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

Enrico Biancalani, Edward Balaban, Ruslan Belikov, Eduardo Bendek, Valeri Frumkin, Israel Gabay, Guangjun Gao, Qian Gong, Christine Gregg, Tyler Groff, Joseph Howard, Omer Luria, Michael McElwain, Lee Mundy, Rachel Ticknor, Sylvain Veilleux, Neil Zimmerman

TL;DR

The paper addresses the challenge of characterizing Earth-like exoplanets by enabling large, stable telescope apertures in space through fluidic shaping of a primary mirror, potentially achieving scale-invariant optics in microgravity. It maps two design pathways—revolutionary fluidic space-assembled mirrors and a JWST-based legacy approach—across four axes (shape, surface continuity, phase of matter, and focal ratio) and outlines tentative post-prime-focus architectures from ~1 m demonstrators to tens-of-meters-class observatories. For spectroscopy, the authors propose a compact dual-configuration spectrograph for Habitable Worlds Observatory that spans 600–1000 nm with interchangeable dispersers, supporting both a low-R integral-field mode and a higher-R single/multi-object mode, while highlighting astro-photonics as a route to compact, high-throughput instrumentation. Pilot simulations using noise models and matched-filter techniques indicate how detector performance and post-processing govern the optimal spectral resolving power for biosignature diagnostics like O2, guiding detector choices such as superconducting photon-counting MKIDs. Collectively, the work outlines a scalable, in-space architectural pathway—combining fluidic primary mirrors and modular spectrographs—to advance exo-Earth biosignature characterization at distant, high-contrast targets.

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.

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

TL;DR

The paper addresses the challenge of characterizing Earth-like exoplanets by enabling large, stable telescope apertures in space through fluidic shaping of a primary mirror, potentially achieving scale-invariant optics in microgravity. It maps two design pathways—revolutionary fluidic space-assembled mirrors and a JWST-based legacy approach—across four axes (shape, surface continuity, phase of matter, and focal ratio) and outlines tentative post-prime-focus architectures from ~1 m demonstrators to tens-of-meters-class observatories. For spectroscopy, the authors propose a compact dual-configuration spectrograph for Habitable Worlds Observatory that spans 600–1000 nm with interchangeable dispersers, supporting both a low-R integral-field mode and a higher-R single/multi-object mode, while highlighting astro-photonics as a route to compact, high-throughput instrumentation. Pilot simulations using noise models and matched-filter techniques indicate how detector performance and post-processing govern the optimal spectral resolving power for biosignature diagnostics like O2, guiding detector choices such as superconducting photon-counting MKIDs. Collectively, the work outlines a scalable, in-space architectural pathway—combining fluidic primary mirrors and modular spectrographs—to advance exo-Earth biosignature characterization at distant, high-contrast targets.

Abstract

| We present a conceptual framework for optically designing space-assembled telescopes whose primary mirror is formed 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. | 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 ~140 integral-field spectrograph and a single- or multi-object spectrograph with on the order of 10. Although ~140 is near-optimal for the absorption -band around 760 nm, higher values of can be utilized with spectral cross-correlation matched-filter techniques to enhance, e.g., HWO's atmospheric characterization capabilities.

Paper Structure

This paper contains 7 sections, 1 equation, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. Fluidic Space Telescopes: Evolution vs Revolution
  3. Pinning, No Spinning: Optics by Fluidic Shaping
  4. FLUTE$\sim$1 & Beyond: An Architectural Compass
  5. Dual Spectrographs $\vert$ Habitability to Biosignatures
  6. Baseline Optical Design & Concept of Operations
  7. Optimal $R$ Region Trade-Off: Pilot Simulations

Figures (6)

