Stray light rejection in giant externally-occulted solar coronagraphs: experimental developments

Venet, M.; Bazin, C.; Koutchmy, S.; Lamy, Philippe

doi:10.1117/12.2309186

Cited by 3 publications

(3 citation statements)

References 7 publications

(13 reference statements)

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“…A&A 626, A1 (2019) The estimation of diffraction produced by the edge of an (external) occulter remains a delicate problem. One first approach relies on experimental measurements, as in the work of Bout et al (2000) for the LASCO C2 coronagraph (Brueckner et al 1995), or measurements on the real sky with a small-scale model as done by Venet et al (2010) for the ASPIICS coronagraph. However, the very large size of the latter instrument prevents us from performing any full-scale experimental characterization.…”

Section: Introductionmentioning

confidence: 99%

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

Rougeot

Flamary

Mary

et al. 2019

A&A

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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.

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Section: Introductionmentioning

confidence: 99%

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

Rougeot

Flamary

Mary

et al. 2019

A&A

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“…We also note that the purely analytical studies of Ferrari (2007) and Ferrari et al (2010) come with the drawback that the Lyot stop must equal the entrance pupil and cannot be reduced. There are also some experimental approaches, such as the works described in Fort et al (1978), Bout et al (2000), Venêt et al (2010) and Landini et al (2010). In contrast, extensive and complete analytic and numerical analysis appears nowadays mandatory, for modern advanced high-contrast instrumentation.…”

Section: Introductionmentioning

confidence: 99%

Performance of the hybrid externally occultedLyotsolar coronagraph

Rougeot

Flamary

Galano

et al. 2017

A&A

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Context. High-contrast hybrid coronagraphs, which combine an external occulter and a Lyot-style coronagraph became a reality in recent years, despite the lack of analytic and numerical end-to-end performance studies. The solar coronagraph ASPIICS which will fly on the future ESA Formation Flying mission Proba-3 is a good example of such a hybrid coronograph. Aims. We aim to provide a numerical model to compute theoretical performance of the hybrid externally occulted Lyot-style coronagraph, which we then aim to compare to the performance of the classical Lyot coronagraph and the externally occulted solar coronagraph. We will provide the level and intensity distribution of the stray light, when the Sun is considered as an extended source. We also investigate the effect of different sizes for the internal occulter and Lyot stop. Methods. First, we have built on a recently published approach, to express the diffracted wave front from Fresnel diffraction produced by an external occulter at the entrance aperture of the coronagraph. Second, we computed the coherent propagation of the wave front coming from a given point of the Sun through the instrument. This is performed in three steps: from the aperture to the image of the external occulter, where the internal occulter is set, from this plane to the image of the entrance aperture, where the Lyot stop is set, and from there to the final image plane. Making use of the axis-symmetry, we considered wave fronts originating from one radius of the Sun and we circularly average the intensities. Our numerical computation used the parameters of ASPIICS. Results. The hybrid externally occulted Lyot coronagraph rejects sunlight below 10 −8 B from 1.3 R -in the particular configuration of ASPIICS. The Lyot coronagraph effectively complements the external occultation. We show that reducing the Lyot stop allows a clear gain in rejection, being even better than oversizing the internal occulter, that tends to exclude observations very close to the solar limb. As an illustration, we provide a graph that allows us to estimate performance as a function of the internal occulter and Lyot stop sizes. Conclusions. Our work consists of a methodological approach to compute the end-to-end performance for solar coronagraph.

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“…This question is being approached by both optical calculations and laboratory tests [6,7] and is still actively studied. Assuming that a high performance external occulter can be implemented, the total stray light that will enter the 10 cm pupil amounts to 5x10 -8 Bsun, to be compared with 10 -6 Bsun that enters the 2.1 cm of LASCO-C2.…”

Section: Stray Light Considerationsmentioning

confidence: 99%

ASPIICS: a giant, white light and emission line coronagraph for the ESA proba-3 formation flight mission

Lamy

Vivès

Curdt

et al. 2017

International Conference on Space Optics — ICSO 2010

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INTRODUCTIONClassical externally-occulted coronagraphs are presently limited in their performances by the distance between the external occulter and the front objective. The diffraction fringe from the occulter and the vignetted pupil which degrades the spatial resolution prevent useful observations of the white light corona inside typically 2-2.5 solar radii (Rsun). Formation flying offers and elegant solution to these limitations and allows conceiving giant, externally-occulted coronagraphs using a two-component space system with the external occulter on one spacecraft and the optical instrument on the other spacecraft at a distance of hundred meters [1,2]. Such an instrument ASPIICS (Association de Satellites Pour l'Imagerie et l'Interférométrie de la Couronne Solaire) has been selected by the European Space Agency (ESA) to fly on its PROBA-3 mission of formation flying demonstration which is presently in phase B (Fig. 1). The classical design of an externally-occulted coronagraph is adapted to the formation flying configuration allowing the detection of the very inner corona as close as ~0.04 solar radii from the solar limb. By tuning the position of the occulter spacecraft, it may even be possible to reach the chromosphere and the upper part of the spicules [3]. ASPIICS will perform (i) high spatial resolution imaging of the continuum K+F corona in photometric and polarimetric modes, (ii) high spatial resolution imaging of the E-corona in two coronal emission lines (CEL): Fe XIV and He I D3, and (iii) two-dimensional spectrophotometry of the Fe XIV emission line. ASPIICS will address the question of the coronal heating and the role of waves by characterizing propagating fluctuations (waves and turbulence) in the solar wind acceleration region and by looking for oscillations in the intensity and Doppler shift of spectral lines. The combined imaging and spectral diagnostics capabilities available with ASPIICS will allow mapping the velocity field of the corona both in the sky plane (directly on the images) and along the line-of-sight by measuring the Doppler shifts of emission lines in an effort to determine how the different components of the solar wind, slow and fast are accelerated. With a possible launch in 2014, ASPIICS will observe the corona during the maximum of solar activity, insuring the detection of many Coronal Mass Ejections (CMEs). By rapidly alternating highresolution imaging and spectroscopy, CMEs will be thoroughly characterized.

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Stray light rejection in giant externally-occulted solar coronagraphs: experimental developments

Cited by 3 publications

References 7 publications

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

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

Performance of the hybrid externally occultedLyotsolar coronagraph

ASPIICS: a giant, white light and emission line coronagraph for the ESA proba-3 formation flight mission

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