Analysis and Suppression of Secondary Reflected Light in the Detection of Spectral Imaging Coronagraph (SICG)

Abstract The coronagraph is a scientific instrument designed for observing extremely faint coronal emissions against the bright solar disk, where stringent stray light suppression is essential for obtaining reliable measurements. This paper primarily investigates various forms of stray light that may be difficult to differentiate within coronagraph systems, including secondary reflected light generated by multiple reflections between the two surfaces of the objective lens, scattering caused by surface defects or contamination on the objective, and diffraction from the aperture stop. The discrete localized foci formed by secondary reflected light are referred to as ghost images. The coronagraph system is simulated using optical design software, and the characteristics of the associated stray light are analyzed.
The results show that the imaging positions of the three types of stray light are in close proximity, leading to significant interference in the detection process and affecting the system’s measurement results. A coronagraph with a field-of-view (FOV) ranging from 1.05 to 2.0 R⊙(R⊙ represents the solar radius), operating wavelengths of 5303 and 6374 Å, a clear aperture of 200 mm, an F-number of
6.7, and an optical length of 3796 mm was used as a representative system for validation. In addition, different types of occulters and ghost image suppression elements are designed for comparative analysis, providing additional options for stray light suppression and thereby improving the measurement accuracy of the coronagraph system.