The dust-scattered stray light in an inner-occulted coronagraph mainly arises from dust particles on the surfaces of the objective lens. Due to the random accumulation of dust on the lens surfaces, it is challenging to monitor this type of stray light and no application can be used for its real-time monitor in the past. In this study, we provide a system and method to overcome this issue, and these have been applied to the Spectral Imaging CoronaGraph (SICG) of the Chinese Meridian Project. The method is based on the relation between the sizes of dust particles and its stray light level at the imaging plane established in the laboratory and the relation between the real size of dust particles and the occupancies on the imaging plane. To monitor the stray light levels accounted for by dusts, one needs only an image of the objective lens that can be provided by the auxiliary imaging system that specially comes with SICG. Our tests show that the errors of the method are less or about 2%, giving a strong confidence in its accuracy. It provides a handy tool to monitor the dust level of the objective lens of SICG and has significantly improved the efficiency of the pipeline of stray light control.

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