardless of whether the host-star image is intrinsically polarized or whether the light is polarized by preoptics, such as the telescope that is located before the polarimeter, which makes exoplanet polarization imaging feasible with any telescope, optical system, and target star. Using minimum-polarization components with a solid-state image LCVR as the key polarization component, our polarimeter is less sensitive to the wavefront phase and amplitude errors than other exoplanet imaging techniques. Based on commercial-grade optical components, we demonstrated for the first time that by combining our polarimeter with a currently available coronagraph, the polarimeter and coronagraph system can deliver a contrast better than 10−11 at a small inner working angle in the visible wavelengths, which paves the way for Earth-like exoplanet imaging for a future space mission.