SCALES: Lenslet-Slicer IFS for Exoplanets

• SCALES is a high-contrast integral field spectrograph that combines a lenslet array with an image slicer (‘slenslit’ approach) to deliver both low- and medium-resolution thermal IR spectroscopy.
• Its design features a fully cryogenic, all-reflective optical train and advanced detector readouts to minimize thermal background and maintain high throughput.
• The dual spectroscopic modes and coronagraphic techniques enable precise direct imaging and atmospheric characterization of cold, low-luminosity exoplanets.

Slicer Combined with an Array of Lenslets for Exoplanet Spectroscopy (SCALES) is a high-contrast integral field spectrograph (IFS) optimized for the thermal infrared (2–5 μm) currently under construction for the W. M. Keck Observatory. SCALES is designed to rapidly advance direct exoplanet imaging and atmospheric characterization by operating at spectral resolutions and wavelengths that probe colder and older exoplanets than previous generations of instruments. Its architecture combines a lenslet array with an image slicer—termed the “slenslit” approach (Editor's term)—enabling both low- and medium-resolution spatially resolved spectroscopy in coronagraphic modes that suppress stellar light.

1. Instrument Architecture and Optical Design

SCALES is constructed around a fully cryogenic, all-reflective optical train to minimize thermal background and maximize throughput (Stelter et al., 2020, Kupke et al., 2022). The principal optical subsystems include:

• Lenslet Array: Custom silicon lenslet arrays subdivide the focal plane into micro-pupils (“spaxels”). In the baseline configuration, two lenslet subarrays are used:
  • A 110×110 grid for low-resolution IFS (R ≲ 250), spanning a field-of-view (FoV) of ∼2.2″×2.2″.
  • An 18×18 grid for medium-resolution (R ≈ 3500–7000), routed to the image slicer/slenslit.
• Slicer (Slenslit): The slenslit module optically reformats selected micro-pupils into a pseudo-slit, which is then dispersed by gratings, significantly extending spectral length without introducing additional aberrations (Stelter et al., 2021, Stelter et al., 2022).
• Coronagraphic Masks: Selectable coronagraph masks (focal-plane and Lyot stops), optimized for high-contrast imaging, suppress the on-axis starlight using designs such as vector vortex coronagraphs (Li et al., 2022).
• Imaging Channel: A complementary imaging system (12.3″×12.3″ at 6 mas/pixel) shares the cryogenic bench, enabling wide-field diffraction-limited observations, pupil alignment, and high-resolution acquisition (Banyal et al., 2022).

All major elements—including slicers, lenslets, dispersive elements, and imaging mirrors—are mounted with materials matched for thermal expansion and manufactured to strict interferometric tolerances, utilizing diamond-turned aluminum optics (Kain et al., 2023).

2. Spectroscopic Modes and Field-of-View

SCALES implements dual spectroscopic modes tailored to exoplanet characterization:

Mode	Spectral Resolution (R)	Field-of-View	Disperser
Low-Res IFS	R ≲ 250	∼2.15″ × 2.15″	LiF prisms
Slenslit	R ≈ 3500–7000	∼0.36″ × 0.34″	Gratings

• Low-Resolution Mode: Delivers integrated spectra across broad bandpasses for surveys and continuum features. Each micro-pupil produces a short, ∼60-pixel spectrum, well suited for initial exoplanet detection and broad molecular absorption bands (Sallum et al., 2023, Skemer et al., 2022).
• Medium-Resolution (Slenslit) Mode: Selected spatial elements are reformatted into a pseudo-slit and dispersed across up to 2000 detector pixels, resolving molecular lines and enabling detailed atmospheric modeling (Stelter et al., 2022).

This architecture allows both high spatial fidelity and extended spectral coverage. The dual-lenslet/slicer configuration preserves the minimal aberration characteristics of lenslet IFS while leveraging the increased dispersion afforded by image slicing (Stelter et al., 2021).

3. Thermal Infrared Optimization and Cryogenic Engineering

Operation in the 2–5 μm regime is enabled by a fully cryogenic system with the following key optimizations:

• Cold Stop and Lyot Stop: Precision-machined pupil masks are designed to maximize throughput and suppress unwanted thermal radiation from telescope structures (Li et al., 2021, Li et al., 2022). Optimal mask geometries (serrated hexagonal, undersized Lyot stop) are chosen via modeled signal-to-noise ratio (SNR) maximization.

$$SNR(R) = \frac{T_{\rm signal}(R)}{\sqrt{T_{\rm signal}(R) + B_{\rm noise}(R)}}$$

• Diamond-Turned Optics: Most foreoptics and benches use RSA 6061 aluminum, diamond-turned to <24 nm RMS figure error, ensuring alignment is maintained from room temperature through cooldown. Cryogenic expansion and wavefront error budgets are quantitatively traced by interferometric and power spectral density analysis (Kain et al., 2023).

