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REVIEW 4 major objections 5 minor 7 references

Proposal for the optical design of three robust and highly performing FPI systems for the European Solar Telescope

T0 review · 4 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash

Pith's one-line read This paper proposes that three dual-etalon filter systems for the European Solar Telescope can meet strict image-quality and spectral-resolution goals in a straight-through, lens-only layout under 4.7 meters.

desk verdict Serious, honestly-flagged design study for EST's three FPI systems, but the >90% Strehl claim doesn't survive the authors' own apodization floor: the 12 µm design has zero margin and the 5 µm design falls short. read the letter →

arxiv 2505.21053 v1 pith:SHSVRRH2 submitted 2025-05-27 astro-ph.IM astro-ph.SR

classification astro-ph.IMastro-ph.SR
keywords Fabry-PerotinterferometersolartelescopeimagingspectropolarimeteropticaldesigntelecentricStrehlratioetalonEuropean
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper argues that the three narrowband Fabry-Perot spectropolarimeters planned for the European Solar Telescope (EST) can be built as comparatively simple, all-lens instruments rather than complex mirror-based systems. The authors present complete optical prescriptions for EST-B, EST-V, and EST-R, each a dual-etalon, telecentric reimaging system with an optical path of 4.4–4.7 m, no folding mirrors, and etalon clear apertures minimized to meet the Science Advisory Group requirements on field, spectral resolution, and Strehl. The central claim is that all three systems are fully feasible to construct and can keep overall Strehl above 90 percent—EST-B, the hardest, reaching a minimum of 0.91–0.95 depending on pixel size and design variant—while remaining modular enough that a future change of camera pixel size requires swapping only the last camera lens. If the proposal is correct, EST obtains three science-grade narrowband channels at lower cost, higher transmission, and greater stability than the more complex alternatives, with a proven refocusing scheme that compensates the telescope's chromatic focus curve without losing duty cycle.

What carries the argument

The load-bearing mechanism is the modular 'mother system': a fixed, telecentric, all-lens reimaging chain (wideband beam-splitter cube, two doublets, the two etalons, another doublet, two pupil stops) whose expensive components are optimized once in a 'mother file' and then left unchanged, plus an interchangeable air-spaced triplet camera lens. Optimizing with an ideal paraxial camera lens first, then designing the real triplet to match, means every change of detector pixel size (the paper designs for 5 and 12 µm) requires only a new camera lens and connecting tube, keeping the mother system intact. Between the etalons the beam is slow (F/110 for EST-B, F/147 for EST-V and R) and telecentric, which makes the apodization from tilted rays small enough to hold Strehl and lets the pupil stops suppress ghosts. The dual-etalon pair—a high-resolution etalon together with a deliberately lower-resolution, lower-reflectivity etalon—mitigates cavity errors, and the camera lens is moved quickly and repeatably to compensate the chromatic focus curve of POP, with small-pixel designs needing only about 1.5 mm of travel for the full EST-B wavelength range.

What would settle it

A Monte Carlo tolerance analysis that adds realistic etalon plate wedges, tilts, and air-slit nonuniformities (with figure-error magnitudes like those reported for the heritage etalons) and recomputes the EST-B Strehl at 380 nm at the edge of the field, in the POP-connected configuration, would settle the claim: if the minimum Strehl drops below 0.90, the central feasibility conclusion fails.

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Extended reading notes

Core claim

The paper's central claim, stated in the concluding section, is that it is fully feasible to construct highly performing, compact and robust FPI systems for EST, which can easily be adopted to future pixel sizes, with the overall Strehl of EST-B kept in excess of 90 percent. The evidence is a set of Zemax-based optical designs and Strehl/spot-diagram tables showing that each system meets the SAG top-level requirements across its full wavelength range and 1-arcminute field, both standalone and when connected to the pier optical path (POP). The design achieves this through four interacting choices: minimize the etalon clear aperture by choosing the smallest F-ratio (110 for EST-B, 147 for EST-V and EST-R) that keeps apodization-induced Strehl loss under 5 percent; use a straight-through lens-only telecentric train so there are no folding mirrors; pair a high-resolution, high-reflectivity etalon with a low-reflectivity, low-resolution etalon to suppress cavity errors; and refocus at each wavelength by translating only the small camera lens. The paper is explicit that this is a conceptual design, not a final one: the Fabry-Perot plates are modeled as ideal glass blocks without wedges, tilts, or air slits, and the authors list the inclusion of real plate details, refined tolerances, ghost images, and polarization fringes as remaining final-design work.

