Medium Resolution Spectrometer Overview

Updated 20 November 2025

Medium Resolution Spectrometers are defined by their ability to achieve resolving powers between ~1,000 and 10,000, effectively isolating key spectral features for diverse applications.
They employ multi-channel IFUs, image slicers, reflective gratings, and dichroic filters to cover broad wavelength ranges with high calibration accuracy.
Instruments like JWST/MIRI MRS and LAMOST-MRS integrate advanced calibration and data cube reconstruction methods, enhancing studies of astrophysical diagnostics and temporal phenomena.

The term Medium Resolution Spectrometer (MRS) refers to a class of spectrographs designed for applications requiring a spectral resolving power intermediate between low- and high-resolution instruments. The MRS achieves instrument resolving power in the range of approximately $R = \lambda / \Delta\lambda \sim 1\text{,}000$ – $10\text{,}000$ , allowing it to isolate atomic and molecular spectral features in a broad set of astrophysical, planetary, and time-series contexts. This article focuses on the current state-of-the-art implementations, with particular emphasis on the Mid-Infrared Instrument (MIRI) MRS on the James Webb Space Telescope (JWST), and contrasts its instrumental characteristics and calibration with ground-based systems such as LAMOST-MRS in optical regimes.

1. Instrumental Architecture and Operational Modes

The MRS realizes its spectral coverage and resolving power through a combination of multi-channel integral field units (IFUs), grating/dichroic selector wheels, and dedicated detector arrays. On JWST/MIRI, the MRS is a four-channel IFU spectrograph covering $4.9$– $28.8\,\mu$ m, with each channel further segmented into three sub-bands (A/B/C: SHORT/MEDIUM/LONG), yielding twelve discrete spectral bands in three exposures (Wells et al., 2015, Argyriou et al., 2023, Jones et al., 2023, Crouzet et al., 15 Apr 2025, Law et al., 23 Sep 2024).

Each MRS channel on MIRI incorporates:

Image slicers that format the incoming field (3″×3.7″ to 7.2″×7.9″ bandwidth-dependent FoV) into slices for dispersion.
First-order reflective diffraction gratings (mounted on wheel mechanisms for sub-band selection).
Dichroic filters to split overlapping orders.
Two 1024×1024 Si:As BIB detectors (MIRIFUSHORT and MIRIFULONG for channels 1–2 and 3–4, respectively).

For the LAMOST-MRS, 16 fiber-fed spectrographs are deployed at the prime focus of a 4-m Schmidt telescope. Each spectrograph is split into blue (495–535 nm) and red (630–680 nm) arms using a dichroic, each arm delivering $R\sim7500$ via volume-phase holographic gratings to dedicated 4k×4k CCDs (Zong et al., 2020, Liu et al., 2020).

2. Spectral Coverage and Resolving Power

The MRS’s defining characteristic is its ability to deliver resolving power sufficient to distinguish key astrophysical diagnostics without compromising survey efficiency.

JWST/MIRI MRS resolving power is empirically characterized by $R(\lambda) \approx 4603 - 128\,\lambda [\mu\text{m}]$ (approximate range $R\sim4000$ at 5 μm to $R\sim1500$ at 28 μm). The spectral coverage of each sub-band and their resolving powers are given in the table below (Jones et al., 2023, Labiano et al., 2021, Wells et al., 2015).

Channel / Band	Wavelength Range (μm)	R (approx)
1A	4.9–5.8	3600–3800
1B	5.6–6.7	3500–3700
1C	6.5–7.7	3400–3600
2A	7.5–8.9	3300–3500
2B	8.6–10.3	3000–3300
2C	9.9–11.9	2800–3100
3A	11.5–13.7	2600–2900
3B	13.2–15.8	2300–2700
3C	15.3–18.2	2000–2400
4A	17.5–21.1	1800–2100
4B	20.4–24.7	1600–1900
4C	23.8–28.8	1400–1700

LAMOST-MRS achieves $R\approx7500$ in both blue and red arms, over narrower spectral windows. The spectral resolution corresponds to FWHM of $0.67\,\mathrm{Å}$ (blue) and $0.84\,\mathrm{Å}$ (red), yielding internal uncertainties in stellar parameters ( $T_{\rm eff}$ : 100 K, $\log g$ : 0.15 dex, [Fe/H]: 0.09 dex, RV: 1 km s $^{-1}$ at $\mathrm{S/N}=10$ ) (Zong et al., 2020).

