To observe magnetic fields in the solar corona, very low levels of instrumental background are required. The primary contributor to instrumental background light in an internally-occulted coronagraph is light scattered from the primary lens or mirror. This is caused by surface roughness, particul... Show moreTo observe magnetic fields in the solar corona, very low levels of instrumental background are required. The primary contributor to instrumental background light in an internally-occulted coronagraph is light scattered from the primary lens or mirror. This is caused by surface roughness, particulate contamination, and (in the case of refractors)imperfections of the glass. To obtain good coronal data, the scattered light intensity in the coronal image should be less than or on the order of the coronal brightness (a few ppm of the disk center). We examine the sources of scattering and estimate the level of scattering from each for both reflectors and refractors. We conclude that refractors have a significant advantage over reflectors for the dominant sources of scattered light. Show less
