Shestov, S. V. ; Zhukov, A. N.

submitted to Astronomy&Astrophysics (2017)

Abstract: Context. ASPIICS is a novel externally occulted coronagraph, which will be launched onboard the PROBA-3 mission of the European Space Agency. The external occulter will be placed on the first satellite ∼ 150 m ahead the second satellite, that will carry an optical instrument. During 6 hours of 19.38 hours orbit, the satellites will fly in a precise (accuracy around a few millimetres) formation, constituting a giant externally occulted coronagraph. Large distance between the external occulter and the primary objective will allow observations of the white-light solar corona starting from extremely low heights ∼ 1:1R . Aims. To analyze influence of shifts of the satellites and misalignments of optical elements on the ASPIICS performance in terms of diffracted light. Based on the quantitative influence of misalignments on diffracted light, we will provide a “recipe” for choosing the size of the internal occulter to achieve a trade-off between minimal height of observations and sustainability to possible misalignments. Methods. We consider different types of misalignments and analyze their influence from optical and computational points of view. We implement numerical model of the diffracted light and its propagation through an optical system, and compute intensities of diffracted light throughout the instrument. Our numerical model is based on the model of Rougeot et al. (2017), who considered axi-symmetrical case. Here we extend the model to include non-symmetrical cases and possible misalignments. Results. The numerical calculations fully confirm main properties of the diffracted light, that we obtained from semi-analytical consideration. We obtain, that relative influences of various misalignments are significantly different. We show that the internal occulter with RIO = 1:694 mm = 1:1R is large enough to compensate possible misalignments, expected to occur in PROBA- 3/ASPIICS. Beside that we show that apodizing the edge of the internal occulter leads to additional suppression of diffracted light. Conclusions. We conclude that the most important misalignment is the tilt of the telescope with respect to the line connecting the center of the external occulter and the entrance aperture. Special care should be taken to co-align the external occulter and the coronagraph, which means co-aligning of the diffraction fringe from the external occulter and the internal occulter. We suggest that the best orientation strategy will be to point the coronagraph to the center of the external occulter.

Keyword(s): Sun: corona ; Instrumentation: high angular resolution ; Telescopes ; Methods: numerical
Funding: 3PRODPROBA3/3PRODPROBA3/3PRODPROBA3