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International audience; 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.

International audience; Aims. This study is carried out in the context of data processing of the solar coronagraph ASPIICS of the future formation-flying mission Proba-3, which is expected to provide images of the corona very close to the limb. There will be a transition zone of the order of 100 arcsec close to the limb, where the telescope aperture suffers a strong vignetting by the external occulter (a disc of 1.42 m at 144 m). The instrument response in this region will vary rapidly both in shape and in integrated intensity, the latter being particular to the external occultation. The aim of this paper is to propose a technique to recover as much as possible of the image of the corona very close to the limb in the vignetting zone.Methods. The object image relationship in this zone is not defined by the usual convolution but by the more general Fredholm integral of the first kind. Theoretical aspects of the problem are detailed in the context of a matrix formalism for the inversion of the Fredholm integral, formalism that we maintain up to the end of the numerical simulations, which is specific to the present work. The iterative Richardson-Lucy algorithm, specially written for the non-constant integrated intensity of the responses is used here for reconstruction. A study of the effect of noise on a photodetected image is made.Results. An important part of the work consisted in calculating the elements of the transfer matrix between the object and the image for a simulation on a small region of size 100 × 100 arcsec sampled over 128 × 128 pixels. This is obtained propagating the light through the system using a previously published approach. On a toy object, the reconstruction is excellent down to about 60 arcsec from the limb, corresponding to a vignetting of 50%. The drawback is that the recovery of a N × N object requires the handling of a N2 × N2 matrix, i.e. a 16384 × 16384 transfer matrix here. However, taking into account radial symmetries of the experiment, we propose the use of a transformation from Cartesian to polar coordinates which allows to apply the same procedure all around the sun as for a small region.

International audience; Context. This study is made in the context of the future solar coronagraph ASPIICS of the ESA formation-flying mission Proba-3. Aims. In the context of solar coronagraphy, we provide a comparative study of the theoretical performance of serrated (or toothed) external occulters by varying the number and size of the teeth, which we compare to the sharp-edged and apodized disks. The tooth height is small (a few centimeters), to avoid hindering the observation of the solar corona near the limb. We first analyze the diffraction pattern produced by such occulters. In a second step, we compute the umbra profile by integration over the Sun. Methods. We explored a few methods to compute the diffraction pattern. Two of them were implemented. The first is based on 2D fast Fourier transformation (FFT) routines and a multiplication by the Fresnel filter of the form exp(−i πλzu 2 ). Simple rules were derived and discussed to set the sampling conditions. The Maggi–Rubinowicz representation is then proposed as an alternative method, and is proven to be very efficient for this study. Results. Serrated occulters tend to create a two-level intensity pattern, the inner being the darker, which perfectly matches a previously reported geometrical prediction. The diffraction in this central region is lower by two to four orders of magnitude when compared to the sharp-edged disk. The achieved umbra level at the center ranges from 10 −4 to below 10 −7 , depending on the geometry of the teeth. Conclusions. Our study shows that serrated occulters can achieve a high rejection and can almost reach the performance of the apodized disk when very many teeth are used. We prove that shaped occulters must be preferred to simple disks in solar and stellar coronagraphy.