Coronagraph

The diffraction rings are removed by a Lyot coronagraph ([Lyot 1930], Fig. 1). The occulting masks in the f/122 beam are inter-changeable, with sizes correponding to $0.22\arcsec$, $0.45\arcsec$ and $0.67\arcsec$. They have been vaccum evaporated through pinholes onto a 1.2 mm thick plate of anti-reflection coated glass. The apodizing masks located at the relayed pupil are pinholes carried by a x-y translation stage. No attempt was made to mask the aperture spider and central obscuration although this should decrease the scattered light significantly.

  

Figure: Wynne corrector for making the speckle pattern wavelength-invariant. A pair of triplets, afocal in yellow light, shrinks the blue pupil while enlarging the red pupil. The exit angle, u', becomes approximately proportional to wavelength, thus ensuring a nearly wavelength-invariant diffraction pattern.