Solar coronagraphs in formation flying require several mechanical and technological constraints to be met. One of the most critical issues is the external occulter design and its optimization. The occulter edge requires special attention in order to minimize the di↵raction while being compatible with the constraints of handling and integrating large delicate space components. Moreover, it is practically impossible to realize a full scale model for laboratory tests. This article describes the results of tests performed with a scaled-model breadboard of the ASPIICS coronagraph disk edge, using the Artificial Sun facility at Laboratoire d'Astrophysique de Marseille.

The design and optimization of the external occulter geometry is one of the most discussed topics among solar coronagraph designers. To improve the performance of future coronagraphs and to stretch their inner fields of view toward the solar limb, the new concept of coronagraphs in formation flight has been introduced in the scientific debate. Solar coronagraphs in formation flight require several mechanical and technological constraints to be met, mainly due to the large dimension of the occulter and to the spacecraft's reciprocal alignment. The occulter edge requires special attention to minimize diffraction while being compatible with the handling and integrating of large delicate space components. Moreover, it is practically impossible to set up a full-scale model for laboratory tests. This article describes the design and laboratory tests on a demonstrator for a coronagraph to be operated in formation flight. The demonstrator is based on the principle of the linear edge, thus the presented results cannot be directly extrapolated to the case of the flying circular occulter. Nevertheless, we are able to confirm the results of other authors investigating on smaller coronagraphs and provide further information on the geometry and tolerances of the optimization system. The described work is one of the results of the ESA STARTIGER program on formation flying coronagraphs ["The STARTIGER's demonstrators: toward a new generation of formation flying solar

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Formation flying opens new perspectives in solar physics, and allow to conceive giant, externally-occulted coronagraphs using a two-component space system with the external occulter on one spacecraft and the optical instrument on the other spacecraft at a distance of hundred meters. ASPIICS (Association de Satellites Pour l'Imagerie et l'Interférométrie de la Couronne Solaire) is a mission proposed to ESA in the framework of its PROBA-3 demonstration program of formation flying which is presently in phase A. ASPIICS is a single coronagraph which will perform both high spatial resolution imaging of the solar corona as well as 2-dimensional spectroscopy of several emission lines from the coronal base out to 3 R using anétalon Fabry-Pérot interferometer. The selected lines will allow to address different coronal regions: the forbidden line of FeXIV at 530.285 nm (hot coronal matter), Fe IX/X at 637.4 nm (coronal holes), HeI at 587.6 nm (cold matter). An additional broad spectral channel will image the white light corona so as to derive electron densities. The classical design of an externally-occulted coronagraph is adapted to the formation flying configuration allowing the detection of the very inner corona as close as 0.01 R from the solar limb.