Stray light in PICARD SODISM instrument: design, check, flight results, and alignment issues

Description

The PICARD satellite is dedicated to the monitoring of solar activity. It carries several imaging and radiometric instruments. One of them, SODISM, is a high-resolution radio-imaging telescope measuring the Sun diameter and total flux in near UV and visible wavelengths. Along with mirrors, SODISM includes highly reflective filters and attenuators, which generate ghost images. These disturb the Sun edge area, the total flux measurement and also the fine aiming channel. This is compounded with tilt tolerances, which shift and modify the ghosts images. Stray light was studied through ASAP simulation, with broad sources and high order splits. Each path was studied separately, checking its effect on instrument performance and the possible effect of tilts. Some design improvements allowed to reduce the most critical paths, while others, although relatively intense, stood clear from the critical areas. However ground tests and flight results show some residual ghosts, which could not be fully suppressed due to mechanical tolerances. They shall be taken into account by image processing.

Abstract

International audience

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