NIRPS: High-Precision NIR Spectrograph

• NIRPS is a high-resolution, adaptive-optics assisted near-infrared spectrograph designed for sub-meter-per-second radial velocity measurements of M dwarfs and terrestrial exoplanets.
• It employs few-mode fibers, innovative modal noise mitigation, and cryogenic stability with multi-source calibration to achieve precision below 1 m/s.
• NIRPS advances exoplanet discovery and atmospheric characterization by enabling simultaneous optical/NIR diagnostics in conjunction with instruments like HARPS.

The Near InfraRed Planet Searcher (NIRPS) is a high-resolution, high-stability, fiber-fed, adaptive optics–assisted near-infrared (NIR) echelle spectrograph installed at the ESO 3.6 m telescope at La Silla Observatory. Designed as the NIR complement to the pioneering HARPS instrument, NIRPS is optimized for precision radial velocity (RV) measurements (better than 1 m/s) of low-mass exoplanets, particularly around cool M dwarfs. The system provides simultaneous spectral coverage in conjunction with HARPS, enabling combined optical/NIR diagnostics, high-throughput operation, and precise atmospheric characterization of exoplanets. NIRPS’s design integrates adaptive optics (AO), few-mode fiber technologies, advanced thermal control, and sophisticated noise-mitigation strategies, culminating in a spectrograph capable of breaking the longstanding 1 m/s precision barrier at NIR wavelengths and advancing both exoplanet discovery and atmospheric studies through an extensive Guaranteed Time Observation (GTO) program (Artigau et al., 12 Jun 2024, Bouchy et al., 29 Jul 2025).

1. Instrument Architecture and Technical Advances

NIRPS covers the 0.98–1.8 µm spectral window, spanning the Y, J, and H bands, with high-resolution capabilities: R ≈ 90,000 in High-Accuracy (HA) mode and R ≈ 75,000 in High-Efficiency (HE) mode. The instrument is installed in a cryogenic vacuum vessel at 75 K with stability better than 1 mK, minimizing mechanical and thermal drifts (intrinsic drift measured at ≈0.1 m/s per day) (Artigau et al., 12 Jun 2024, Bouchy et al., 29 Jul 2025). The front end includes a dedicated AO system (Shack-Hartmann WFS, deformable mirror with up to 15×15 actuators operating at 1 kHz), enabling fiber injection into a 0.4″ or 0.9″ acceptance (HA/HE modes). NIRPS uses few-mode octagonal fibers (29 µm core ΔHA; 66 µm core HE) to balance coupling efficiency, throughput, and modal noise control (Blind et al., 2017, Blind, 2022).

Innovative modal noise mitigation strategies are implemented: an AO-assisted tip–tilt scanning of the fiber core and a dedicated fiber stretcher, which dynamically redistributes modal phase relationships, are used in both laboratory and on-sky operations, delivering sub-m/s RV illumination stability (Blind, 2022). The calibration unit features uranium–neon lamps, a Fabry–Pérot etalon (17800 lines across the full range), and is being upgraded with a laser frequency comb to achieve wavelength calibrations at the ≈55 cm/s level (Bouchy et al., 29 Jul 2025). The Hawaii-4RG detector is read using up-the-ramp sampling, reducing effective noise according to $\mathrm{RoN} = \sqrt{245/N_\mathrm{samples} + 100}$ (Bouchy et al., 29 Jul 2025).

2. Scientific Motivation and Objectives

NIRPS is optimized for RV measurements of M dwarfs, which comprise ≈70% of the galactic stellar population and emit the bulk of their flux in the NIR (Artigau et al., 12 Jun 2024, Bouchy et al., 29 Jul 2025). Such stars are prime exoplanet-search targets, as their lower masses and smaller radii increase the RV amplitude and transit depth induced by Earth-mass planets in their short-period habitable zones (Benatti, 2018). However, they present challenges due to magnetic activity and spot-induced jitter, which is mitigated in the NIR owing to lower spot contrast (Benatti, 2018). By providing high photon collection in the relevant bands—and in combination with HARPS for near-continuous optical/NIR coverage—NIRPS facilitates robust detection and mass determination of terrestrial planets, confirmation of transit candidates (e.g., from TESS), and detailed exoplanet atmospheric characterization (Artigau et al., 12 Jun 2024, Bouchy et al., 29 Jul 2025).

The GTO program (720 nights starting April 2023) targets nearby M dwarfs, transit follow-ups, and atmospheric spectroscopy, establishing a large legacy dataset for planet demographics and atmospheric studies (Artigau et al., 12 Jun 2024, Bouchy et al., 29 Jul 2025).

3. Modal Noise, AO-Assisted Few-Mode Fiber Injection, and Throughput

NIRPS employs few-mode fibers (10–35 modes per fiber, determined by $V = 2\pi\,\mathrm{NA}\,a/\lambda$), which offer higher coupling efficiency (≥50% up to I=12) than single-mode fibers, in part due to increased aberration and seeing tolerance. The AO system, operating at 1 kHz, concentrates starlight into the fiber with high Strehl ratios at 1400 nm (measured ~35% on-sky) (Blind et al., 2022, Blind et al., 2017).

Modal noise, arising from speckle and phase instability between guided modes, would otherwise limit RV precision to the 10 m/s level. This is addressed through a combination of AO tip–tilt scanning (modulation at ~20 Hz), a mechanical fiber stretcher (~7 mm dynamic modulation), and a double scrambler (Blind, 2022, Blind et al., 2017). Laboratory and system-level validation shows RV instability is reduced from ~10 m/s (unscrambled) to ≲1 m/s, with further gains realized when combining methods (Blind, 2022).

