Classical externally-occulted coronagraphs are presently limited in their performances by the distance between the external occulter and the front objective. The diffraction fringe from the occulter and the vignetted pupil which degrades the spatial resolution prevent useful observations of the white light corona inside typically 2-2.5 solar radii. Formation flying offers an elegant solution to these limitations and allows conceiving 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. Such an instrument, ASPIICS (Association de Satellites Pour l'Imagerie et l'Interférométrie de la Couronne Solaire), has just been selected by the European Space Agency (ESA) to fly (launch expected in 2014) on its third PROBA (Project for On-Board Autonomy) mission of formation flying demonstration which is presently in phase B. It will perform both high spatial resolution imaging of the solar corona as well as 2- dimensional spectroscopy of several emission lines (in particular the forbidden line of FeXIV at 530.285 nm) from the coronal base out to 3 solar radii. For this, it will use filters, polarisers and a solid Fabry-Perot interferometer ("étalon"). 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 0.04-0.05 solar radii (40-50 arcsec) from the solar limb. By tuning the position of the occulter spacecraft, it may even be possible to reach the chromosphere and the upper part of the spicules.

Date Published: 5 August 2010
PDF: 12 pages
Proc. SPIE 7731, Space Telescopes and Instrumentation 2010: Optical, Infrared, and Millimeter Wave, 773118 (5 August 2010);

Published in SPIE Proceedings Vol. 7731:
Space Telescopes and Instrumentation 2010: Optical, Infrared, and Millimeter Wave
Jacobus M. Oschmann Jr.; Mark C. Clampin; Howard A. MacEwen, Editor(s)