Design and Stray Light Suppression in Spectral Imaging Coronagraph (SICG), Meridian Project Phase II

The Spectral Imaging Coronagraph (SICG), deployed at Lijiang Observatory (3200M), is China’s first ground-based coronagraph and is part of the Meridian Project Phase II. It is an internal occulted coronagraph, and this paper describes its design and stray light suppression details. The SICG has a 200mm aperture, an F-number of 6.7, and dual-channel observation capabilities at 530.3 nm (Fe XIV) and 637.4 nm (Fe X), with a Field of View(FOV) ranging from 1.05 to 2R☉ (R☉ represents the solar radius). Detailed stray light suppression methods are presented, including the use of ultra-smooth polishing techniques on the primary lens, five group of baffles and a Lyot Spot is employed to suppress corresponding stray light. An unique design was the addition of a primary lens monitoring system in the optical path, allowing the observation of dust adhesion on the lens simultaneously. Stray light measurement was conducted in the laboratory, with a inner FOV of 6×10-6 B☉ (B☉ represents the mean brightness of the solar disk) at 1.05 R☉ and an outer FOV of 2×10-6 B☉ at 2 R☉. These designs ensure that SICG can achieve stable and clear coronagraphic observations, and successfully captured its first image of the solar corona in October 2023.