Two-channel polarization interferometric coronagraph to improve the detectability of extrasolar planets

A dual-channel nulling coronagraph to improve the detectability of extrasolar planets is demonstrated. We have been developing a nulling coronagraph with a four-quadrant polarization mask, and confirmed its performance with monochromatic and polychromatic light sources. However, the imperfections of the mask cause the leakage of the starlight, which is obstructive to the detection of faint companions. Here, we propose a two-channel nulling coronagraph, where s- and p-polarized components of the incident light are separated. The light scattered and reflected from the atmosphere of an extrasolar planet is expected to be partially polarized, while the light from the parent star is usually unpolarized. Thus, the differential method, in which subtraction is taken between coronagraphic images of s- and p-polarized lights, is very useful for direct detection of extrasolar planets. In this approach, cancellation of the residual unpolarized starlight is realized when the coronagraphic performance of the two channels is identical. We constructed the two-channel instrument. The experimental results confirm that the two-channel coronagraph can suppress the residual stellar noise, and improve the detectability of faint companions. The effects of the difference between two channels on the detectability are also discussed.