WST, the Wide-field Spectroscopic Telescope: Mechanical Design and FE Analyses for the High Resolution Spectrograph

Abstract: The Wide-field Spectroscopic Telescope (WST) is a planned 12-meter-class dedicated spectroscopic facility designed to address key scientific challenges through large spectroscopic surveys. This paper presents the current status of Work Package 4.5, which focuses on the High-Resolution Multi-Object Spectrograph (MOS-HR) module for WST. The MOSHR instrument is expected to provide a resolving power of R = 40,000 with a multiplexing capability of about 2,000 targets. The mechanical design activities carried out for the development of the HR spectrograph and for the definition of its optomechanical architecture are described. To account for both the scientific requirements of the spectrograph and the manufacturability constraints associated with such a complex instrument, the mechanical layout has been organized into four larger modules, each containing two sub-modules. Guided by feasibility considerations, such as mechanical performance, available volume, and fabrication and assembly aspects, each sub-module adopts a vertical optical bench configuration with optical elements mounted on both sides. Starting from the baseline optical design, the mechanical configuration has been developed to achieve the required alignment accuracy, structural stability, and environmental robustness. The workflow includes the translation of the optical prescription into a complete mechanical model, the definition of the main mounting and alignment interfaces, and preliminary static, modal, and seismic analyses to evaluate performance under operational and survival loads. As an outcome, the proposed design provides architecture that enables preliminary estimates of mass, volume, cost, and mechanical performance in terms of deformation, stress, and modal behavior of the modules.

• The paper presents a robust design for the WST MOS-HR spectrograph, achieving a resolving power of R=40,000 and high multiplexing via a modular vertical bench architecture.
• It details an integrated optical-mechanical strategy that includes a reflective collimator, a pseudo-slit assembly, and a double-skin, rib-reinforced structure to ensure precise alignment and stability.
• Finite Element analyses validate the design through static, modal, and seismic evaluations, confirming that deformation and vibrational sensitivities remain within acceptable operational ranges.

Mechanical Design and Finite Element Analyses of the WST High-Resolution Spectrograph

Introduction

The design of the High-Resolution Multi-Object Spectrograph (MOS-HR) for the Wide-field Spectroscopic Telescope (WST) is driven by the dual imperative of achieving high optical performance and mechanical robustness within the tight spatial and operational constraints of a 12-m class wide-field survey facility. The MOS-HR is tasked with delivering a resolving power of $R = 40,000$ and a multiplexing capability of approximately 2,000 simultaneous targets. This essay provides a detailed, technically focused overview of the proposed opto-mechanical architecture, the underlying mechanical design philosophy, and the preliminary structural verification activities as described in "WST, the Wide-field Spectroscopic Telescope: Mechanical Design and FE Analyses for the High Resolution Spectrograph" (2607.01132).

Optical and Mechanical Architecture

The optical concept for MOS-HR utilizes a reflective collimator with a 1-m spherical mirror and aspherical corrector, coupled with a folded optical path compactified via a 45-degree flat mirror containing a central aperture for the pseudo-slit. The pseudo-slit, approximately cylindrical (266 mm length, 2–3 mm thick), incorporates a finely packed linear fiber arrangement (pitch: 100 μm, core: 73 μm), utilizing fiber slicing and empty pitches to limit spectral crosstalk. The spatial and functional replication required for 2,000-object multiplexing mandates a modular approach, with each spectrograph module handling $\sim$330 fibers.

The mechanical solution adopts a vertically oriented optical bench architecture. Each module comprises two sub-modules sharing a common structural core, with optical elements populated symmetrically on both sides. This configuration optimizes platform footprint and mass distribution while maximizing accessibility to critical alignment and integration interfaces. Structural considerations are dominated by the mass and inertia of large optical components, the compact central zone accommodating the slit and mirror, and the need to accommodate cryogenic and detector interfaces within strict volume limits.

Modularization and Integration Strategy

Modularity is a central tenet, both for scalability (required by high multiplexing) and for practical integration and maintenance. Inspired by precedent (MOONS/VLT), the system designates each double module as an independent, factory-alignable and testable entity. Vertical orientation of the benches creates a more compact envelope and facilitates interface management. The bench itself is realized as a lightweight double-skin shell reinforced internally by a regular rib network, targeting high bending and torsional stiffness relative to mass, and allowing clear passage for cabling and auxiliary subsystems.

The handling of the pseudo-slit region is of particular note: the folding mirror's central aperture, through which the slit assembly passes, represents a critical alignment and load interface, and concept designs for this subsystem already include integrative features such as a compact slit shutter/back-illumination module necessary for calibration and alignment.

Structural Assessment: Static, Modal, and Seismic Analyses

A first-principles structural evaluation is conducted via simplified Finite Element (FE) modeling (using COMSOL Multiphysics), focusing on the core vertical bench architecture. The FE mass model includes lumped/differentiated loading based on as-designed optical and mechanical part estimates, using conservative upscaling derived from analogs in existing large-scale spectrographs and instrument best practice.

Static analysis investigates gravitational deformation, maximum external skin deflection, and critical relative displacement metrics. The double-skin plus rib approach yields bending/torsional deformation envelopes compatible with preliminary alignment and image stability tolerances associated with high-resolution fiber-fed echelle spectrographs.

Modal analysis identifies principal eigenmodes dominated by large-scale bending and torsion of the vertical bench, reinforcing the necessity for robust internal ribbing. The lowest eigenfrequency (mode at $\approx$10 Hz) demonstrates a dynamic stiffness profile typical for monolithic large-instrument modules, setting a reference point for further optimization, especially regarding vibration isolation versus telescope-induced excitation frequencies.

The preliminary structural results support the viability of the vertical bench/double-module configuration as a scalable architectural solution and affirm that deformation and vibrational sensitivity are—at this stage—within expected operational ranges for the high multiplexing, high-resolution spectroscopic context.

Implications and Future Directions

The presented work establishes a reference architecture for high-density, high-stability spectroscopy on large telescopes. Practically, the modular vertical bench solution enables distributed manufacturing, streamlined integration at the telescope platform, and operational maintainability—key for massively multiplexed systems. The FE analyses, though simplified, reveal primary load vectors and deformation modes, providing guidance for the next round of detail design, particularly in the refinement of critical optical/mechanical interfaces and the introduction of thermal/thermo-mechanical modeling.

Specifically, future work must address:

• Detailed interface modeling for refined stress, deformation, and optical tolerance propagation
• Robust thermo-mechanical modeling in context of the telescope environment and instrument duty cycle
• Higher-fidelity modal analyses and interface-level seismic design, particularly under realistic support constraints
• Optimization of modular design for further mass reduction and assembly simplification
• Development of system-level tolerance budgeting for combined optical and mechanical errors

Given the complexity and demands of next-generation spectroscopic surveys, the design philosophy exemplified here—combining stringent mechanical engineering, explicit modularity, and rigorous early-phase structural analysis—will inform similar initiatives in ELT-class, space-based, and highly multiplexed instrumentation.

Conclusion

The mechanical and structural design pathway for the WST MOS-HR spectrograph—as elucidated in the referenced work—demonstrates a coherent approach to the challenge of combining high optical resolving power, extreme multiplexing, and mechanical integration within a compact and manufacturable envelope. Vertical optical benches, modularization, and cautious, FE-informed preliminary assessment establish a viable baseline for further maturation. Progress in this direction provides not just a framework for MOS-HR, but a reference for future developments in high-throughput, high-stability spectroscopic instrumentation for astrophysical survey science.