NIGHT Spectrograph: Exoplanet Atmosphere Analysis

• NIGHT spectrograph is a compact, fibre-fed instrument optimized for detecting the helium triplet in exoplanet atmospheres during transit observations.
• It employs a two-channel design with a custom VPH grating to achieve a resolution of ~75,000 over a narrow 1081–1085 nm bandpass for precise, time-resolved measurements.
• Its stable, high-throughput and cost-effective design supports statistical surveys of atmospheric mass loss, complementing space-based observations.

The NIGHT spectrograph ("Near-Infrared Gatherer of Helium Transits") is a compact, fibre-fed, high-resolution astronomical spectrograph optimized for precise and repeatable measurement of the metastable helium triplet (He I 1083.3 nm) during exoplanet transits. Specifically designed to enable a statistical survey of atmospheric escape in highly irradiated exoplanet systems, NIGHT leverages high throughput and spectral stability while offering a cost-effective and flexible deployment profile suitable for 1.5–2 m class telescopes. Its instrumental architecture, performance metrics, and survey strategy are tightly coupled to its core scientific objectives: time-resolved characterization of helium-bearing exoplanet atmospheres and detailed exploration of the “hot Neptune desert” and planetary mass-loss mechanisms (Jentink et al., 2023, Jentink et al., 2024).

1. Scientific Motivation and Survey Scope

The primary science driver for NIGHT is the ground-based detection and temporal monitoring of He I 1083.3 nm triplet absorption as a tracer of hydrodynamic atmospheric escape. The metastable helium state acts as a proxy for extended, escaping atmospheres in highly irradiated exoplanets. Temporal changes in line shape and depth directly probe the variability of mass-loss rates and wind geometry, offering constraints on both planetary evolutionary processes and star–planet interaction dynamics.

The instrument’s survey strategy targets close-in exoplanets (periods ≲30 days, radii ≳1.5 R⊕, host stars J ≲ 10–12), with a focus on planets most susceptible to photoevaporation and core-powered mass loss. By systematically monitoring He I absorption in a large (>100) and homogeneous exoplanet sample, NIGHT aims to quantify which planetary systems are experiencing efficient atmospheric loss, mapping their properties in the radius-irradiation-age parameter space, and thereby illuminating the origin and boundaries of the hot-Neptune desert (Jentink et al., 2023, Jentink et al., 2024).

2. Instrument Architecture and Optical Design

NIGHT employs a fibre-fed, two-channel configuration: one science fibre for starlight and one sky fibre for simultaneous sky subtraction. The front-end module features dual 60 μm-core fibres (F/4, NA=0.125), providing an on-sky sampling of 2″ diameter per fibre when coupled to a 1.5–2 m telescope. Starlight is precisely injected using a near-infrared guiding channel with guiding precision ≲0.1″ RMS and mechanical stability better than 0.05 mm over hours.

The spectrograph itself is bench-mounted within a temperature- and pressure-stabilized vacuum enclosure, which isolates the instrument from environmental fluctuations (thermal/mechanical stabilization to ≲0.1 K over hours, alignment drift ≲10 μm per 10 °C). The vacuum tank provides the thermal inertia required for throughput stability, while allowing the spectrograph to operate predominantly at room temperature—only the HgCdTe HAWAII-1 detector array and a narrow short-pass filter are cryogenically cooled to 85 K (Jentink et al., 2024).

The core dispersive element is a volume phase holographic (VPH) grating (1407 lines/mm, AR-coated for ~90% single-pass efficiency), implemented in a unique double-pass layout. This compact configuration yields high resolving power (R ≈ 75,000) over a narrow bandpass (1081–1085 nm), with a collimated beam diameter illuminating ∼53 mm of the grating and no cross-dispersion—optimal for the helium triplet's isolated spectral location. Imaging onto the 1024×1024 HAWAII-1 array is achieved via off-the-shelf camera singlets, with field flattening for minimal distortion across the ±2 nm science band (Jentink et al., 2024).

3. Throughput, Calibration, and Stability

Optical throughput is maximized by minimizing surface losses (AR-coated interfaces, 99.8% or better per lens), high-reflectivity fold mirrors (99.9% per reflection, six reflections per channel), and efficient VPH grating operation. The cumulative internal throughput (from fibre output to detector) is calculated as ∼71%. Incorporating sky transparency (95%), telescope (70%), injection (∼92%), fibre transmission (82%), sky subtraction, and detector quantum efficiency, the modeled end-to-end throughput is ≈34% at 1083 nm—substantially higher than that of multi-band high-resolution nIR spectrographs (e.g., SPIRou, NIRPS, typically 4–13%) on 4 m telescopes (Jentink et al., 2024).

Wavelength calibration is performed with a Uranium–Neon hollow-cathode lamp, providing a dense line spectrum for channel-by-channel mapping to sub-pixel precision (<50 m/s stability). Telluric OH nightglow lines further refine zero-point tracking. Flat-fielding utilizes a Tungsten–Halogen source. Calibration light is injected via stepper-motor-actuated fibres and a combiner, with positional repeatability better than 5 μm. Instrumental stability requirements for transit spectroscopy are notably relaxed compared to radial velocity work: velocity drift |Δv_drift| ≲ 40 m/s over a full transit is sufficient to keep systematic transmission artifacts below the millipercent level (Jentink et al., 2023).

