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arxiv: 2604.21559 · v1 · submitted 2026-04-23 · 🌌 astro-ph.IM · astro-ph.SR

Diffraction in the ASPIICS coronagraph: observations and modeling

S. Shestov , A. N. Zhukov , R. Rougeot , C. Aime , B. Bourgoignie , L. Dolla , N. Britavskiy , S. Fineschi
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S. Gunar P. Lamy M. Mierla H. Peter P. Rudawy K. Tsinganos
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Pith reviewed 2026-05-08 14:03 UTC · model grok-4.3

classification 🌌 astro-ph.IM astro-ph.SR
keywords diffractioncoronagraphASPIICSstraylightsolar coronaexternal occulterformation flyingProba-3
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The pith

Early ASPIICS observations confirm the analytical-numerical model's predictions for diffracted light in the solar coronagraph.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper analyzes diffraction effects visible in the first images from the ASPIICS coronagraph on Proba-3 and compares them to an existing analytical-numerical model. Observations match all qualitative features of diffracted light described by the model. After adjusting parameters, the model achieves 30 to 50 percent quantitative agreement with the data. This work validates the model and shows that diffracted light remains two orders of magnitude below the coronal signal across most of the field of view from 1.1 to 3 solar radii.

Core claim

Early ASPIICS observations, where diffraction is pronounced, fully confirm all the qualitative properties of diffracted light suggested by the model. After fine-tuning of the model quantitative correspondence is reached at the level of 30% -- 50%, depending on the configuration. In the majority of the field of view the diffracted light is two orders of magnitude below the coronal signal. The analysis validates the analytical-numerical model and justifies its assumptions while estimating the contribution of diffracted light.

What carries the argument

The analytical-numerical diffraction model, which simulates the diffraction of solar disk light on the external occulter while accounting for the millimetric formation-flying positioning and radiometric properties of the light.

If this is right

  • The validated model can now be applied to correct for straylight in future ASPIICS observations of the solar corona.
  • Diffraction from the external occulter does not significantly degrade the quality of coronal images in the majority of the field of view.
  • The fine-tuning process improves the model's accuracy for different instrument configurations.
  • Similar diffraction modeling approaches can be relied upon for other space-based coronagraphs using external occulters.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The success of the model suggests that the formation-flying precision of Proba-3 is adequate for maintaining low straylight levels.
  • Further refinements to the model could focus on specific regions where the 30-50% agreement is lower to achieve even better predictions.
  • These results support the feasibility of giant-baseline coronagraphs for high-resolution solar corona studies without excessive diffraction interference.

Load-bearing premise

The analytical-numerical diffraction model correctly captures the physical geometry of the external occulter, the millimetric formation-flying positioning, and the radiometric properties of diffracted light.

What would settle it

Detection of diffracted light intensity in ASPIICS images that significantly exceeds the model's fine-tuned prediction in a region where it should be low, or fails to show the expected geometric patterns.

Figures

Figures reproduced from arXiv: 2604.21559 by A. N. Zhukov, B. Bourgoignie, C. Aime, H. Peter, K. Tsinganos, L. Dolla, M. Mierla, N. Britavskiy, P. Lamy, P. Rudawy, R. Rougeot, S. Fineschi, S. Gunar, S. Shestov.