Load-bearing premise

The performance numbers assume that real Fabry-Perot plates—with their wedges, tilts, and air gaps—will not degrade image quality below the idealized 'two glass blocks' model; the paper defers that analysis to the final design phase and supports the assumption only by citing the earlier instruments' experience.

Editorial extensions

If this is right

  • EST-B, EST-V, and EST-R can meet the SAG top-level requirements—1 arcmin field, spectral resolutions 50,000 to 100,000, Strehl above 90 percent, image scales 0.010 to 0.017 arcsec/pixel—with total optical lengths of 4.4–4.7 m and no folding mirrors.
  • A future change in camera pixel size is absorbed by replacing only the last lens: the 5 µm EST-B design needs only 1.5 mm of camera-lens travel to compensate POP's 6.4 mm focus curve, while the 12 µm design needs 9.3 mm, and both keep minimum Strehl near or above 0.95.
  • Refocusing can be fast enough to hide behind pre-filter changes, so the chromatic focus curve of POP does not cost observing time.
  • The wideband companion channels can be matched to the narrowband image scale during refocusing by using slightly different camera lenses, preserving the wideband data as a stable reference for image reconstruction.
  • EST-V and EST-R are the easier systems: their Strehl values are 97–100 percent across essentially all field points and wavelengths with both 6.5 µm and 12 µm pixels, and their POP focus curves are nearly flat.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the final tolerance analysis confirms the assumptions, the mother-system modularity means the same core optics could serve other 4-meter-class solar telescopes (or multiple port configurations of EST) with only a new input lens and camera lens.
  • The strong preference for small pixels in the paper suggests an even smaller detector—say 3–4 µm—would nearly eliminate the refocus-image-scale problem, since the required camera-lens travel scales with the square of the magnification.
  • The constraint that narrowband and wideband systems share the same image scale is currently driven by the image-reconstruction software; if software that handles different scales is adopted, the image-scale-stabilized 12 µm camera lens (with its lower, 0.87–0.90 Strehl) becomes unnecessary.
  • A concrete next step would be to build one prototype camera lens using the proposed alignment-bonding assembly and measure its delivered wavefront; this would retire the largest manufacturing risk identified in the tolerances.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 5 minor

Summary. The paper presents a preliminary optical design for three dual Fabry-Perot imaging spectropolarimeters for the European Solar Telescope (EST-B, EST-V, and EST-R), covering 380-500 nm, 500-680 nm, and 680-1000 nm. The systems are compact, lens-only, telecentric designs shorter than 4.7 m with no folding mirrors, using two doublets, two etalons, a doublet, and an air-spaced triplet camera lens; the camera lens is replaceable to adapt the image scale. The authors report Zemax-derived Strehl values of 0.91-0.96 for EST-B in the 380-500 nm range and values of 0.97 or higher for EST-V and EST-R. They conclude in Sect. 9 that 'the overall Strehl of EST-B ... can be kept in excess of 90%', with the apodization contribution from the etalon F-ratios set to a 95% floor in Sect. 1 using F-ratios of 110 (EST-B) and 147 (EST-V/R) taken from a companion paper.

Significance. If fully established, the design would meet the SAG top-level requirements (1 arcmin field, spectral resolutions of 50,000-100,000, Strehl above 90%, image scales 0.010-0.017 arcsec/pixel) in a modular and mechanically robust layout. The paper's strengths include complete Zemax prescriptions and Strehl tables in the appendices, an explicit two-configuration optimization that forces the narrowband and wideband systems to share the same image scale, a Monte Carlo tolerance study for EST-V, and a candid statement of the remaining design work. However, the central Strehl claim is not supported by the paper's own arithmetic once the 0.95 apodization floor is multiplied into the optical-design Strehl values, and the estimates omit the FPI plates themselves. The engineering value of the full prescriptions and the transparency about open items are genuine merits, but the headline quantitative conclusion is not yet established.