3. Calibration, Data Processing, and Systematics

a. Fringing and Flat-Fielding

MRS systems—particularly those using Si:As detectors—are susceptible to high-frequency spectral fringes. In JWST/MIRI, periodic modulations arise from two low-finesse Fabry–Pérot cavities in the detector stack (AR coating–buried contact, and buried contact–pixel metallization) and a third, higher-frequency component from upstream dichroics (Crouzet et al., 15 Apr 2025).

The JWST Science Calibration Pipeline employs a two-step fringe-removal protocol. A static fringe flat, modeled as the product of two low-finesse etalons (Airy function expansion for $F \ll 1$ ) and fit to extended-source NGC 7027 mosaics, reduces peak fringe amplitudes from $2$– $15\%$ to sub-percent levels for extended sources.
For point sources, after the static flat, residuals of $1$– $5\%$ persist (depending on dither and spaxel), which are mitigated to $1$– $2\%$ (per spaxel, after two-dimensional residual correction).
Bands 1A–1B remain limited by ground-test flats, and a third (dichroic) fringe is only partially corrected (Crouzet et al., 15 Apr 2025).

b. Geometric and Wavelength Calibration

MRS instruments require detailed distortion correction for high-fidelity 3D cube reconstruction. On JWST/MIRI:

Distortion is modeled by per-slice, 2D polynomials mapping detector coordinates to local MRS $(\alpha, \beta, \lambda)$ .
Mapping to sky coordinates (V2, V3) is realized via a second-order polynomial.
Pipeline reference files provide $<0.1$ resolution element astrometric accuracy, with RMS $<23$ mas at $28\,\mu$ m. Total per-exposure astrometric uncertainty is estimated at $\sim50$ mas (Patapis et al., 2023, Argyriou et al., 2023).
Wavelength calibration on JWST/MIRI uses a combination of ground-based etalon spectra, sharp cutoff references, and on-orbit observations of unresolved emission lines, yielding mapping accuracy better than 0.1 pixel and line centroid accuracy of $<30$ km s $^{-1}$ (Labiano et al., 2021, Jones et al., 2023).

4. Data Cube Construction and Error Propagation

3D-Drizzle algorithms (as implemented for JWST/MIRI MRS) reconstruct data cubes by evaluating separate 1D (spectral) and 2D (spatial) overlaps between detector pixels and target voxels (Law et al., 2023). The mathematical mapping employs an overlap matrix $W[i,j]$ :

$F[j]=\sum_i f[i] W[i,j]$ for voxel radiance.
Variances and covariances scale according to the pixel mixing induced by the drizzle kernel.
Empirical multiplicative factors (1.5–3.0) account for increased variance in 1D spectra extracted from cubes (due to covariance between voxels).

Undersampling in spatial/spectral axes produces resampling artifacts (modulations up to $20\%$ in single-spaxel spectra); four-point dithering and extraction radii $>1.5\times$ FWHM suppress artifacts below $1\%$ .

5. Photometric, Spectral, and Temporal Performance

Photometric calibration of JWST/MIRI MRS is tied to high-S/N (600–1000) observations of standard stars, with 5–18 μm absolute accuracy $<1\%$ and 18–28 μm bands calibrated to $2$– $3\%$ (degrading to $5\%$ at the long end). Extended source calibration is cross-validated against Cassini/CIRS and Voyager/IRIS, agreeing at the $<5$ – $6\%$ level (Law et al., 23 Sep 2024).

Sensitivity reaches $5\sigma$ line flux limits on point sources of $10^{-19}$ – $10^{-20}$ W m $^{-2}$ in $10\,000$ s across channels (Bonato et al., 2017), with repeatability $<1\%$ below $18\,\mu$ m and S/N $\sim330$ (at $5\,\mu$ m) to $\sim10$ (at $28\,\mu$ m).

In time-series modes (e.g., exoplanet transit spectroscopy), the MRS achieves nearly photon-limited performance in $5.2$– $28\,\mu$ m, with temporal Allan deviation slopes consistent with white noise ( $m\approx-0.5$ ) and residual noise within $1.5\times$ of the photon noise (Deming et al., 22 Jul 2024).

LAMOST-MRS delivers radial velocity precision of $1$ km s $^{-1}$ (co-evaluated with Gaia and APOGEE), and element abundance accuracy competitive with other medium-resolution facilities, enabled by stable calibration and a high-throughput, multi-object approach (Zong et al., 2020, Liu et al., 2020).

