  We conducted a number of multi-band coronagraph experiments using a vacuum
chamber and a binary-shaped pupil mask which in principle should work at all
wavelengths, in the context of the research and development on a coronagraph to
observe extra-solar planets (exoplanets) directly. The aim of this work is to
demonstrate that subtraction of Point Spread Function (PSF) and multi-band
experiments using a binary-shaped pupil mask coronagraph would help improve the
contrast in the observation of exoplanets. A checkerboard mask, a kind of
binary-shaped pupil mask, was used. We improved the temperature stability by
installing the coronagraph optics in a vacuum chamber, controlling the
temperature of the optical bench, and covering the vacuum chamber with thermal
insulation layers. We evaluated how much the PSF subtraction contributes to the
high contrast observation by subtracting the images obtained through the
coronagraph. We also carried out multi- band experiments in order to
demonstrate a more realistic observation using Super luminescent Light Emitting
Diodes (SLEDs) with center wavelengths of 650nm, 750nm, 800nm and 850nm. A
contrast of 2.3x10-7 was obtained for the raw coronagraphic image and a
contrast of 1.3x10-9 was achieved after PSF subtraction with a He-Ne laser at
632.8nm wavelength. Thus, the contrast was improved by around two orders of
magnitude from the raw contrast by subtracting the PSF. We achieved contrasts
of 3.1x10-7, 1.1x10-6, 1.6x10-6 and 2.5x10-6 at the bands of 650nm, 750nm,
800nm and 850nm, respectively, in the multi-band experiments. The results show
that contrast within each of the wavelength bands was significantly improved
compared with non-coronagraphic optics. We demonstrated PSF subtraction is
potentially beneficial for improving contrast of the coronagraph, and this
coronagraph produces a significant improvement in contrast with multi-band
light sources.