major comments (4)
  1. [Sect. 9, Tables A.11 and A.13] The conclusion that 'the overall Strehl of EST-B ... can be kept in excess of 90%' does not follow from the paper's own numbers. The Strehl tables are optical-design values for the lens system only; the FPI plates are modeled as glass blocks without wedges, tilts, or air slits (Sect. 2.2), and the apodization contribution from the finite F-ratio is a separate 0.95 floor set in Sect. 1 by the adopted F=110. For the 5 micron EST-B design, the minimum Strehl with POP is 0.91 at 380 nm and the 30" field edge (Table A.11), so 0.91 x 0.95 = 0.86, below the SAG requirement of >0.90. For the 12 micron image-quality design, the minimum is 0.95 (Table A.13), giving 0.95 x 0.95 = 0.90 with essentially zero margin. Thus the 'in excess of 90%' claim holds only for the 12 micron design in the idealized glass-block model, and only marginally at the worst field point; the 5 micron design, which the text explicitly endorses as needing no further improvement, fails the requirement even under the idealized assumption.
  2. [Sects. 1 and 3.1, Tables 4 and 12] The load-bearing F-ratios between the etalons (110 for EST-B, 147 for EST-V and EST-R) and the corresponding 95% apodization Strehl floor are taken from the companion paper (Scharmer et al. 2025, in prep.), which is not available to the reader. These values set the etalon clear apertures, the field diameter between the etalons, and the apodization term in the Strehl budget. Without at least a summary of the apodization calculation, or the companion paper being made accessible, the central design parameters cannot be independently checked. The essential derivation should be included as an appendix or in the main text.
  3. [Sect. 2.2] The FPI plates are explicitly modeled 'as two glass blocks without any wedges, tilts or air slits', and the paper states that including the details of the FPI plates, refining the tolerance analysis, and modeling ghost images and polarization fringes remain for the final design. At the proposed clear apertures (140-200 mm) and the short wavelengths of EST-B (380-500 nm), surface figure error, wedge, coating nonuniformity, and cavity errors will enter the Strehl budget, and the paper provides no quantitative estimate of these contributions. The CRISP and CHROMIS heritage is invoked qualitatively, but those systems operate at smaller apertures and longer wavelengths; they do not quantify the EST-B margin. Given that the idealized margin for the 12 micron design is only about 0.0025 above the requirement and the 5 micron design is below it, the missing etalon error budget is load-bearing for the paper's main conclusion.
  4. [Sect. 8 and Appendix D] The tolerance analysis is preliminary and does not cover the systems where the Strehl claim matters most. For EST-B, Appendix D.1 lists tolerance values but provides no Monte Carlo result or end-to-end Strehl after manufacturing and assembly errors, and the camera lens (L4) is described as having very demanding tolerances that may force a redesign 'at the prize of a small reduction of the Strehl' (Sect. 9). For EST-R, Sect. 5.2 states that 'A tolerance analysis for EST-R has not yet been carried out'. Since the conclusion refers to 'design, manufacture and alignment of the optics', the absence of a quantified manufacturing and alignment error budget for EST-B and EST-R leaves the 'in excess of 90%' claim unsubstantiated for those systems.
minor comments (5)
  1. [Abstract] There are two typos in the abstract: 'for for' appears twice, and the final sentence ends with a stray 's.'.
  2. [Tables B.5, B.7, B.10, B.11, B.13] Several table captions contain '?? mm focus curve' placeholders; the actual camera-lens travel values are given in the text (0.3 mm, 0.92 mm, etc.) and should be inserted in the tables.
  3. [Figs. 7 and 8] Figure 7's caption says 'the entire narrowband EST-V system' but the figure is labeled 'FPI-R'; Figure 8 similarly describes EST-V camera lenses for figures showing EST-R layouts. The captions and labels should be made consistent.
  4. [Table 18] The EST-B pixel size is listed as '5.0' in Table 18 but as '5 um' everywhere else; use one consistent notation.
  5. [Throughout] There are several typos: 'presxented' in Sect. 3.3.2, 'prize' for 'price' in Sect. 9, and 'adoption' where 'adaptation' is meant in the abstract and Sect. 9.

Circularity Check

1 steps flagged · score 2.0 of 10

No circular reduction found: Strehl values are computed from the paper's own Zemax prescriptions, not fit to data. But the '>90% overall Strehl' claim is arithmetically fragile (5 µm EST-B: 0.95 × 0.91 = 0.86 < 0.90) and its 0.95 apodization floor is load-bearing, taken from the authors' own in-prep companion paper.

  1. self citation load bearing [Sect. 1 (apodization/F-ratio constraint); Table 4 note; Sect. 9 conclusion]
    "The apodization effects depend primarily on the spectral resolution required and the F-ratio of the system at the etalons, and were evaluated through calculations that are reported on separately (Scharmer et al. 2025). Given the spectral resolution required, it was possible to determine the smallest possible F-ratio that allows a Strehl of at least 95%, based on these apodization effects."