6. Scientific Applications

JWST/MIRI MRS has established new standards for spatially resolved 3D mid-infrared spectroscopy. Key applications include:

Detection and abundance analysis of atomic/molecular lines in planetary nebulae, H II regions, protoplanetary disks, and AGNs.
Unraveling dust chemistry (e.g., SiC, PAHs, fullerenes), with long-term stability confirmed via invariant SiC emission in SMP LMC 058 over 17 years (Jones et al., 2023).
Direct exoplanet characterization and molecular mapping (e.g., H $_2$ O, CO, CH $_4$ , NH $_3$ bands) via cross-correlation and likelihood-ratio tests in resolved systems (Patapis et al., 2021).
Precision time-series exoplanet and stellar eclipse spectroscopy, achieving the spectral stability levels required to search for CO $_2$ signatures in temperate exoplanets (Deming et al., 22 Jul 2024).
Reconstruction of the dust-obscured star-formation history through deep, serendipitous MRS line detections in faint galaxies at $z\sim 1$ –$3$, hundreds per FoV in survey mode (Bonato et al., 2017).

LAMOST-MRS provides the timescale coverage and radial velocity precision necessary for Galactic archaeology, asteroseismology, binary star demography, and time-domain stellar physics in the optical (Liu et al., 2020, Zong et al., 2020).

7. Best Practices and Pipeline Recommendations

For optimum MRS data quality and fidelity:

Always apply the static fringe flat correction as the first step after detector-level processing.
For point-like sources, complement static fringe removal with a two-dimensional “residual fringe” fit, and, if possible, a matched calibration observation (Crouzet et al., 15 Apr 2025).
Use four-point dither strategies and appropriate extraction radii during cube reconstruction to suppress spatial/spectral undersampling artifacts (Law et al., 2023).
For requirements on line-flux precision $\lesssim2\%$ , propagate the full error budget including residual fringe and flat-field uncertainties through extraction and subsequent analysis (Crouzet et al., 15 Apr 2025, Law et al., 23 Sep 2024).
Employ pipeline-provided covariance scale factors when extracting 1D spectra from 3D cubes, and validate photometric repeatability on standard sources (Law et al., 2023, Law et al., 23 Sep 2024).
For time-domain and exoplanet transit observations, leverage group-level PSF-weighted extraction and custom ramp processing for optimal handling of detector systematics and charge migration (Deming et al., 22 Jul 2024).

References

(Crouzet et al., 15 Apr 2025) Extended source fringe flats for the JWST MIRI Medium Resolution Spectrometer
(Law et al., 23 Sep 2024) The James Webb Space Telescope Absolute Flux Calibration. III. Mid-Infrared Instrument Medium Resolution IFU Spectrometer
(Law et al., 2023) A 3D Drizzle Algorithm for JWST and Practical Application to the MIRI Medium Resolution Spectrometer
(Argyriou et al., 2023) JWST MIRI flight performance: The Medium-Resolution Spectrometer
(Patapis et al., 2023) Geometric distortion and astrometric calibration of the JWST MIRI Medium Resolution Spectrometer
(Jones et al., 2023) Observations of the Planetary Nebula SMP LMC 058 with the JWST MIRI Medium Resolution Spectrometer
(Labiano et al., 2021) Wavelength Calibration and Resolving Power of the JWST MIRI Medium Resolution Spectrometer
(Patapis et al., 2021) Direct emission spectroscopy of exoplanets with the medium resolution imaging spectrometer on board JWST MIRI: I. Molecular mapping and sensitivity to instrumental effects
(Zong et al., 2020) Phase II of the LAMOST-Kepler/K2 survey. I. Time series of medium-resolution spectroscopic observations
(Liu et al., 2020) LAMOST Medium-Resolution Spectroscopic Survey (LAMOST-MRS): Scientific goals and survey plan
(Deming et al., 22 Jul 2024) Toward Exoplanet Transit Spectroscopy Using JWST/MIRI's Medium Resolution Spectrometer
(Bonato et al., 2017) Exploring the evolution of star formation and dwarf galaxy properties with JWST/MIRI serendipitous spectroscopic surveys
(Wells et al., 2015) The Mid-Infrared Instrument for the James Webb Space Telescope, VI: The Medium Resolution Spectrometer