  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 broad-band mid-infrared observations in
space, while high-precision optics is required for coronagraphy. For the
coronagraph instrument originally proposed for the next-generation infrared
astronomical satellite project SPICA (SCI: SPICA Coronagraph 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^(-5.4) as a coronagraph in the visible light. At a
wavelength of 5 um, the coronagraphic system is expected to achieve a contrast
of ~10^(-7) based 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.
