SPICA infrared coronagraph for the direct observation of exo-planets

Abstract

We present a mid-infrared coronagraph to target the direct observation of extrasolar planets, for Space Infrared telescope for Cosmology and Astrophysics (SPICA). SPICA is a proposed JAXA–ESA mission, which will carry a telescope cooled to 5 K with a 3.5 m diameter aperture, and is planned to be launched in 2018 by an H II family rocket. The SPICA mission gives us a unique opportunity for high-contrast observations because of the large telescope aperture, the simple pupil shape, and the capability for infrared observations from space. We have commenced studies for a coronagraph for SPICA, in which this coronagraph is currently regarded as an option of the focal plane instruments. The primary target of the SPICA coronagraph is the direct observation of Jovian exo-planets. A strategy of the baseline survey and the specifications for the coronagraph instrument for the survey are introduced together. The main wavelengths and the contrast required for the observations are 3.5–27 μm, and 10 −6 , respectively. Laboratory experiments were performed with a visible laser to demonstrate the principles of the coronagraphs. In an experiment using binary-shaped pupil coronagraphs, a contrast of 6.7 × 10 −8 was achieved, as derived from the linear average in the dark region and the core of the point spread function (PSF). A coronagraph by a binary-shaped pupil mask is a baseline solution for SPICA because of its feasibility and robustness. On the other hand, a laboratory experiment of the phase induced amplitude apodization/binary-mask hybrid coronagraph has been executed to obtain an option of higher performance (i.e., smaller inner working angle and higher throughput), and a contrast of 6.5 × 10 −7 was achieved with active wavefront control. Potentially important by-product of the instrument, transit monitoring for characterization of exo-planets, is also described. We also present recent progress of technology on a design of a binary-shaped pupil mask for the actual pupil of SPICA, PSF subtraction, the development of free-standing binary masks, a vacuum chamber, and a cryogenic deformable mirror. Considering SPICA to be an essential platform for coronagraphs and the progress of key technologies, we propose to develop a mid-infrared coronagraph instrument for SPICA and to perform the direct observation of exo-planets with it.