Characterizing edge generated stray light sources for TPF coronagraph pupil masks

The edge generated stray-light from corner boundary conditions, interactions with the lower mask structure, and surface plasmon polaritons that may limit Terrestrial Planet Finder Coronagraph performance are characterized. Previously a number of stray light sources, unaccounted for by the ideal thin mask theory used to design the pupil-plane masks, were identified. ¹ In this paper we illustrate and quantify the most important outstanding stray-light sources in the near-field in order to improve the model of pupil-plane mask transmission used by the Integrated Telescope Model. ² Corner spikes, caused by the need to bring the ideal top-hat field into compliance with the boundary conditions set forth by Maxwell's equations, form the strongest source of stray-light, accounting for up to a 1λ shift in the effective opening width per edge. Undercutting mask edges by 20° reduces this source of stray-light by more than a factor of five. Interactions between light and the lower mask structure, a secondary effect, account for only a few percent of the stray-light in the TE polarization but account for up to 50% of the stray-light in the TM polarization due to surface plasmon polaritons. Surface plasmon polaritons, surface waves that run for tens of microns and radiate at corners, form the final stray-light source. On thin masks they may account for up to a 1A shift in the effective opening width; however, their effects can be easily mitigated by choosing a poor surface plasmon material, such as Chrome. The results presented here are being used to facilitate end-to-end system modeling through the Integrated Telescope Model. ³.