Influence of surface roughness on diffraction in the externally occulted Lyot solar coronagraph

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.

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

“…This may be especially important for observations of polarized light. Recent consideration of diffraction and scattering together (Rougeot et al 2018a) shows that, due to scattering, the diffracted light increases its intensity in the outer corona (this is not taken into account in other models). This possibility even further increases the importance of the diffracted light and reduces relative contributions of other sources.…”

“…Thus, scattering of the bright light from the inner corona to the greater heights was less efficient. Currently an advanced investigation of the lens scattering characteristics is being performed (Rougeot et al 2018a).…”

Expected performances of the PROBA-3/ASPIICS solar coronagraph: Simulated data

“…We also tried to extend the model of Rougeot et al (2019) and included the effects of misalignments, such as solar shift and the telescope tilt (following the approach of SZ18). We found that misalignments did not change the scattered signal.…”

Топография Поверхности И Пропускание Света Кварцевыми Окнами После Экспозиции В Высокочастотном Разряде В Дейтерии И Смеси Дейтерия С Азотом

“…В работе [16] показано, что в задаче по прохождению света через границу воздух (или вакуум)/прозрачная среда (рис. 1) с показателем преломления n, вместо числа 4 в формуле (2) следует использовать 2 (n − 1), а вместо коэффициента отражения R -коэффициент пропускания T = 1 − R.…”