Optical Design Pathways to Fluidic Space-Assembled Reflectors & Dual-Configuration Spectrographs for Characterizing Exo-Earths

Abstract: $\textbf{Fluidic Telescopes}$ | We present a conceptual framework for optically designing space-assembled telescopes whose primary mirror is formed $\textit{in situ}$ via the enabling, scale-invariant technology of fluidic shaping. In-space assembly of optical reflectors can solve light-gathering aperture scaling, which currently limits space-borne optical telescopes. Our compass reduces the top-level optical design trade to three types of avenues---a fluidic pathway, a legacy one building upon the James Webb Space Telescope, and hybrid solutions---with a focus on exo-Earths. A primarily fluidic pathway leads, in the first place, to a post-prime-focus architecture. We apply this configuration to propose the tentative optical design for a ~1-m technology demonstrator and pathfinder for fluidic-telescope apertures scaling up to many tens of meters in diameter. $\textbf{Dual-Configuration Spectrographs}$ | The Habitable Worlds Observatory (HWO) will be the first mission equipped for the high-contrast direct imaging and remote spectral characterization, in reflected starlight, of exo-Earths in our galactic neighborhood. We present a novel concept for a compact, dual-configuration HWO spectrograph tailored for a broad wavelength range covering at least 600--1000 nm. Our design can interchange dispersive elements via a slider mechanism while preserving the rest of the optical path, enabling both a spectral resolving power $R$~140 integral-field spectrograph and a single- or multi-object spectrograph with $R$ on the order of 10$^3$. Although $R$~140 is near-optimal for the $O_2$ absorption $A$-band around 760 nm, higher values of $R$ can be utilized with spectral cross-correlation matched-filter techniques to enhance, e.g., HWO's atmospheric characterization capabilities.

• The paper demonstrates that fluidic shaping enables large, space-assembled reflectors that overcome gravity limitations for telescope design.
• It employs fluid pinning to form smooth optical surfaces, integrating interchangeable prism and grism elements for flexible spectrographic analysis.
• Pilot simulations reveal that adjustable spectral resolution improves exo-Earth biosignature detection, emphasizing the need for enhanced detector performance.

Optical Design Pathways to Fluidic Space-Assembled Reflectors & Dual-Configuration Spectrographs for Characterizing Exo-Earths

Introduction

The scientific paper "Optical Design Pathways to Fluidic Space-Assembled Reflectors & Dual-Configuration Spectrographs for Characterizing Exo-Earths" presents a novel approach to telescope design, emphasizing fluidic shaping technologies and dual-configuration spectrographs. The focus is on addressing the challenges associated with the characterization of exo-Earths using innovative optical architectures and spectrographic techniques.

Fluidic Space Telescopes: Evolution vs. Revolution

The paper explores the concept of fluidic shaping, where in-space assembled telescopes leverage the scale-invariant properties of fluids to form large optical components for space telescopes. This approach aims to overcome the limitations imposed by gravity on Earth, which currently restrict the diameter of deployable space telescopes.

Figure 1: Diachronic macro-evolution graph of optical telescopes' effective primary aperture's diameter.

Optics by Fluidic Shaping

The fluidic shaping technique involves pinning a liquid to a geometrical boundary, allowing surface tension to form smooth optical surfaces naturally. This method enables the creation of both refractive and reflective optical components. The potential of fluidic shaping is demonstrated by successful lens fabrication aboard the International Space Station (ISS).

FLUTE$\sim$1 {content} Beyond: An Architectural Compass

Fluidic shaping offers a scale-invariant method that could disrupt traditional telescope designs by enabling large-scale, space-assembled reflectors that don't require the extensive folding mechanisms like JWST. The proposed architecture for FLUTE$\sim$1 is a strategic step in demonstrating the feasibility of this approach.

Figure 2: FLUTE$\sim$1 Optical Architecture - Tentative post-prime-focus optical design layout.

Dual Spectrographs: Habitability to Biosignatures

The paper also introduces a dual-configuration spectrograph designed for the Habitable Worlds Observatory (HWO). This spectrograph can switch between low and moderate spectral resolving power modes, enabling comprehensive characterization of exo-Earths by capturing both habitability indicators and potential biosignatures.

Figure 3: Dual spectrograph optical design layout, showcasing low-$R\sim$140 prismatic mode and Grismatic mode with moderate $R\sim 10^3$.

Baseline Optical Design & Operations

The optical design of the dual spectrograph leverages both prism and grism elements to achieve different spectral resolutions. The modular design allows for interchangeable dispersive elements via a slider mechanism, optimized for a reference wavelength of 760 nm. This adaptability facilitates robust exoplanetary spectral analysis under varying observational conditions.

Optimal $R$ Region Trade-Off: Pilot Simulations

The paper conducts pilot simulations to identify the optimal spectral resolving power $R$ for detecting biosignatures in exoplanet atmospheres under different noise conditions. The findings suggest that higher $R$ values, although beneficial, demand improvements in detector technology to minimize dark current and read noise impacts.

Figure 4: Simulated matched-filter S/N over 1000 hours as a function of $R$, dark current, and read noise for modern exo-Earth observations.

Conclusion

The research presents a forward-looking framework for deploying next-generation telescopes using fluidic shaping technology, potentially offering significant advances in mirror scalability and optical performance. Furthermore, the dual-configuration spectrographs propose a flexible solution for both initial surveys of habitable exoplanets and detailed biosignature analyses. As astronomical technology evolves, these concepts could redefine observational capabilities and open new frontiers in exoplanet research.