Widefield Spectroscopic Telescope (WST): coating strategy to achieve high optical throughput

Benoit Sassolas; Christophe Michel; Corentin Cudennec; Laurent Pinard; Matthieu Coulon; Philippe Dierickx; Roland Bacon

arxiv: 2606.24179 · v1 · pith:GCLDTCB7new · submitted 2026-06-23 · 🌌 astro-ph.IM

Widefield Spectroscopic Telescope (WST): coating strategy to achieve high optical throughput

Benoit Sassolas , Matthieu Coulon , Christophe Michel , Laurent Pinard , Roland Bacon , Corentin Cudennec , Philippe Dierickx This is my paper

Pith reviewed 2026-06-25 23:15 UTC · model grok-4.3

classification 🌌 astro-ph.IM

keywords telescope coatingsoptical throughputwidefield spectroscopyantireflective coatingsprotected silverdielectric stacksgraded-index coatingslarge optics

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The pith

A size-based coating assignment for the WST's mirrors and lenses can deliver the target throughput above 83 percent across 370-1600 nm.

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

The paper sets out a practical coating plan for the thirteen mirrors and five lenses of a 12-meter spectroscopic telescope that must maintain high throughput from ultraviolet through near-infrared wavelengths. Large mirrors receive enhanced metallic layers that reach more than 90 percent reflectivity in the ultraviolet and above 99 percent in the near-infrared. Smaller mirrors are assigned niobium-oxide and silica dielectric stacks that provide near-total reflection, tunable response, and good long-term stability. The largest lenses, up to 1.6 meters across, are the hardest element; the authors propose exploring graded-index antireflection layers because conventional multilayer coatings lose performance at wide angles and across such a broad band.

Core claim

By matching coating type to component size and optical role, the telescope can meet its throughput goals: protected metallic coatings on mirrors larger than 2 meters, Nb2O5/SiO2 dielectric stacks on the remaining mirrors, and graded-index antireflective coatings on the large lenses.

What carries the argument

Size-dependent coating assignment that pairs enhanced metallic layers with large mirrors, dielectric multilayers with small mirrors, and graded-index antireflection with lenses up to 1.6 m diameter.

If this is right

Overall system throughput exceeds 83 percent at the wide-field focus and 76 percent at the integral-field focus.
Large-mirror reflectivity stays above 90 percent in the ultraviolet and above 99 percent in the near-infrared.
Dielectric stacks on smaller mirrors supply both high reflection and spectral tunability without rapid degradation.
Graded-index layers on the lenses overcome the bandwidth and angle limits that restrict conventional multilayer antireflection coatings.

Where Pith is reading between the lines

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

The same size-based split could be tested on other large spectroscopic instruments that share the same wavelength range and optic diameters.
Early coating trials on smaller witness samples would be required to confirm the graded-index process before committing to full-size lenses.
If the graded-index approach succeeds, it would reduce the number of surfaces that must be replaced during the telescope's operational life.

Load-bearing premise

Graded-index antireflective coatings can be manufactured and applied to 1.6-meter lenses while keeping high transmission over the full 370-1600 nm range and at varying angles of incidence.

What would settle it

A test showing that no graded-index coating process reaches the required transmission on a 1.6 m substrate across the band at angles up to 30 degrees, or that the coating degrades under observatory conditions within the expected lifetime.

read the original abstract

The Wide-field Spectroscopic Telescope (WST) is a 12-m class facility designed for simultaneous wide-field multi-object and integral-field spectroscopy across 370-1600 nm, targeting a throughput above 83% its wide-field focus and 76% at its integral field one. Its 13 mirrors and 5 lenses require optimized coatings balancing feasibility, operational needs, and long-term durability. Large mirrors (>2 m) may use enhanced metallic coatings, such as the protected-silver solution from the Vera C. Rubin Observatory, achieving over 90% reflectivity in UV and above 99% in NIR. Smaller mirrors would use Nb2O5/SiO2 dielectric stacks, offering 99% reflectivity, tunable spectral response, and excellent stability. Coating the large lenses (up to 1.6 m) is challenging due to the ultra-broadband range, traditional multilayer antireflective coatings show limitations from manufacturing and incidence-angle effects. Graded-index AR coatings are being explored for superior performance across broad wavelengths and angles.

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.

Desk Editor's Note private letter to a colleague

This is a descriptive engineering note on coatings for the WST that applies existing solutions but supplies no measurements or calculations to support the critical graded-index AR claims for the large lenses.

read the letter

The paper lays out a coating plan for the 12-m WST to reach its throughput targets. Large mirrors would use protected silver like the Rubin Observatory solution, smaller mirrors would use Nb2O5/SiO2 stacks, and the five lenses up to 1.6 m would need graded-index antireflective coatings to handle the 370-1600 nm band.

