Influence of misalignments on performance of externally occulted solar coronagraphs. Application to PROBA-3/ASPIICS

摘要

ASPIICS is a novel externally occulted coronagraph that will be launchedonboard the PROBA-3 mission of ESA. The external occulter (EO) will be placedon one satellite ~150 m ahead of the second satellite with an opticalinstrument. During part of each orbit, the satellites will fly in a preciseformation, constituting a giant externally occulted coronagraph. Large distancebetween the EO and the primary objective will allow observations of thewhite-light solar corona starting already from ~1.1RSun. We analyze influenceof shifts of the satellites and misalignments of optical elements on diffractedlight. Based on the quantitative influence of misalignments on diffractedlight, we will provide a "recipe" for choosing the size of the internalocculter (IO) to achieve a trade-off between the minimal height of observationsand sustainability to possible misalignments. We implement a numerical model ofthe diffracted light and its propagation through the optical system, andcompute intensities of diffracted light throughout the instrument. Ournumerical model extends axi-symmetrical model of Rougeot et al. 2017 tonon-symmetrical cases. The computations fully confirm main properties of thediffracted light obtained from semi-analytical consideration. Results: relativeinfluences of various misalignments are significantly different. We show that:the IO with R=1.1RSun is large enough to compensate possible misalignments inASPIICS, apodizing the edge of the IO leads to additional suppression of thediffracted light. Conclusions: the most important misalignment is the tilt ofthe telescope WRT the line connecting the center of the EO and the entranceaperture. Special care should be taken to co-align the EO and the coronagraph,i.e. co-aligning the diffraction fringe from the EO and the IO. We suggest thatthe best orientation strategy is to point the coronagraph to the center of theEO.