A New Method for Monitoring Scattered Stray Light of an Inner-occulted Coronagraph

Abstract

The scattered stray light of coronagraph is a type of stray light that is generated by the objective lens as its surface defects irradiated by sunlight. The defects mainly include dust, blemishes on the lens surface, microroughness of the lens surface, and impurity and inhomogeneity of the glass. Unlike the other types of relatively-stable defects that occur after the objective lens is manufactured, the scattered stray light caused by dusts on the lens surface is difficult to quantify accurately due to the disorder and randomness of the dust accumulation. The contribution of this type of stray light to the overall stray light level is difficult to determine through any simulations or examinations. This can result in a continuous deterioration of the stray light level of a coronagraph and thus affect the observing capabilities of the instrument. To solve this issue, through analyzing the mechanism of scattered stray light and ghost image generated by the inner-occulting coronagraph, we propose a novel method to monitor the scattered stray light of dusts by utilizing different stray light correlation coefficients. In this method, we first simulate and measure the level of stray light from the ghost image of the objective lens, and then determine the flux ratio of scattered light and ghost image on the conjugate plane. Although the flux ratio varies with the accumulation of dusts on the lens surface, it keeps constant on the image plane. Therefore, the level of dust scattering light on the image plane can be obtained by using this ratio together with the level of ghost image stray light. We also validate the accuracy of this method in laboratory by applying objective lens with numerous surface cleanliness levels.