  • Figure 2: Diachronic macro-evolution graph of optical telescopes' effective primary aperture's diameter, i.e., accounting for secondary-optics geometrical obscurations. Dashed lines indicate potential or future telescopes. Annotated acronyms and the like for modern telescopes: OAO-2 = Orbiting Astronomical Observatory 2; BTA = Bolshoi Teleskop Alt-azimutalnyi (romanized Cyrillic script, meaning "Large Alt-azimuth Telescope" in Russian); HST = Hubble Space Telescope; LBTI = Large Binocular Telescope Interferometer; VLTI = Very Large Telescope Interferometer; JWST = James Webb Space Telescope; RST = Roman Space Telescope; ELT = Extremely Large Telescope; FLUTE = Fluidic Telescope (with $\sim\!\!1$-m, $\sim\!\!12$-m, and $\sim\!\!50$-m primary apertures); GMT = Giant Magellan Telescope; TMT = Thirty Meter Telescope; HWO = Habitable Worlds Observatory. The minimum cost estimates, adjusted for inflation to recent years, are sourced from the internet.
  • Figure 3: Frame of the video that displays the first lens that was fabricated in outer space, aboard the ISS, thanks to fluidic shaping Bercovici_Fluidic_Technologies_Lab.
  • Figure 4: \ref{['FLUTE-1 Optical Architecture']}) Tentative post-prime-focus optical design layout---in Ansys Zemax OpticStudio---for FLUTE$\sim\!$1, with a prolate-ellipsoidal $\sim\!$1-m primary mirror on the right and, in order, central optical baffle and corrector on the left; rays trace the central field angle. The corrector consists of two holed conical mirrors; the instrumentation package would be connected on the back of the left-most mirror and pick up light from the internal focus. With an effective focal length of $\sim$2.4 m, it is diffraction-limited at 1 µm over a circular field of view of $\sim$5 arcminutes in diameter, with $\sim$97$\%$ fraction of rays unvignetted over this field of view. The diffraction limit here refers to a nominal root-mean-square (RMS) wavefront-error (WFE) threshold of $\sim\!\!\lambda/13.4$ or $\sim$0.0745 waves, which is a standard WFE criterion (fully valid for an ideal clear aperture): i.e., 74.5 nm at a reference wavelength of 1 µm. Fluidic shaping provides a sub-nanometric RMS surface roughness. \ref{['FLUTE-1 Opto-Fluidic Analysis']}) Opto-fluidic analysis of FLUTE$\sim\!$1 undergoing a 10$°$ slew maneuver, in terms of RMS WFE with respect to the optical centroid. At a slew rate of 0.4$°$/min, FLUTE$\sim\!$1 would be diffraction-limited throughout operations; faster slew rates may require active stabilization or solidification---for reference, JWST's requirement is $\sim$2$°$/min. With no fluctuations, the fiducial primary is an ellipsoidal cap. The main fluidic parameters defining the primary mirror's ionic liquid---according to Israel Gabay's purely fluidic underlying model---are: a 5-mm initial film thickness (which varies throughout operations), a surface tension of 50 mN/m, a mass density of 1700 kg/m$^3$, and a kinematic viscosity of 10$^{-4}$ m$^2$/s.
  • Figure 5: \ref{['JWST']}) Optical design layout of the $\sim$6.6-m-aperture JWST, which has a folded three-mirror anastigmat configuration. As for the FLUTE designs in Fig. \ref{['FLUTE-1 Optical Architecture']} and in Fig. \ref{['FLUTE-50']}, JWST has a prolate-ellipsoidal primary mirror---shown without tessellation here---close to a paraboloidal shape, which is a perfect geometrical focuser on-axis at infinity: this deviation helps reduce the coma optical aberration for off-axis field angle points. JWST has an $\sim\!\!f$/20 optical system with a slight field angle bias (with respect to the central field, shown here) to accommodate the fine-steering mirror in front of the tertiary mirror. Its rectangular diffraction-limited field of view at $\sim\!$1 µm is $\sim\!$18$\times$9 arcminutes$^2$. \ref{['FLUTE-50']}) Tentative post-prime-focus optical design layout---in Ansys Zemax OpticStudio---for the $\sim$50-m-aperture FLUTE$\sim$50, with a prolate-ellipsoidal primary mirror, a central optical baffle, and the corrector; rays trace the central field angle. The corrector consists of two holed conical mirrors; the instrumentation package would be connected to the focus behind the secondary mirror. This is an $\sim\!\!\!f$/3 optical system with a much more compact optical folding than JWST in Fig. \ref{['JWST']}, which leaves the primary mirror continuous. On the other hand, it gives less margin of maneuver to correct aberrations for off-axis field angle points: the price to pay is on the diffraction-limited field of view (at 1 µm), which is $\sim$0.8 arcminutes in diameter---a circular field of view---at the current iteration.
  • Figure 6: Dual spectrograph optical design layout (with central-field rays), starting from a design by Qian Gong---in Ansys Zemax OpticStudio. It has fixed front-end (left) and back-end (right) optics, while dispersers are interchangeable. Top: low-$R\!\sim$140 prismatic mode. Bottom: grismatic mode with a moderate $R\!\sim$10$^3$.
  • ...and 1 more figures