---
title: 'NIRPS: High-Precision NIR Spectrograph'
url: https://www.emergentmind.com/topics/near-infra-red-planet-searcher-nirps
type: topic
---

# NIRPS: High-Precision NIR Spectrograph

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 [2406.08304][2507.21767].

## 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) [2406.08304][2507.21767]. 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 [1711.00835][2207.14136].

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 [2207.14136]. 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 [2507.21767]. The Hawaii-4RG detector is read using up-the-ramp sampling, reducing effective noise according to \( \mathrm{RoN} = \sqrt{245/N_\mathrm{samples} + 100} \) [2507.21767].

## 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 [2406.08304][2507.21767]. 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 [1808.02302]. However, they present challenges due to magnetic activity and spot-induced jitter, which is mitigated in the NIR owing to lower spot contrast [1808.02302]. 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 [2406.08304][2507.21767].

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 [2406.08304][2507.21767].

## 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) [2207.14143][1711.00835].

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 [2207.14136][1711.00835]. 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 [2207.14136].

The overall throughput peaks at ≈13% in the H band, with high coupling efficiency and stability even under variable seeing [2507.21767]. The cryogenic design and compact spectrograph minimize instrumental drifts and maximize light concentration [1711.00835].

## 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) [2507.21751][2507.21767]. Short-term (minute to hour-long) sequences yield RV binned dispersions of ~77 cm/s, and multi-week drifts remain below 1 m/s [2507.21767]. 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 [2507.21751][2406.08304].

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) [2507.21767].

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 [2406.08304]. 

## 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 [2507.21751][2507.21767]. 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 [2507.21284]. 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 [2507.21767][2507.21284].

## 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 [2406.08304][2507.21767]. 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 [1711.00835][1808.02302]. 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 [1808.02302][2507.21767].

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 [2207.14143]. 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 [2207.14136]. 

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 [2507.21767]. 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.

Source: https://www.emergentmind.com/topics/near-infra-red-planet-searcher-nirps