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For future space infrared astronomical coronagraphy, we perform experimentalstudies on the application of aluminum mirrors to a coronagraph. Cooledreflective optics is required for broad-band mid-infrared observations inspace, while high-precision optics is required for coronagraphy. For thecoronagraph instrument originally proposed for the next-generation infraredastronomical satellite project SPICA (SCI: SPICA Coronagraph Instrument), wefabricated and evaluated the optics consisting of high-precision aluminumoff-axis mirrors with diamond-turned surfaces, and conducted a coronagraphicdemonstration experiment using the optics with a coronagraph mask. We firstmeasured the wave front errors (WFEs) of the aluminum mirrors with a He-NeFizeau interferometer to confirm that the power spectral densities of the WFEssatisfy the SCI requirements. Then we integrated the mirrors into an opticalsystem and evaluated the overall performance of the system. As a result, weestimate the total WFE of the optics to be 33 nm (rms), each mirrorcontributing 10-20 nm (rms) for the central 14 mm area of the optics, andobtain a contrast of 10^(-5.4) as a coronagraph in the visible light. At awavelength of 5 um, the coronagraphic system is expected to achieve a contrastof ~10^(-7) based on our model calculation with the measured opticalperformance. Thus our experiment demonstrates that aluminum mirror optics isapplicable to a highly WFE-sensitive instrument such as a coronagraph in space.

Coronagraphic demonstration experiment using aluminum mirrors for space infrared astronomical observations