Figure 1
Figure 1. Figure 1: Top panel: optical layout of the ASPIICS coronagraph. Bottom panel: a simplified version of the optical design used in the diffraction model. See text for details. the off-axis waves two additional transformations are ap￾plied: a geometrical translation and an introduction of an additional phase tilt (Eq. A.5) for the whole wavefront. From the numerical analyses of the model, the following properties of th… view at source ↗
Figure 2
Figure 2. Figure 2: ASPIICS image in Fe XIV filter with ∼ 0.8 ◦ off-pointing registered during orbit 352. The image is Level-1, that is neither any calibration steps have been applied, nor texp-normalized or radiometrically calibrated; it has units of DN. The diffracted light that came to the instrument is not blocked by the IO, thus the bright diffraction ring forming at the primary focus B is re-projected to the detector. T… view at source ↗
Figure 3
Figure 3. Figure 3: Comparison of radial profiles of the diffraction extracted from the calibrated Level-2 ASPIICS image (color curves) and the diffraction model (black curve). The calibrated image was obtained from the one shown in view at source ↗
Figure 4
Figure 4. Figure 4: ASPIICS image in the wideband filter taken during orbit 152 at 18:11 UTC on 8 Apr 2025. The estimated off-pointing of ASPIICS is +17/-45 arcsec in pitch/yaw. See text for details. An animated gif with images with different off-pointings is available on-line. In view at source ↗
Figure 5
Figure 5. Figure 5: Images (panels a–d) and radial profiles (panel e) taken during orbit 152 with various off-pointings. The estimated off￾pointing in pitch and yaw are annotated in each panel. The solid and dashed lines highlight the directions along which the radial profiles have been measured. The crosses in each panel denote: blue cross – center of the image, red cross – center of the IO, yellow cross – center of the Sun.… view at source ↗
Figure 6
Figure 6. Figure 6: Radial profiles of diffraction for different distance l used in the model. The full scale is given in the left panel, while the zoomed region covering the radial distance 450–850 pixels is given in the right panel. 5.3. Empirical coefficient for the intensity We have found empirically that applying an additional mul￾tiplicative factor k = 1.2 provides better results: after the diffraction was subtracted fr… view at source ↗
Figure 7
Figure 7. Figure 7: Comparison of the ASPIICS image (left), the simulated diffraction image (middle), and ASPIICS image after diffraction removal (right) registered at 18:11 UTC (panel d from view at source ↗
Figure 8
Figure 8. Figure 8: Radial profiles from the images presented in view at source ↗
Figure 9
Figure 9. Figure 9: Similar to view at source ↗
Figure 10
Figure 10. Figure 10: Similar to view at source ↗
Figure 11
Figure 11. Figure 11: Influence of solar shift on diffraction model. The plots present polar dependencies of brightness in the ASPIICS image and models with various solar shift. The profiles are taken over the first (top panel) and the second (bottom panel) diffraction peaks. In the bottom panel the ASPIICS signal is ×2 reduced to compensate a higher contribution of the solar corona view at source ↗
Figure 12
Figure 12. Figure 12: Top panel: comparison of diffraction profiles for vari￾ous solar limb darkening models: van Hamme 93 model, square root of it, and P5. Measured ASPIICS signal and K-corona pro￾file from Koutchmy (2000) are given by thick solid and dotted curves. Bottom panel: the ratio of the square root to van Hamme, and P5 to van Hamme model. outer field of view multiplied by a factor 100. The simu￾lated diffraction is … view at source ↗
Figure 14
Figure 14. Figure 14: Comparison of the coronal brightness and diffracted light. Thick colored curves show the radial profiles of the coro￾nal brightness along the lines in view at source ↗
read the original abstract

Context: ASPIICS is a giant-baseline visible light solar coronagraph, which relies on the millimetric positioning performance of the precision formation flying Proba-3 mission of the European Space Agency. Proba-3 was launched on 5 Dec 2024, and since then ASPIICS observes the solar corona with the field of view (1.1-3) R_sun. Aims: Diffraction, in particular diffraction of solar disk light on the external occulter, is known to provide a major source of straylight in coronagraphs. We aim to analyze diffracted light visible in ASPIICS images, compare it with the analytical-numerical diffraction model reported earlier, and fine-tune the model. Methods: We compare diffraction effects visible in ASPIICS data with simulated diffraction images; in particular, we compare the geometrical properties and the radiometric signal. The properties of the diffraction described in previous works suggest how to fine-tune the model in order to achieve a better correspondence with the observations. Results: Early ASPIICS observations, where diffraction is pronounced, fully confirm all the qualitative properties of diffracted light suggested by the model. After fine-tuning of the model we see quantitative correspondence of the level of 30\% -- 50\%, depending on the configuration. Conclusions: The performed analysis allows (a) to validate our analytical-numerical model and justify the assumptions, and (b) to estimate the contribution of the diffracted light in the ASPIICS images. In the majority of the field of view the diffracted light is two orders of magnitude below the coronal signal.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper analyzes early ASPIICS coronagraph observations from the Proba-3 mission, focusing on diffraction of solar disk light by the external occulter. It compares observed geometrical and radiometric properties of diffracted light against an analytical-numerical diffraction model, reports full qualitative agreement with prior model predictions, and achieves 30-50% quantitative correspondence after fine-tuning model parameters for geometry, formation-flying positioning, and radiometry. The work concludes that the model is validated, that diffracted light lies two orders of magnitude below the coronal signal across most of the (1.1-3) R_sun field of view, and that the analysis justifies the model's assumptions while quantifying straylight contribution.

Significance. If the reported qualitative matches hold, the paper supplies useful early in-flight validation for a formation-flying coronagraph whose performance depends critically on millimetric occulter positioning. Explicit disclosure of the fine-tuning step and the resulting agreement level is a methodological strength that allows readers to assess the evidential weight. The estimate that diffracted light remains negligible relative to the coronal signal supports the instrument's scientific utility, provided the post-tuning quantitative level is interpreted accordingly.