    The 'overall Strehl ... in excess of 90%' claim (Sect. 9) multiplies the in-paper optical Strehl (Tables A.11/A.13) by a 0.95 apodization floor that is not computed here. Table 4 enters F/110 as 'Obtained from numerical simulations, see Scharmer et al. (2025)', an in-prep companion paper by the same authors, and Sect. 1 says the F-ratio was chosen so apodization 'allows a Strehl of at least 95%'. The margin (0.95 × 0.95 = 0.9025) is inherited from an adopt-by-construction value in an unverifiable self-citation; load-bearing, but not a full reduction, since the cited apodization calculation is physical and the in-paper designs are independent.

full rationale

The optical-design Strehl values (Tables A.8-A.21) are genuine outputs of Zemax ray tracing of the full prescriptions in Appendices A-C, with pupil geometry, glass data and focus-compensation logic specified; they are not fitted to data, and no parameter is tuned to match a measured Strehl. The central content - lens prescriptions, image-scale analysis, tolerance tables (Appendix D), and the EST-V Monte-Carlo check (Table 14) - is self-contained and checkable, so the derivation chain for the designs does not reduce to its inputs. The 'overall Strehl in excess of 90%' conclusion (Sect. 9) is, however, a combination (per the Table 1 footnote) of the in-paper optical Strehl and a 0.95 apodization floor. That floor is not derived here: Table 4 enters 'F-number between FPIs 110' with the note 'Obtained from numerical simulations, see Scharmer et al. (2025)', and Sect. 1 states the F-ratio was chosen so that apodization 'allows a Strehl of at least 95%'. The margin of the headline claim (0.95 × 0.95 = 0.9025 for the 12 µm system) is therefore inherited from an adopt-by-construction input in an unverifiable, in-preparation, same-author companion paper. I flag this as load-bearing self-citation rather than full circularity: the apodization calculation is a parameter-free physical simulation, and the in-paper optical content is independent. Two non-circular concerns weigh in the verdict: (i) using the paper's own tables, the recommended 5 µm EST-B narrowband system gives 0.95 × 0.91 = 0.86 at 380 nm field edge, below the SAG 90% requirement, while the 12 µm system clears 90% with zero margin; and (ii) Sect. 2.2 models the FPI plates 'as two glass blocks without any wedges, tilts or air slits' and defers real plate details, ghost images and polarization fringes to final design, so real-etalon errors are absent from the budget. These are correctness/fragility concerns, not circularity.

Assumptions & free parameters 1 free parameters · 5 assumptions · 0 invented entities

No new physical entities are introduced. The only postulates are design choices (F-numbers, lens prescriptions) and engineering simplifications, which are captured as free parameters and axioms above.

free parameters (1)
  • F-number between etalons (adopted design values) = 110 (EST-B), 147 (EST-V and EST-R)
    Chosen from apodization simulations in the companion paper (Scharmer et al. 2025, in prep.) to minimize etalon clear aperture while keeping Strehl >= 95%; the derivation is not included in this paper.
assumptions (5)
  • domain assumption The apodization simulations in Scharmer et al. (2025, in prep.) correctly determine the minimum F-ratio (110 for EST-B, 147 for EST-V/R) for Strehl >= 95%.
    Section 1 and Table 4 adopt these F-numbers as inputs; the simulation details are not reproduced here, so the entire aperture budget rests on this external companion.
  • domain assumption The POP design of 23 April 2024, with F/50 telecentric output and focus curve of 6.4 mm (EST-B) or 7.5 mm (EST-R), is correct and representative.
    Section 2.1 states the design uses this accepted version 'as of 23 April 2024'; if POP focus curve or telecentricity changes, the refocusing ranges and Strehl results change.
  • ad hoc to paper Real FPI plates with wedge, tilt and air slits will not degrade Strehl below the values computed with the glass-block model.
    Section 2.2 models FP plates as glass blocks and defers actual plate details to the final design, relying on 'our experience with earlier designs (CRISP, CHROMIS and CRISP2)'; this is an unproven transfer assumption.
  • domain assumption The SAG top-level requirements (Table 1) are accepted as fixed: 1 arcmin FOV, spectral resolutions of 50,000/100,000/80,000, Strehl > 90%, and specified image scales.
    These external requirements set the design targets; they are not derived or validated in the paper.
  • domain assumption The performance heritage of CRISP and CHROMIS at the Swedish 1-m Solar Telescope transfers to the larger EST systems.
    The introduction states the proposal 'relies heavily on the heritage' and uses it as support for feasibility; no quantitative extrapolation is given.