The overall throughput peaks at ≈13% in the H band, with high coupling efficiency and stability even under variable seeing (Bouchy et al., 29 Jul 2025). The cryogenic design and compact spectrograph minimize instrumental drifts and maximize light concentration (Blind et al., 2017).

4. Radial Velocity Precision, Stability, and Spectrograph Performance

NIRPS breaks the 1 m/s precision barrier at NIR wavelengths, achieving a standard deviation of ≈1.69 m/s and a median internal uncertainty of 55 cm/s in RV measurements (demonstrated on Proxima Centauri) (Mascareño et al., 29 Jul 2025, Bouchy et al., 29 Jul 2025). Short-term (minute to hour-long) sequences yield RV binned dispersions of ~77 cm/s, and multi-week drifts remain below 1 m/s (Bouchy et al., 29 Jul 2025). Data reduction is handled with dual pipelines—the native NIRPS-DRS and APERO—building on heritage from ESPRESSO and exploiting least-squares matching in high-resolution template space (Mascareño et al., 29 Jul 2025, Artigau et al., 12 Jun 2024).

The spectral format comprises 71 cross-dispersed echelle orders, extending from 972.4 to 1919.6 nm. Long-term RV stability benefits from multi-source calibration (FP etalon, uranium–neon, and a pending LFC), 1 mK thermal stability, and mechanical design (optics epoxied with custom thermal recipes) (Bouchy et al., 29 Jul 2025).

The photon-noise-limited RV uncertainty scales as $$\sigma_\mathrm{RV} \propto c/(\lambda\,R\,\sqrt{N})$$ (where $N$ is photon count), favoring high-resolution, high-flux operation in the NIR for M dwarfs (Artigau et al., 12 Jun 2024).

5. Observational Results: Planet Detection and Atmospheric Characterization

NIRPS’s early science results include robust RV detections and atmospheric studies for benchmark systems:

• Proxima Centauri: 420 high-S/N spectra over 159 nights yielded the detection of Proxima b (orbital period $P_b \approx 11.18$ days, $m_b \sin i = 1.055 \pm 0.055\,M_\oplus$), confirmation of Proxima d ($P_d \approx 5.12$ days, $m_d \sin i = 0.260 \pm 0.038\,M_\oplus$), and RV residuals of 80 cm/s post–activity correction—all achieved with lower dispersion than HARPS for this target (Mascareño et al., 29 Jul 2025, Bouchy et al., 29 Jul 2025). Combined analysis with HARPS/ESPRESSO improved detection significance and parameter stability.
• Atmospheric escape from WASP-69b: Three transit observations with NIRPS enabled time-resolved detection of helium 1083 nm absorption (average 3.17 ± 0.05%), dynamic velocity shifts up to –29.5 km/s, and post-egress persistence attributed to a cometary tail. EVE 3D simulations constrained the mass-loss rate to 2.25×10¹¹ g/s and indicated complex wind interactions (Allart et al., 28 Jul 2025). The RM Revolutions technique, operating on disc-integrated cross-correlation functions, enabled detailed reconstruction of the star–planet spin–orbit geometry (ψ ≈ 29°), demonstrating RV precision on par with HARPS.
• Broadband atmospheric studies: Joint NIRPS/HARPS observations have delivered broad wavelength coverage from 378 nm to 1920 nm, facilitating transmission and emission spectroscopy with identification of molecules such as water and helium in exoplanet atmospheres (Bouchy et al., 29 Jul 2025, Allart et al., 28 Jul 2025).

6. Impact, Legacy Programs, and Comparison with Predecessors

NIRPS is the first facility to demonstrate stable sub-m/s NIR RV precision suitable for Earth-mass planet detection around M dwarfs (Artigau et al., 12 Jun 2024, Bouchy et al., 29 Jul 2025). Its performance is attributed to a combination of AO-assisted fiber injection, effective modal noise mitigation, thermal/mechanical stability, and a broad calibration approach.

Relative to previous NIR instruments (e.g., CRIRES, GIANO, CARMENES), NIRPS is distinguished by its high spectral resolution, compact cryogenic design, AO+few-mode fiber implementation, and extended simultaneous coverage with HARPS (Blind et al., 2017, Benatti, 2018). The demonstrated throughput and RV precision in the NIR are substantial improvements over both single-mode and seeing-limited solutions.

Joint operation with HARPS expands science cases to include: robust disentanglement of stellar activity (exploiting wavelength dependence), simultaneous atmospheric and mass characterization, and improved photometric/spectroscopic cross-diagnostics (Benatti, 2018, Bouchy et al., 29 Jul 2025).

The five-year GTO legacy will deliver an unprecedented RV dataset on nearby M dwarfs, providing population statistics and atmospheric spectra essential for future comparative exoplanetology and the design of ELT-class NIR precision spectrographs.

7. Lessons Learned and Future Directions

Commissioning revealed the importance of mechanical tolerancing (notably with the ADC), AO system drift management, and rigorous metrology for fiber/optics alignment (Blind et al., 2022). AO tip–tilt scanning and fiber stretchers are validated as robust strategies for modal noise suppression, especially for few-mode fibers where double scramblers have less impact (Blind, 2022).

Thermal management via epoxy-bonded optics and cryogenic operation is crucial for pixel/line stability and is directly responsible for the system’s long-term RV precision (Bouchy et al., 29 Jul 2025). Integration of a laser frequency comb is expected to boost calibration accuracy to the ∼10 cm/s regime.

In summary, NIRPS’s architecture, throughput, and science output establish a new standard for NIR RV precision and exoplanet characterization, with ongoing legacy programs and instrumental upgrades poised to shape the next decade of small-planet discovery and exoplanet atmospheric research around the Galaxy’s most common stars.