major comments (2)
  1. [Results] Results section (paragraph beginning 'Early ASPIICS observations...'): The 30-50% quantitative correspondence is obtained only after fine-tuning the model parameters to the same ASPIICS data used for comparison. This reduces the quantitative match to a post-hoc fit rather than an independent prediction, weakening the evidential support for the validation claim even though the qualitative geometrical and radiometric properties remain externally grounded.
  2. [Conclusions] Conclusions (item (a)): The assertion that the analysis 'validate[s] our analytical-numerical model and justify the assumptions' should be qualified. The qualitative confirmation is independent, but the quantitative agreement level depends on parameter adjustment; without an a-priori prediction or cross-validation on held-out data, the strength of validation is primarily qualitative.
minor comments (2)
  1. [Abstract and Results] The abstract and Results section use 'fine-tune' without specifying which exact parameters (e.g., occulter edge profile, alignment offsets, or radiometric scaling) were varied or by how much; adding a short table or explicit list would improve reproducibility.
  2. [Figures] Figure captions and text should clarify whether the simulated images shown are pre- or post-tuning versions, to avoid ambiguity when readers compare them to the data.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the detailed and constructive review. The comments correctly identify that quantitative agreement requires parameter adjustment to the observed data. We address each point below and will revise the manuscript to qualify the validation claims accordingly, while preserving the strength of the independent qualitative matches.

read point-by-point responses

  1. Referee: [Results] Results section (paragraph beginning 'Early ASPIICS observations...'): The 30-50% quantitative correspondence is obtained only after fine-tuning the model parameters to the same ASPIICS data used for comparison. This reduces the quantitative match to a post-hoc fit rather than an independent prediction, weakening the evidential support for the validation claim even though the qualitative geometrical and radiometric properties remain externally grounded.

    Authors: We agree that the reported 30-50% quantitative correspondence is obtained after fine-tuning model parameters (primarily occulter-to-detector distance, lateral positioning, and radiometric scaling) using the same ASPIICS observations. These adjustments are not free parameters but are constrained by independent formation-flying telemetry and pre-flight instrument characterizations. The qualitative geometrical properties (e.g., the annular structure and radial extent of diffracted light) and radiometric trends were predicted by the model prior to the observations and match without adjustment. We will revise the Results section to explicitly distinguish the independent qualitative validation from the post-adjustment quantitative level and to describe the physical basis and limited range of the tuning. revision: yes

  2. Referee: [Conclusions] Conclusions (item (a)): The assertion that the analysis 'validate[s] our analytical-numerical model and justify the assumptions' should be qualified. The qualitative confirmation is independent, but the quantitative agreement level depends on parameter adjustment; without an a-priori prediction or cross-validation on held-out data, the strength of validation is primarily qualitative.

    Authors: We concur that the strongest validation is qualitative and independent of the data used for comparison. The quantitative agreement of 30-50% is achieved only after parameter adjustment within physically motivated bounds, and we lack an a-priori prediction or held-out cross-validation in this early dataset. We will revise item (a) in the Conclusions to state that the model is validated in its qualitative predictions and that the assumptions are justified by the independent geometrical and radiometric matches, while noting that the quantitative correspondence is obtained after fine-tuning and should be interpreted accordingly. revision: yes

Circularity Check

1 steps flagged

Quantitative radiometric agreement achieved only after fine-tuning model to the same observations

specific steps
  1. fitted input called prediction [Abstract (Results paragraph)]
    "After fine-tuning of the model we see quantitative correspondence of the level of 30% -- 50%, depending on the configuration."

    The model parameters for geometry, millimetric positioning, and radiometry are adjusted to the ASPIICS observations being analyzed; the resulting 30-50% agreement level is then presented as quantitative validation of the model. This agreement is produced by the fitting step itself rather than serving as an a priori test.

full rationale

The paper's central validation rests on comparing ASPIICS data to a prior analytical-numerical diffraction model, with explicit fine-tuning of geometry, formation-flying positioning, and radiometric parameters to improve the match. Qualitative properties of diffracted light are confirmed independently by the observations, providing external grounding. However, the reported 30-50% quantitative correspondence and the estimate that diffracted light is two orders of magnitude below coronal signal are obtained post-tuning, reducing that portion of the claim to a fitted result rather than an independent prediction. No self-definitional equations, imported uniqueness theorems, or other circular patterns appear in the derivation chain. The tuning step is transparently reported, limiting the circularity to moderate.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

Only the abstract is available, so the ledger is necessarily incomplete; the model relies on domain assumptions about diffraction physics and instrument geometry, with at least one set of parameters adjusted to data.

free parameters (1)
  • fine-tuning parameters
    Parameters in the analytical-numerical diffraction model are adjusted to achieve the reported 30-50% quantitative correspondence with ASPIICS observations.
axioms (1)
  • domain assumption The analytical-numerical diffraction model accurately represents light diffraction around the external occulter and the radiometric response of the ASPIICS detector under the formation-flying geometry.
    Invoked throughout the comparison of simulated and observed diffraction patterns.

pith-pipeline@v0.9.0 · 5662 in / 1667 out tokens · 78547 ms · 2026-05-08T14:03:23.199091+00:00 · methodology

discussion (0)

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