These design choices maintain diffraction-limited imaging and minimize optical aberrations and sky background, which is crucial for faint exoplanet spectroscopy.

4. High-Contrast Imaging Strategies

SCALES’ coronagraphic capabilities are central to exoplanet isolation:

• Vector Vortex Coronagraphs: Spiral phase ramp masks (topological charge 2) redistribute on-axis starlight, which is then suppressed by downstream Lyot stops (Li et al., 2022). The design yields an order of magnitude improvement in raw contrast at separations from the host star.
• Sparse Aperture Masking (SAM): Non-redundant mask arrays in the pupil plane establish an interferometric baseline set, enabling closure phase and squared visibility extraction with immunity to phase errors. Mask design is performed via the NRM-artist package, and sensitivity is validated by recovery of simulated planet and disk signals through the scalessim and SQUEEZE pipelines (Lach et al., 2023, Lach et al., 2022).

Contrast ratios of $C(\theta) \sim 4 \times 10^{-4}$ (Δm ~8.5) at separations of 0.25″–1″ are feasible under twilight conditions (Kain et al., 16 Sep 2025).

5. Detector Technology and Readout Innovations

Infrared spectroscopy at SCALES’ wavelengths requires advanced detector management:

• H2RG Detectors: The imaging and IFS arms utilize Teledyne H2RG detectors (2040×2040 pixels), originally built for JWST requirements (Banyal et al., 2022). Standard slow mode (100 kHz) would saturate due to high IR backgrounds.
• Hybrid Fast/Slow Readout: SCALES implements a custom buffer cable and firmware supporting SIDECAR ASIC and MACIE controller, enabling pixel clock rates up to 1.8 MHz, reducing full-frame readout times from 10.5 s to ∼0.58 s (Benac et al., 28 Aug 2025). The formula for frame time is:

$$T_{\text{frame}} = \frac{2040 \times 2040}{4 \times 1.8 \times 10^6} \approx 0.58~\text{s}$$

This innovation prevents detector saturation from the infrared sky, preserves dynamic range, and enables time-resolved spectroscopic studies.

6. Data Acquisition, Simulations, and Differential Imaging Techniques

• End-to-End Simulation: The scalessim Python suite models the complete optical train, point spread function (PSF), spectral extraction, and noise, supporting rapid pipeline development, instrument trades, and observation planning (Sallum et al., 2023).
• Differential Imaging: Medium-resolution mode employs angular differential imaging (ADI) and reference differential imaging (RDI) strategies to disentangle exoplanet signals from residual stellar speckles. In simulations, both approaches allow successful recovery of injected planet signals, with comparative efficacy dependent on separation and contrast (Desai et al., 2023).
• Quick-Look Pipeline: Sparse matrix multiplication methods extract spectral cubes from simulated detector frames in sub-second timescales, supporting real-time data quality assessment (Sallum et al., 2023).

7. Observing Modes, Twilight Science, and Broader Applications

• Twilight Observing Program: SCALES is well suited to operate during astronomical twilight due to infrared sky brightness remaining low, enabling “snapshot” surveys of bright stars and Solar System targets within a ∼40 minute morning window. Optimized AO system backgrounds and automated acquisition routines maximize science yield (Kain et al., 16 Sep 2025).
• Science Cases: Key applications include:
  • Atmospheric characterization of exoplanets as cold as ~300 K.
  • Surveys for wide-separation, low-luminosity planets unreachable by transit or RV techniques.
  • Spectroscopic investigations of Solar System bodies, disks, and extragalactic features (Sallum et al., 2023).
• Instrument Development: Early science demonstration surveys are planned for first light (~2025), leveraging the unique spectral and spatial capabilities to paper benchmark systems and extend planetary atmosphere modeling.

References and Key Papers

• “Update on the Preliminary Design of SCALES: the Santa Cruz Array of Lenslets for Exoplanet Spectroscopy” (Stelter et al., 2020)
• “Design of SCALES: A 2-5 Micron Coronagraphic Integral Field Spectrograph for Keck Observatory” (Skemer et al., 2022)
• “Weighing Exo-Atmospheres: A novel mid-resolution spectral mode for SCALES” (Stelter et al., 2022)
• “Fabrication of Pupil Masks for a New Infrared Exoplanet Imager at Keck Observatory” (Li et al., 2022)
• “Simulating medium-spectral-resolution exoplanet characterization with SCALES angular/reference differential imaging” (Desai et al., 2023)
• “Increasing science yield with a twilight observing program with the SCALES instrument at Keck” (Kain et al., 16 Sep 2025)
• “Characterization of diamond-turned optics for SCALES” (Kain et al., 2023)

SCALES’ integrated design—combining a slicer with an array of lenslets, coronagraphic starlight suppression, innovative pupil mask geometry, and fast-readout detectors—positions it as a transformative facility for exoplanet spectroscopy in the thermal infrared at Keck Observatory.