Development of a high-contrast imaging system, especially toward direct detection of habitable Earth-like exoplanets, would be one of the most challenging themes in modern astronomy. For direct detection of exo-Earth, coronagraphic devices are required for suppressing bright diffracted light from a parent star. In addition, residual stellar speckles, caused by imperfection of optical components, have to be also rejected by wavefront control such as the speckle nulling technique. It is important to construct a dedicated testbed at which we can comprehensively develop the high-contrast imaging system for future era of space coronagraphs aimed at searching for exo-Earths. Recently, we have started construction of a new testbed in Japan which is called EXIST (Exoplanet Imaging System Testbed). The EXIST is planned to be compatible with various types of coronagraphs, such as phase-mask coronagraphs based on the photoniccrystal technology, common-path visible nulling coronagraph, and so on. In addition, optimally designed pupil apodizers will be installed into the testbed for maximizing the performance of the phase-mask coronagraphs with arbitrary aperture telescopes. We plan to utilize a spatial light modulator (SLM) for conducting the speckle nulling control. Thanks to a large pixel format of the SLM, we expect that a huge dark hole can be created against the residual speckles. Here, we report our recent progress on the construction of the new testbed and results of some preparatory experiments related to the coronagraphs and the speckle nulling control using the SLM.
Subaru telescope has been operating a high-contrast imaging instruments called Subaru coronagraphic extreme adaptive optics (SCExAO) which is used for exoplanet research. We are developing phase mask coronagraphs using photonic crystal wave plates inside the SCExAO. An eight-octant phase mask (8OPM) of three-layer achromatic structure has been fabricated as a second generation. It was designed for J and H band to reach 10-5 contrast, and Ks band to 10-4. A retardation and a coronagraphic performance of the 8OPM were confirmed almost as designed at 1550nm. An apodised (binary shaped) pupil to be used with the 8OPM was also studied to suppress diffracted light by the secondary shadow and spiders. We confirmed a performance of the combination of the apodizer and the 8OPM at visible wavelengths in a lab. We optimized the apodizer for a pupil of the SCExAO where we obtained a transmission of 50 % and a contrast of 10-4 the center and 10-6 at outer region. We manufactured the designed apodizer to be installed in SCExAO for infrared observations.
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