The role of narrow-angle forward surface scatter and particulate scatter in exoplanet exploration

It is often thought that because of the very small solid angle subtended by the field-of-view of the coronagraph, scattered light from optical surfaces will have no effect on images recorded for terrestrial exoplanet spectroscopy. In this paper, we examine mirror surface scatter and particulate contamination scatter as sources of background light or noise signal in large aperture terrestrial exoplanet telescope/coronagraph systems. Scattered light control to one part in 10+10 or better is required for exoplanet exploration. We will discuss the optical fabrication tolerances necessary to minimize narrowangle forward scatter and their relative effects upon direct imaging coronagraph instruments used to characterize terrestrial exoplanets.