Multi-Color Coronagraph Experiment in a Vacuum Testbed with a Binary Shaped Pupil Mask

Abstract

We conducted a number of multi-color/broadband 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 of a stellar coronagraph to observe extra-solar planets (exoplanets) directly. The aim of this work is to demonstrate that subtraction of the Point Spread Function (PSF) and multi-color/broadband experiments using a binary-shaped pupil mask coronagraph would help to improve the contrast in observations 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, while controlling the temperature of the optical bench, and covering the vacuum chamber with thermal insulation layers. Active wavefront control was not applied in this work. We evaluated how much the PSF subtraction contributed to the high-contrast observation by subtracting images obtained through the coronagraph. We also carried out multi-color/broadband experiments in order to demonstrate a more realistic observation using Super luminescent Light Emitting Diodes (SLEDs) with center wavelengths of 650 nm, 750 nm, 800 nm, and 850 nm. A contrast of 2.3 $\times$ 10$^{-7}$ was obtained for the raw coronagraphic image and a contrast of 1.3 $\times$ 10$^{-9}$ was achieved after PSF subtraction with a He-Ne laser at 632.8 nm wavelength. Thus, the contrast was improved by around two orders of magnitude from the raw contrast by subtracting the PSF. We achieved contrasts of 3.1 $\times$ 10$^{-7}$, 1.1 $\times$ 10$^{-6}$, 1.6 $\times$ 10$^{-6}$, and 2.5 $\times$ 10$^{-6}$ at the bands of 650 nm, 750 nm, 800 nm, and 850 nm, respectively, in multi-color/broadband experiments. The results show that the contrast within each of the wavelength bands was significantly improved compared with the non-coronagraphic optics. We demonstrated that PSF subtraction is potentially beneficial for improving the contrast of a binary-shaped pupil mask coronagraph; this coronagraph produces a significant improvement in contrast with multi-color/broadband light sources.