What it does is match coating types to component scale and requirements in a clear way. The metallic option for mirrors over 2 m gives the needed UV-to-NIR coverage, and the dielectric stacks for smaller optics add stability and tunability. That matching is practical and draws directly from demonstrated work.

The soft spot is the lenses. The text correctly flags that conventional multilayers hit manufacturing and angle-sensitivity limits over this wide band, then pivots to graded-index coatings as the fix. Yet it gives no performance data, deposition references, or arguments showing these coatings can be applied uniformly at 1.6 m diameter while keeping reflectance low across the full range and at the actual incidence angles. The headline throughput numbers rest on that step.

Nothing here is new in coating physics or technique. The work is a strategy document that references prior implementations without adding verification.

This is for instrumentation groups planning large spectroscopic facilities who need a concrete example of how one team is thinking about the coating trade-offs. A reader already working on similar broadband optics might pick up the component-by-component breakdown, but the paper does not move the field forward on its own.

It deserves a serious referee because the topic is relevant to an actual facility proposal and the choices are laid out plainly, even though the evidence for the lens solution is thin.

Referee Report

2 major / 2 minor

Summary. The manuscript presents a coating strategy for the 12-m Widefield Spectroscopic Telescope (WST) to meet throughput targets of >83% in wide-field mode and >76% in integral-field mode over 370-1600 nm. It proposes protected-silver enhanced metallic coatings (modeled on Rubin Observatory) for mirrors >2 m, Nb2O5/SiO2 dielectric stacks for smaller mirrors, and graded-index antireflective coatings for the five lenses up to 1.6 m diameter, while noting manufacturing and angle-sensitivity limitations of conventional multilayers.

Significance. If the graded-index AR coatings can be scaled to 1.6 m apertures with the required broadband performance and uniformity, the strategy would support the WST's high-throughput goals and inform coating choices for other large spectroscopic facilities. The paper supplies no new measurements, error budgets, or scaling calculations, so its contribution is primarily descriptive and dependent on external references and future implementation success.

major comments (2)

[Abstract / lens coatings paragraph] Abstract and lens-coating discussion: The throughput targets (83% wide-field, 76% IFU) are stated to require near-ideal transmission from the five lenses up to 1.6 m, yet the text only notes that graded-index AR coatings 'are being explored' without supplying reflectance curves, deposition-process references, uniformity data, or incidence-angle performance estimates for that aperture size and 370-1600 nm band. This assumption is load-bearing for the headline numbers.
[Coating strategy description] Mirror and overall throughput sections: No quantitative error analysis, coating-loss budget, or end-to-end throughput calculation is provided that combines the cited reflectivities (>90% UV / >99% NIR for protected silver; 99% for dielectric stacks) with realistic contamination, angle, and polarization effects across the 13 mirrors and 5 lenses.

minor comments (2)

The manuscript would benefit from explicit section headings or numbered subsections to improve navigation between mirror and lens discussions.
[lens coatings paragraph] Add at least one reference or citation for any prior successful application of graded-index AR coatings on apertures approaching 1 m or larger.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for their detailed review and constructive comments on our manuscript. The paper presents a coating strategy for the WST based on existing and emerging technologies. We address the major comments point-by-point below, agreeing where revisions are needed to clarify assumptions and limitations.

read point-by-point responses

Referee: [Abstract / lens coatings paragraph] Abstract and lens-coating discussion: The throughput targets (83% wide-field, 76% IFU) are stated to require near-ideal transmission from the five lenses up to 1.6 m, yet the text only notes that graded-index AR coatings 'are being explored' without supplying reflectance curves, deposition-process references, uniformity data, or incidence-angle performance estimates for that aperture size and 370-1600 nm band. This assumption is load-bearing for the headline numbers.

Authors: We acknowledge that the lens coatings are a critical and challenging component. The manuscript is intended as a high-level overview of the coating strategy rather than a detailed technical report on coating performance. Graded-index AR coatings for large apertures are an emerging technology, and specific data for 1.6 m optics in the full 370-1600 nm range are not yet available in the literature. We will revise the abstract and relevant sections to include additional references to ongoing research on graded-index coatings (such as those using atomic layer deposition or sol-gel methods) and to explicitly state that the throughput targets assume successful development and scaling of these coatings. This will better contextualize the assumptions. revision: partial
Referee: [Coating strategy description] Mirror and overall throughput sections: No quantitative error analysis, coating-loss budget, or end-to-end throughput calculation is provided that combines the cited reflectivities (>90% UV / >99% NIR for protected silver; 99% for dielectric stacks) with realistic contamination, angle, and polarization effects across the 13 mirrors and 5 lenses.

