We report laboratory demonstrations of an eight-octant phase-mask (EOPM) coronagraph for direct detection of exoplanets. The EOPM coronagraph is a family of a four-quadrant phase-mask (FQPM) one, and shows better coronagraphic performance for partially resolved stars. We manufactured an eight-octant ferroelectric liquid-crystal (FLC) mask. The FLC mask is composed of eight-segmented half-wave plates whose principal axes are different between adjacent segments. The mask operates as a fully achromatic EOPM when the FLC mask is placed between crossed polarizers. We carried out laboratory experiments on the EOPM coronagraph by using partially resolved whitelight source, and compared the performance with that of the FQPM one. As a result, we confirmed that the EOPM shows higher contrast than the FQPM. A drawback of the proposed method is that the FLC mask can be used only for one component of polarization of incoming light because it is necessary to use the polarizer in front of the FLC mask. To solve this problem, a two-channel coronagraph, based on two polarizing beam splitters instead of the polarizers, is proposed. Observational efficiency can significantly be improved because the two-channel coronagraph enables us to detect both components of polarizations from exoplanets. We also report preliminary experimental results of laboratory demonstrations of the two-channel coronagraph.

Date Published: 10 August 2010
PDF: 6 pages
Proc. SPIE 7731, Space Telescopes and Instrumentation 2010: Optical, Infrared, and Millimeter Wave, 77314E (10 August 2010); doi: 10.1117/12.856395

Published in SPIE Proceedings Vol. 7731:
Space Telescopes and Instrumentation 2010: Optical, Infrared, and Millimeter Wave
Jacobus M. Oschmann Jr.; Mark C. Clampin; Howard A. MacEwen, Editor(s)