Shown below is the optical design of the OSCA coronographic unit. The system consists of the following. The converging beam from the NAOMI port is interrupted by a steering mirror, which diverts the focus across the optical table. At the focus is a focal plane stop assembly where a selection of focal plane stops on 3mm thick transmissive substrates can be inserted into the focal plane. The beam is then collimated by an off axis parabolic mirror and in the collimated beam at a pupil conjugate a Lyot stop is placed that can be rotated to match the secondary strut position in the pupil image. A second paraboloid reconverges the beam and this is relayed to the original NAOMI focus via a dihedral mirror pair.

The substrate on which the focal plane stop is mounted is a 3mm thick optical wedge with a wedge angle of ~1 degree to remove ghosting effects.

Figure 1. Zemax optical design of the OSCA system

The spot diagrams are shown below for various wavelengths and at three different fields positions corresponding to on-axis, and 11 arcseconds off-axis. Also shown is the size of the diffraction pattern at 0.6 microns.

Figure 2 OSCA spot diagrams