Performance of the hybrid externally occulted<i>Lyot</i>solar coronagraph

Rougeot, R.; Flamary, Rémi; Galano, Damien; Aime, Claude

doi:10.1051/0004-6361/201629259

Cited by 15 publications

(44 citation statements)

References 24 publications

(30 reference statements)

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“…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).…”

Section: Functional Optical Layoutmentioning

confidence: 99%

“…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).…”

Section: Functional Optical Layoutmentioning

confidence: 99%

“…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.…”

Section: Sources Of Stray Lightmentioning

confidence: 99%

“…In inter-nally occulted coronagraphs the light diffracted by the entrance aperture produces an intense, sharply decreasing stray light pattern on the detector. The intensity of the diffraction in the region of the inner corona becomes greater than the intensity of the corona itself (Rougeot et al 2017). Furthermore, the situation is worsened by the scattering of direct sunlight on the primary objective (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…Externally occulted coronagraphs place the primary objective (PO) in the umbra of the external occulter, which alleviates the problem of scattering, but this provides no improvement of diffraction in the innermost corona (see Fig. 10 in Rougeot et al 2017). The remaining diffraction is removed by the internal occulter (IO), which is one of the key optical elements of externally occulted coronagraphs.…”

Section: Introductionmentioning

confidence: 99%

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

Shestov

Zhukov

Seaton

2019

A&A

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Context. ASPIICS is a novel externally occulted solar coronagraph, which will be launched onboard the PROBA-3 mission of the European Space Agency. The external occulter will be placed on the first satellite ∼ 150 m ahead of the second satellite that will carry an optical instrument. During 6 hours per orbit, the satellites will fly in a precise 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 optical ghost images formed inside the telescope and develop an algorithm for their removal. Methods. We implement the optical layout of ASPIICS in Zemax and study the ghost behaviour in sequential and non-sequential regimes. We identify sources of the ghost contributions and analyze their geometrical behaviour. Finally we develop a mathematical model and software to calculate ghost images for any given input image.Results. We show that ghost light can be important in the outer part of the field of view, where the coronal signal is weak, since the energy of bright inner corona is redistributed to the outer corona. However the model allows to remove the ghost contribution. Due to a large distance between the external occulter and the primary objective, the primary objective does not produce a significant ghost. The use of the Lyot spot in ASPIICS is not necessary.

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Section: Functional Optical Layoutmentioning

confidence: 99%

Section: Functional Optical Layoutmentioning

confidence: 99%

Section: Sources Of Stray Lightmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Shestov

Zhukov

Seaton

2019

A&A

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Influence of misalignments on the performance of externally occulted solar coronagraphs

Shestov

Zhukov

2018

A&A

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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.

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Theoretical performance of serrated external occulters for solar coronagraphy

Rougeot

Aime

2018

A&A

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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(−iπλzu2). 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.

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Performance of the hybrid externally occultedLyotsolar coronagraph

Cited by 15 publications

References 24 publications

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

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

Influence of misalignments on the performance of externally occulted solar coronagraphs

Theoretical performance of serrated external occulters for solar coronagraphy

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