Abstract
Context. The solar coronagraph ASPIICS will fly on the future ESA formation flying mission Proba-3. The instrument combines an external occulter of diameter 1.42 m and a Lyot solar coronagraph of 5 cm diameter, located downstream at a distance of 144 m. Aims. The theoretical performance of the externally occulted Lyot coronagraph has been computed by assuming perfect optics. In this paper, we improve related modelling by introducing roughness scattering effects from the telescope. We have computed the diffraction at the detector, that we compare to the ideal case without perturbation to estimate the performance degradation. We have also investigated the influence of sizing the internal occulter and the Lyot stop, and we performed a sensitivity analysis on the roughness. Methods. We have built on a recently published numerical model of diffraction propagation. The micro-structures of the telescope are built by filtering a white noise with a power spectral density following an isotropic ABC function, suggested by Harvey scatter theory. The parameters were tuned to fit experimental data measured on ASPIICS lenses. The computed wave front error was included in the Fresnel wave propagation of the coronagraph. A circular integration over the solar disk was performed to reconstruct the complete diffraction intensity. Results. The level of micro-roughness is 1.92 nm root-mean-square. Compared to the ideal case, in the plane of the internal occulter, the diffraction peak intensity is reduced by ≃0.001%. However, the intensity outside the peak increases by 12% on average, up to 20% at 3 R⊙, where the mask does not filter out the diffraction. At detector level, the diffraction peak remains ≃10−6 at 1.1 R⊙, similar to the ideal case, but the diffraction tail at large solar radius is much higher, up to one order of magnitude. Sizing the internal occulter and the Lyot stop does not improve the rejection, as opposed to the ideal case. Conclusions. Besides these results, this paper provides a methodology to implement roughness scattering in the wave propagation model for the solar coronagraph.
Highlights
The study of the corona of the Sun in white light requires highcontrast, as the coronal brightness in this spectral band ranges from 10−6 to 10−10 with respect to the mean solar brightness (Cox 2000)
This paper provides a methodology to implement roughness scattering in the wave propagation model for the solar coronagraph
Our analysis focuses on the telescope, that is, the first optical element, as it is known to be the most stringent for straylight in coronagraphy (Brueckner et al 1995)
Summary
The study of the corona of the Sun in white light requires highcontrast, as the coronal brightness in this spectral band ranges from 10−6 to 10−10 with respect to the mean solar brightness (Cox 2000). The first actual observation of the corona of the Sun in the absence of a natural eclipse was performed by the French astronomer Bernard Lyot (Lyot 1939). Solar coronagraphy was further developed with the external occultation technique introduced by Evans (1948), and the use of serrated or multiple disks (Purcell & Koomen 1962; Newkirk & Bohlin 1965). Koutchmy (1988) gives a review of the early advent of spaceborne solar coronagraphy and associated development Solar coronagraphy was further developed with the external occultation technique introduced by Evans (1948), and the use of serrated or multiple disks (Purcell & Koomen 1962; Newkirk & Bohlin 1965). Koutchmy (1988) gives a review of the early advent of spaceborne solar coronagraphy and associated development
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