Influence of misalignments on the performance of externally occulted solar coronagraphs

Abstract

Context. ASPIICS is a novel externally occulted coronagraph that will be launched onboard the PROBA-3 mission of the European Space Agency. The external occulter will be placed on one satellite ∼ 150 m ahead of the second satellite that will carry an optical instrument. During 6 hours out of 19.38 hours of 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 (IO) to achieve a trade-off between the 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 a numerical model of the diffracted light and its propagation through the 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 the axi-symmetrical case. Here we extend the model to include non-symmetrical cases and possible misalignments. Results. The numerical computations 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 R IO = 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 the 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 the diffraction fringe from the external occulter and the internal occulter. We suggest that the best orientation strategy is to point the coronagraph to the center of the external occulter.

Modeling and removal of optical ghosts in the PROBA-3/ASPIICS externally occulted solar coronagraph

“…The O ′ plane is situated at a distance z 1 , or ∼ 0.76 mm further than B. The major part of the diffracted light is cut out by the internal occulter, the radius of which must be carefully chosen to obtain a compromise between good rejection of diffraction, minimum observational height and vulnerability of the coronagraph to possible tilts and other misalignments (Rougeot et al 2017;Shestov & Zhukov 2018). The IO is deposited directly on the surface of the field lens O2 (a singlet lens).…”

“…Finally, both O2 and O3 project the primary focus B onto the detector plane D, making the effective focal length f eff ≈ 734.6 mm. An extensive discussion of the functional optical layout is given in Rougeot et al (2017); Shestov & Zhukov (2018).…”

“…Diffracted light has the shape of a very bright and defocused ring, the size and the position of which correspond to the edge of the IO projected to the detector plane (Rougeot et al 2017;Shestov & Zhukov 2018). Since the effect of diffraction consists in interference of wavefront regions propagating through the opening in the apertures (Huygens-Fresnel principle), the resulting diffraction weakly depends on the lens characteristics, such as geometry and roughness, which is opposite to the behaviour of ghost and scattered light.…”

“…The coronal scene is vignetted in the inner zone due to vignetting produced by the IO. The diffracted light scene was calculated following the algorithm of Shestov & Zhukov (2018) for the symmetrical ASPIICS configuration and the IO size r IO = 1.662 mm. The diffraction pattern has a bright ring coinciding with the size of the IO, and a smaller bright ring that corresponds to the internal opening in the IO.…”

“…The difficulty of observing the low solar corona in white-light coronagraphic observations is caused by the two factors: the very high dynamic range of the corona and the presence of intense light from the solar disk diffracted by the external occulter (EO) (Koutchmy 1988;Shestov & Zhukov 2018).…”