Design and development of a near-IR integral field spectrograph for the HWO Coronagraph Instrument

Abstract: The primary mission of the Habitable Worlds Observatory (HWO) is to identify and characterise potentially habitable worlds. Spectra across a wide wavelength range are needed to cover multiple spectral features per molecule of interest. An integral field spectrometer (IFS), fed by a coronograph system, can be used to measure spectra from any planets within the nulled field of the coronograph, while also characterizing the residual speckles as a function of wavelength, enabling the contrast ratio to be further enhanced. We present design trades for an infrared IFS (0.8 to 1.7 μm) for the HWO Coronagraph Instrument, including assessment of the relative merits of lenslet and image slicer based architectures. Key requirements include full sampling of the speckle field at all wavelengths, maximized optical throughput, and control of spectral cross talk and stray light. We identify technology developments needed to advance the instrument design to the required technology readiness level.

• The paper outlines the design and trade studies for a near-IR IFS, emphasizing lenslet architectures for enhanced throughput in low-flux regimes.
• It compares lenslet and image slicer approaches, evaluating trade-offs in sensitivity, field-of-view, and manufacturability under stringent space conditions.
• The study employs innovative sampling strategies and anamorphic magnification to optimize spectral performance for detecting biosignatures in exoplanet atmospheres.

Near-Infrared Integral Field Spectrograph Design for the HWO Coronagraph Instrument

Scientific Motivation and Instrument Requirements

The Habitable Worlds Observatory (HWO) is architected to identify and characterize potentially habitable exoplanets, with a particular emphasis on Earth analogs orbiting FGK-type stars. The principal driver for the instrument design is the detection and interpretation of atmospheric biosignatures through simultaneous, broad-wavelength spectroscopy. No individual atmospheric signature unambiguously implies biosignature gases; hence, the requirement is set for spectral coverage from approximately 0.3 to 1.7 μm. This continuous coverage enables the simultaneous observation of absorption bands of molecular species such as O$_2$, O$_3$, H$_2$O, CH$_4$, and CO$_2$ across multiple passbands, drastically reducing the ambiguity associated with single-band analysis.

A coronagraph enables suppression of host star light, achieving contrast levels on the order of $10^{10}$ and allowing for spatially resolved spectroscopy of faint planetary companions. The integral field spectrograph (IFS), integrated after the coronagraph, performs three crucial roles: (1) spatially resolved planetary and disk spectroscopy, (2) characterization of residual speckle noise as a function of wavelength, and (3) the potential for simultaneous multi-planet or disk-planet system studies.

Key requirements for the NIR IFS are:

• Wavelength range: 0.8 to 1.7 μm, divided into ~20% passbands aligned with coronagraph transmission.
• Spectral resolution: Nominally $R = 100$, balancing biosignature detectability with photon-limited sensitivity.
• Field of view: $\sim40~\lambda/D$, driven by coronagraphic dark hole area.
• Detector format: $\geq2\text{k}\times2\text{k}$ arrays, guided by sampling requirements at the shortest wavelength.
• Spaxel size: Max $\sim15$ mas/pixel, ensuring Nyquist sampling at 900 nm on a 6-m aperture.

Architecture Trade Study: Lenslet versus Image Slicer

Two main architectures are evaluated for the IFS:

Lenslet Array: Each lenslet samples a spaxel and forms a micro-pupil image, generating a sparse grid of input beams for the spectrograph. Advantages include insensitivity to post-lenslet aberrations and reduced optical surfaces, directly translating to higher throughput — a critical metric for HWO's low-flux regime. Limitations include inefficient detector usage due to the spatial separation of spectra and potential difficulty in minimizing cross-talk without specialized interventions (e.g., BIGRE dual-lenslet arrays).

Image Slicer: The focal plane is dissected into slices, re-imaged as a continuous pseudo-slit at the spectrograph entrance. This architecture allows for high packing efficiency, potentially enabling a larger field or higher spectral resolution per detector area. Space heritage is robust — notably, JWST MIRI and NIRSpec. However, complexity in manufacture and alignment, especially meeting HWO-centric stray light and surface roughness specifications, is nontrivial. Advanced fabrication techniques (e.g., ultrafast laser-etched glass, hydroxide catalysis bonding) are proposed but not yet validated for space-qualification at the required precision.