Authors: The focus of the paper is on selecting appropriate coating technologies for different optical elements rather than performing a full end-to-end throughput calculation, which would involve detailed ray-tracing and contamination modeling not included in this work. However, we agree that a basic loss budget would be useful. We will add a paragraph in the throughput section providing a preliminary estimate based on the cited reflectivities, with conservative allowances for angle of incidence and contamination effects drawn from similar projects like the Rubin Observatory. We will also note that a more comprehensive error analysis is planned for future detailed design studies. revision: yes

standing simulated objections not resolved

Specific measured reflectance curves, uniformity data, and incidence-angle performance for graded-index AR coatings on 1.6 m diameter lenses over 370-1600 nm, since such coatings have not yet been manufactured at this scale.

Circularity Check

0 steps flagged

No significant circularity; descriptive engineering strategy only

full rationale

The manuscript is a descriptive engineering document outlining coating choices for mirrors and lenses in the WST project. It contains no equations, derivations, fitted parameters, or predictions that could reduce to inputs by construction. References to external solutions (e.g., protected-silver coatings from the Vera C. Rubin Observatory) are independent citations. No self-citations are load-bearing, no ansatzes are smuggled, and no uniqueness theorems are invoked. The throughput targets are stated goals rather than derived results, so the document is self-contained with no circular steps.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

This is an engineering design document focused on practical coating selection. No free parameters, mathematical axioms, or new physical entities are introduced or required for the central claim.

pith-pipeline@v0.9.1-grok · 5733 in / 1156 out tokens · 25753 ms · 2026-06-25T23:15:50.190342+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

6 extracted references

[1]

WST - Widefield Spectroscopic Telescope: motivation, science drivers and top level requirements for a new dedicated facility

1 R. Bacon et al., “WST - Widefield Spectroscopic Telescope: motivation, science drivers and top level requirements for a new dedicated facility”, Proc. SPIE 13094, Ground-based and Airborne Telescopes X, 130941O (2024)

2024
[2]

Vera C. Rubin Observatory - Final Coating Results over the Main Telescope Mirrors

T. Vučina et al., “Vera C. Rubin Observatory - Final Coating Results over the Main Telescope Mirrors”, 67th Annual Technical Conference Proceedings, (2024)

2024
[3]

Thin-film optical filters

A.H. MacLeod, “Thin-film optical filters” 5th Edition, pp188
[4]

Empirical expression for the minimum residual reflectance of normal - and oblique -incidence antireflection coatings

T. V. Amotchkina “Empirical expression for the minimum residual reflectance of normal - and oblique -incidence antireflection coatings”, Applied Optics 47(17) 3109-3113 (2008)

2008
[5]

Grass-like Alumina with Low Refractive Index for Scalable, Broadband, Omnidirectional Antireflection Coatings on Glass Using Atomic Layer Deposition

C. Kauppinen, K. Isakov and M. Sopanen, “Grass-like Alumina with Low Refractive Index for Scalable, Broadband, Omnidirectional Antireflection Coatings on Glass Using Atomic Layer Deposition”, ACS Appl ied Materials and Interfaces 9, 15038−15043 (2009)

2009
[6]

Al2O3 anti-reflection coatings with graded-refractive index profile for laser applications

C. Yin “Al2O3 anti-reflection coatings with graded-refractive index profile for laser applications”, Optical Materials Express 11(3), pp 875-883 (2021)

2021

[1] [1]

WST - Widefield Spectroscopic Telescope: motivation, science drivers and top level requirements for a new dedicated facility

1 R. Bacon et al., “WST - Widefield Spectroscopic Telescope: motivation, science drivers and top level requirements for a new dedicated facility”, Proc. SPIE 13094, Ground-based and Airborne Telescopes X, 130941O (2024)

2024

[2] [2]

Vera C. Rubin Observatory - Final Coating Results over the Main Telescope Mirrors

T. Vučina et al., “Vera C. Rubin Observatory - Final Coating Results over the Main Telescope Mirrors”, 67th Annual Technical Conference Proceedings, (2024)

2024

[3] [3]

Thin-film optical filters

A.H. MacLeod, “Thin-film optical filters” 5th Edition, pp188

[4] [4]

Empirical expression for the minimum residual reflectance of normal - and oblique -incidence antireflection coatings

T. V. Amotchkina “Empirical expression for the minimum residual reflectance of normal - and oblique -incidence antireflection coatings”, Applied Optics 47(17) 3109-3113 (2008)

2008

[5] [5]

Grass-like Alumina with Low Refractive Index for Scalable, Broadband, Omnidirectional Antireflection Coatings on Glass Using Atomic Layer Deposition

C. Kauppinen, K. Isakov and M. Sopanen, “Grass-like Alumina with Low Refractive Index for Scalable, Broadband, Omnidirectional Antireflection Coatings on Glass Using Atomic Layer Deposition”, ACS Appl ied Materials and Interfaces 9, 15038−15043 (2009)

2009

[6] [6]

Al2O3 anti-reflection coatings with graded-refractive index profile for laser applications

C. Yin “Al2O3 anti-reflection coatings with graded-refractive index profile for laser applications”, Optical Materials Express 11(3), pp 875-883 (2021)

2021