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Cite this review

Pith. "Pith review of Proposal for the optical design of three robust and highly performing FPI systems for the European Solar Telescope." pith.science (2026). https://pith.science/paper/SHSVRRH2

@misc{pith2026250521053,
  author       = {Pith},
  title        = {Pith review of: Proposal for the optical design of three robust and highly performing FPI systems for the European Solar Telescope},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/SHSVRRH2}},
  note         = {Machine review of arXiv:2505.21053}
}
read the original abstract

We describe a proposal for the optical design of three dual Fabry-Perot based narrowband filter systems for for the future European Solar Telescope (EST). These are intended to constitute the core elements of three imaging spectropolarimeters, foreseen to become amongst the most important science instruments for EST. The designs proposed here rely heavily on the heritage of CRISP and CHROMIS, developed for the Swedish 1-m Solar Telescope and described in detail in a companion paper (Scharmer et al. 2025, in prep.). The outstanding performance of these systems, and the simplicity of their designs, provide strong support of our proposal to build similar systems for EST. The design concepts involve i) minimising the FPI clear aperture diameter by means of numerical simulations based on constraints on Strehl and spectral resolution set by the EST Science Advisory Group (SAG); ii) a compact telecentric optical design with an optical path length of less than 4.7 m; iii) a straight-through optical system based on lenses and without any folding mirrors; iv) the combination of a high resolution etalon with high reflectivity and a low reflectivity, low resolution etalon, to mitigate the effects of cavity errors (Scharmer 2006, Scharmer et al. 2025); v) flexibility in terms of image scale by simple replacement of the last lens (the camera lens) of the FPI system. We propose to compensate for the focus curve of ESTs Pier Optical Path (POP) by focusing the camera lenses of the FPI systems. s. The proposed systems should offer several advantages over other much more complex systems, including manufacture, alignment, stability, flexibility of changes of image scale, and costs. The underlying design concepts also make the proposed FPI systems robust and highly performing in terms of image quality, overall transmission, and fidelity of the spectral transmission profile.

Figures

Figures reproduced from arXiv: 2505.21053 by the authors.

Figure 1
Figure 1. Idealized model of the telescope Idealized model of the EST telescope and POP. [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Shows the layout of the narrowband (top) and wideband (bottom) ESTB systems, de pixel size. The figures are stretched 2x in the vertical direction for clarity. Both camera lenses a pansion of the glasses are well matched and that an elastic ce [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. The rear (top) and central (bottom) of the 12 μm pixel narrowband EST-B system. 3.2 OPTIMIZATION OF EST-B The model of the FPI system consists of a number of optical components in the following order (starting on the input side): one 80 mm beam splitter cube (used to deflect a small fraction of the light to the wideband re-imaging system), two doublet lenses, two FPI etalons, one doublet lens, one triplet lens (the … view at source ↗
Figures from the paper (6 more)
Figure 2
Figure 2. Figure 2: Layout of the FPI-V system. The 80 mm wideband beam splitter on the input side is located before F3 and is not show here but in [PITH_FULL_IMAGE:figures/full_fig_p008_2.png]
Figure 5
Figure 5. Figure 5: The layout of the camera lenses designed for the 6.5µm pixel size (top) and for the 12 µm pixel size system (bottom) for EST-V. Shown is also the polarising beam splitter. The scaling is different for the two panels. unaffected by these tolerances, but the whole beam w…
Figure 7
Figure 7. Figure 7: The layout of the entire narrowband EST-V system (top), de￾signed for 12 µm pixel size. The middle and bottom panels show the first and rear parts of the same system in more detail. The figures are stretched 2x in the vertical direction for clarity. F1-F3 are focal pla…
Figure 8
Figure 8. Figure 8: The layout of the camera lenses designed for the 6.5µm pixel size (top) and for the 12 µm pixel size system (bottom) for EST-V. Shown is also the polarising beam splitter. The scaling is different for the two panels. Notes, page 11 of 38 [PITH_FULL_IMAGE:figures/full_…
Figure 4
Figure 4. Figure 4: shows the spot diagrams for each wavelength and field position separately for FPI-V alone (without POP) with 6.5 μm pixels. The box size is 40x40 μm. The focus position is common to all wavelengths. 500 530 560 590 620 650 680 [PITH_FULL_IMAGE:figures/full_fig_p025_4.png]
Figure 8
Figure 8. Figure 8: shows the layout of the wideband system as designed for a pixel size of 6.5 μm. The lenses used are the same as in the TIS-V system except for the singlet lens that was re-optimized to slightly different radii. All wavelengths have a common focus. The 50 mm beam splitt…

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