An explicit decision is made to focus design trades on the lenslet architecture, as field size and packing do not currently dominate sensitivity requirements. The hybrid "mirrorlet" concept, combining aspects of both architectures to achieve maximal detector economy, is earmarked for future study.

Baseline Optical Design and Performance Optimization

A working concept, adapted from the PISCES spectrograph, establishes a refractive design employing high-purity CaF$_3$0, BaF$_3$1, and fused silica doublets, with compound prism dispersion (fused silica and ZnS compensator) to mitigate resolution diversity across bands. The design supports diffraction-limited imaging and is compatible with both lenslet and slicer IFUs. Radiation hardening of optics is considered via material selection, referencing empirical survivability studies in simulated space environments.

Cross-talk minimization is tackled using micro-pupil masking (pinhole arrays), and the potential adoption of the BIGRE dual-lenslet approach – shown to suppress spectral contamination efficiently at the cost of increased component complexity.

Sampling Strategies Across Broad Wavelength Coverage

Fixed spatial sampling, determined by the shortest wavelength, leads to oversampling at longer wavelengths, degrading sensitivity due to increased read noise from additional detector pixels. To address this, three strategies are to be assessed:

1. Single fixed spaxel scale: Simpler but incurs SNR penalties at long wavelengths.
2. Interchangeable lenslet arrays: Permits optimal sampling for distinct wavelength bands, but demands precise mechanical positioning.
3. Interchangeable relay optics: Provides different plate scales without moving the lenslet array; this relaxes alignment tolerances but complicates optomechanical integration.

Detailed wavefront and detector noise modeling, incorporating HARMONI/ELT heritage, will guide the selection process. The potential of oversampled data for post-acquisition rebinning is explicitly conditioned on ultra-low detector read noise performance.

Signal-to-Noise Maximization: Anamorphic Magnification

A critical concern is to minimize spectral width in the non-dispersion direction, thereby concentrating collimated light onto the minimum detector real estate. Approaches leveraging prism-induced or optically-engineered anamorphic magnification — via toroidal or cylindrical surfaces in collimator/camera or even the lenslet array itself — are proposed. This is technically aggressive due to non-circular symmetry requirements and places stringent demands on manufacturing and alignment. Evaluation criteria include optical performance under diffraction, compatibility with interchangeable sampling, and manufacturability, especially with advances in additive micro-manufacturing (e.g., two-photon polymerization).

Path Forward and Technology Development

Instrument sensitivity is ultimately limited by detector performance at single-photon flux levels relevant for exoplanetary reflected light. The initiative includes developing an ultra-low background testbed for characterizing candidate NIR detectors, such as avalanche photodiode arrays. The study further anticipates design scaling for larger apertures (future HWO architectures up to 10 m primaries) and advanced NIR detector arrays (up to 4k$_3$24k), as these directly impact characterization yield for Earth analogs.

Design trades on lenslet configuration, sampling optimization, and spectral concentration will be coupled with prototyping efforts, including novel lenslet array manufacturing. The baseline will iterate as empirical detector performance and technological readiness mature.

Conclusion

This study details the foundational design considerations, trade studies, and technology drivers for a near-IR integral field spectrograph optimized for exoplanet spectroscopy with the HWO Coronagraph Instrument. The focus on broad simultaneous wavelength coverage, alignment of sampling strategies to instrumental and astrophysical constraints, and strategies for maximizing signal-to-noise ratio constitute the core pillars of the design approach. Emphasis is placed on hardware prototyping and verification, with explicit recognition of the pivotal role of detector performance and opto-mechanical manufacturability. The approaches and trade-offs outlined will inform future IR-IFS conceptual baselines on HWO and set benchmarks for analogous instruments targeting high-contrast, low-light exoplanetary research.