Diffraction in the ASPIICS coronagraph: Observations and modeling

Context . ASPIICS is a giant-baseline visible light solar coronagraph, which relies on the millimetric positioning performance of the precision formation flying Proba-3 mission of the European Space Agency. Proba-3 was launched on 5 December 2024, and since then ASPIICS has observed the solar corona with the field of view (1.1–3) R ⊙ . Aims . Diffraction, in particular diffraction of solar disk light on the external occulter, is known to provide a major source of stray light in coronagraphs. Our aims were to analyze diffracted light visible in ASPIICS images, to compare it with the analytical-numerical diffraction model reported earlier, and to fine-tune the model. Methods . We compared the diffraction effects visible in the ASPIICS data with simulated diffraction images; in particular, we compared the geometrical properties and the radiometric signal. The properties of the diffraction described in previous works suggest how to fine-tune the model in order to achieve a better correspondence with the observations. Results . Early ASPIICS observations, where diffraction is pronounced, fully confirm all the qualitative properties of diffracted light suggested by the model. After fine-tuning the model we found a quantitative correspondence of the level of 30–50%, depending on the configuration. Conclusions . The performed analysis allows us to validate our analytical-numerical model and justify the assumptions, and to estimate the contribution of the diffracted light in the ASPIICS images. Over most of the field of view the diffracted light is two orders of magnitude below the coronal signal.