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Context.A high contrast coronagraph is expected to provide one of the promising ways to directly observe extra-solar planets. We present the newest results of our laboratory experiment investigating “rigid” coronagraph with a binary shaped checkerboard pupil mask, which should offer a highly stable solution for telescopes without adaptive optics (AO) for wavefront correction in space missions.

Context.We present the results of the first laboratory experiment of checkerboard shaped pupil binary mask coronagraphs using visible light, in the context of the R&D activities for future mid-infrared space missions such as the 3.5 m SPICA telescope.

For future space infrared astronomical coronagraphy, we perform experimental studies on the application of aluminum mirrors to a coronagraph. Cooled reflective optics is required for broadband mid-infrared observations in space, while high-precision optics are required for coronagraphy. For the coronagraph instrument originally proposed for the next-generation infrared astronomical satellite project SPICA(SCI: SPICACoronagraph Instrument), we fabricated and evaluated the optics consisting of high-precision aluminum off-axis mirrors with diamond-turned surfaces, and conducted a coronagraphic demonstration experiment using the optics with a coronagraph mask. We first measured the wave front errors (WFEs) of the aluminum mirrors with a He-Ne Fizeau interferometer to confirm that the power spectral densities of the WFEs satisfy the SCI requirements. Then we integrated the mirrors into an optical system and evaluated the overall performance of the system. As a result, we estimate the total WFE of the optics to be 33 nm (rms), each mirror contributing 10-20 nm (rms) for the central 14 mm area of the optics, and obtain a contrast of 10-5.4as a coronagraph in the visible light. At a wavelength of 5 ?m, the coronagraphic system is expected to achieve a contrast of ?10-7based on our model calculation with the measured optical performance. Thus our experiment demonstrates that aluminum mirror optics is applicable to a highly WFE-sensitive instrument such as a coronagraph in space.

This paper, first, presents introductory reviews of the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) mission and the SPICA Coronagraph Instrument (SCI). SPICA will realize a 3m class telescope cooled to 6K in orbit. The launch of SPICA is planned to take place in FY2018. The SPICA mission provides us with a unique opportunity to make high dynamic-range observations because of its large telescope aperture, high stability, and the capability for making infrared observations from deep space. The SCI is a high dynamic-range instrument proposed for SPICA. The primary objectives for the SCI are the direct coronagraphic detection and spectroscopy of Jovian exoplanets in the infrared region, while the monitoring of transiting planets is another important target owing to the non-coronagraphic mode of the SCI. Then, recent technical progress and ideas in conceptual studies are presented, which can potentially enhance the performance of the instrument: the designs of an integral 1-dimensional binary-shaped pupil mask coronagraph with general darkness constraints, a concentric ring mask considering the obscured pupil for surveying a wide field, and a spectral disperser for simultaneous wide wavelength coverage, and the first results of tests of the toughness of MEMS deformable mirrors for the rocket launch are introduced, together with a description of a passive wavefront correction mirror using no actuator.

We present our first results on the development and evaluation of a cryogenic deformable mirror (DM) based on Micro Electro Mechanical Systems (MEMS) technology. A MEMS silicon-based DM chip with 32 channels, in which each channel is 300 ?m × 300 ?m in size, was mounted on a silicon substrate in order to minimize distortion and prevent it from being permanently damaged by thermal stresses introduced by cooling. The silicon substrate was oxidized to obtain electric insulation and had a metal fan-out pattern on the surface. For cryogenic tests, we constructed a measurement system consisting of a Fizeau interferometer, a cryostat cooled by liquid N2, zooming optics, electric drivers. The surface of the mirror at 95 K deformed in response to the application of a voltage, and no significant difference was found between the deformation at 95 K and that at room temperature. The power dissipation by the cryogenic DM was also measured, and we suggest that this is small enough for it to be used in a space cryogenic telescope. The properties of the DM remained unchanged after five cycles of vacuum pumping, cooling, warming, and venting. We conclude that fabricating cryogenic DMs employing MEMS technology is a promising approach. Therefore, we intend to develop a more sophisticated device for actual use, and to look for potential applications including the Space Infrared Telescope for Cosmology & Astrophysics (SPICA), and other missions.

We present the results of high-precision measurements of the thermal expansion of sintered SiC, SiC-100, intended for use in cryogenic space telescopes, in which minimization of thermal deformation of the mirror is critical, and precise information of the thermal expansion is needed for the telescope design. The temperature range of the measurements extends from room temperature down to ?10 K. Three samples, Nos. 1, 2, and 3, were manufactured from blocks of SiC produced in different lots. The thermal expansion of the samples was measured with a cryogenic dilatometer, consisting of a laser interferometer, a cryostat, and a mechanical cooler. The typical thermal expansion curve is presented using an eighth-order polynomial of the temperature. For the three samples, the coefficients of thermal expansion (CTE), ? 1, ? 2, and ? 3, were derived for temperatures between 293 and 10 K. The average and the dispersion (1 ? rms) of these three CTEs are 0.816 × 10?6and 0.002 × 10?6K?1, respectively. No significant difference was detected in the CTE of the three samples from the different lots. Neither inhomogeneity nor anisotropy of the CTE was observed. Based on the CTE dispersion obtained, we performed a finite-element method (FEM) analysis of the thermal deformation of a 3.5 m diameter cryogenic mirror made of six SiC-100 segments. It was shown that the present CTE measurement has an accuracy that is sufficient for the design of the 3.5 m cryogenic infrared telescope mission SPICA(Space Infrared Telescope for Cosmology and Astrophysics).

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