Demonstrator of the formation flying solar coronagraph ASPIICS/PROBA-3

Formation Flying opens the possibility to conceive and deploy giant solar coronagraphs in space permanently reproducing the optimum conditions of a total eclipse of the Sun ("artificial" eclipse) thus giving access to the inner corona with unprecedented spatial resolution and contrast (low stray light). The first opportunity to implement such a coronagraph "ASPIICS" will be offered by the European Space Agency (ESA) PROBA-3 technology mission devoted to the in-orbit demonstration of formation flying technologies. Two spacecrafts separated by about 150 m form a giant externally-occulted coronagraph: the optical part hosted by one spacecraft remains entirely protected from direct sunlight by remaining in the shadow of an external occulter hosted by the other spacecraft. We developed and tested a scale-model 'breadboard' (i.e., 30m) of the PROBA-3/ASPIICS Formation Flying coronagraph. The investigations focused on two metrology systems capable of measuring both the absolute pointing of the coronagraph (by sensing the projected shadow and penumbra produced by the external occulting disk) and the alignment of the formation (by re-imaging light sources located on the rear-side of the occulting disk with the optical part of the coronagraph). In this contribution, we will describe the demonstrator and report on our results on the crucial question of the alignment and pointing in space of long instruments (> 100 m) with an accuracy of a few arcsec. This study has been conducted in the framework of an ESA "STARTIGER" Initiative, a novel approach aimed at demonstrating the feasibility of a new and promising technology on a very short time scale (six months).