A High Efficiency Coronagraph for Astronomical Applications

Direct detection of extra-solar planetary systems is one of a class of astronomical problems requiring a significant reduction of diffracted light within a few arcseconds of a bright point source. Traditional approaches to diffraction reduction control have included pupil plane apodization, which extracts enormous penalties in effective collecting area to obtain small angle performance, and coronagraphs, in which high efficiency requires extensive occultation of the central source giving up a valuable portion of the field of view. We have developed a hybrid, high efficiency, coronagraph which is optimized for diffraction reduction within a few Airy radii of a bright unresolved source. The coronagraph utilizes a graded, or apodized transmission mask to occult the central source. Theoretical models have shown that the mask renders the Lyot stop more efficient while allowing some transmission very close to the parent star. For targets near the parent star we have found that the coronagraphic efficiency remains virtually independent of radius even when transmission losses through the mask are taken into account. Instead of being used to reduce the wings of a focal plane diffraction pattern, apodization is used in this design to reduce the diffraction wings of the pupil plane image. With a more compact pupil plane image, the application of a Lyot stop becomes much more efficient resulting in greatly improved performance without the loss of a valuable portion of the field of view. Theoretical results will be presented as well as data from a breadboard experiment.