Component	Internal Throughput [%]	Cumulative Throughput [%]
Spectrograph optics (fibre to detector)	71	71
+ Telescope, sky, injection, sky fibre	–	34

4. Detector System and Noise Performance

The detector subsystem centers on a Teledyne HAWAII-1 1024×1024 HgCdTe array repurposed from the TRIDENT instrument, with operational temperature stabilized at 85 K by an LN₂ bath cryostat. The narrow-band science window (1081–1085 nm) permits the entire spectrograph to remain at room temperature, due to negligible thermal background at these wavelengths. Typical read noise is ≈10 e⁻ rms per single read (further suppressed via up-the-ramp sampling for long exposures), and dark current is ≲0.02 e⁻/s/pixel; for t_exp = 300 s exposures, dark current noise is ≈2.5 e⁻ per pixel—negligible compared to read noise except for the faintest targets or very long integrations.

For a J=11 star on a 2 m telescope, the expected photon flux at the detector is ≈5×10⁵ photon/s, yielding S/N ≈200 per 300 s per resolution element. For bright targets (J<10), single-transit sensitivities to 1% (10 ppt) absorption depths at S/N >5 are achieved (Jentink et al., 2024).

5. Observing Strategy and Sensitivity

The baseline survey allocates ≈70–75 nights per year on a 1.5–2 m telescope, targeting over 100 exoplanets with J<12 and requiring two transits each for 30–40 of the most temporally variable systems. Criteria for target selection include host star brightness (J≲8–10 for high S/N), sufficient transit depth (S/N_transit>700), planet radius (≳1.5 R⊕), and orbital period (≲30 d for favorable observational cadence).

S/N for a single exposure is computed as:

$$\mathrm{SNR}(\lambda) = \frac{F_\star(\lambda)\,\sqrt{\eta\,A\,t_{\rm exp}\,\Delta\lambda}} {\sqrt{F_\star(\lambda) + F_\mathrm{sky}(\lambda) + D + R^2}},$$

where $F_\star$ is the stellar photon flux, $F_\mathrm{sky}$ the background, $D$ the detector dark current, $R$ the read noise, $\eta$ the total throughput, $A$ the telescope area, $t_{\rm exp}$ the exposure time, and $\Delta\lambda$ the width of one resolution element (Jentink et al., 2023).

For a 2 m telescope and typical noise parameters, single-transit sensitivity reaches:

• S/N ≈708 per resolution element for an 8th-mag host (1% helium absorption at 5σ).
• S/N ≈1073 for two-transit difference photometry (0.4% variation at 3σ).

Simulated yield from the set of all known transiting exoplanets (P<30 d, R>1.5 R⊕):

• 118 planets: 1% peak He I absorption detectable at 5σ in a single transit.
• 66 planets: 0.4% temporal changes detectable at 3σ between two transits (Jentink et al., 2023).

6. Implementation, Modularity, and Cost Considerations

NIGHT is engineered as a modular “visitor” instrument optimized for rapid deployment. The optical design is cost-efficient by virtue of the narrow science band: optics and filters are simpler than those in broad-band or cross-dispersed nIR spectrographs. Most optics (collimator, camera lenses, fold mirrors) are off-the-shelf or standard catalog items; the VPH grating is custom, but less expensive than multi-order echelles, and filters are minimal.

The fibre-injection interface is designed for straightforward adaptation to typical F/8–F/10 Cassegrain or Coude foci. Minimal fore-optics and standardized fibre connectors facilitate rapid mounting/dismounting, enabling efficient sharing between observatories or telescopes and maximizing telescope productivity, particularly for follow-up of JWST, TESS, and PLATO targets (Jentink et al., 2023, Jentink et al., 2024).

7. Science Impact and Complementarity

NIGHT’s statistical survey is poised to expand the sample of exoplanets with resolved He I escape signatures from ≲20 to ≳100, significantly improving empirical constraints on atmospheric mass-loss rates versus irradiation, host age, and stellar wind conditions. The high-cadence temporal monitoring capability addresses the core questions of escape variability and wind–magnetosphere coupling.

Its measurements complement space-based ultraviolet (Lyman-α via HST) and mid-infrared (JWST/NIRSPEC) transit observations, allowing joint multi-layer studies of planetary escape physics. Velocity resolution (Δv ≈4.3 km/s for R=70,000) is more than sufficient to trace planetary wind outflows and bulk motions. The anticipated first light was scheduled for 2024, enabling ground-based follow-up campaigns of exoplanets discovered or characterized in the JWST and ongoing transit survey era (Jentink et al., 2023, Jentink et al., 2024).

The instrument’s detailed sensitivity forecasts and modular, high-throughput narrow-band architecture establish a new regime for cost-effective, high-precision exoplanet atmosphere observations from small- to mid-aperture ground-based telescopes.

---

References:

• "NIGHT: a compact, near-infrared, high-resolution spectrograph to survey helium in exoplanet systems" (Jentink et al., 2023)
• "The Near-Infrared Gatherer of Helium Transits (NIGHT)" (Jentink et al., 2024)