Medium-resolution spectroscopy, an observing mode for JWST's Mid-Infrared Instrument (MIRI), obtains spatially resolved spectroscopic data between 4.9 to 27.9 μm over a FOV up to 6.6'' × 7.7''.

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The JWST MIRI medium-resolution spectrometer (MRS) (Wells et al. 2015; Argyriou et al. 2023) observes simultaneous spatial and spectral information between 4.9 to 27.9 μm over a contiguous field of view up to 6.6'' × 7.7'' in size. This is the only JWST configuration offering medium-resolution spectroscopy (with spectral resolving power R ~ 1,500 to 3,500) longward of 5.2 μm.

MRS observations are carried out using a set of 4 integral field units (IFUs), each of which covers a different portion of the MIRI wavelength range. MRS IFUs split the field of view into spatial slices, each of which produces a separate dispersed "long-slit" spectrum. Post-processing produces a composite 3-dimensional (2 spatial dimensions and one spectral dimension) data cube combining the information from each of these spatial slices. This process is illustrated schematically in Figure 1.

Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.

MRS operations have been designed to allow for efficient observations of point sources, compact sources, and fully extended sources. The observer has control over 3 primary parameters: (1) wavelength coverage, (2) dither pattern, and (3) detector read out mode and exposure time (via the number of frames and integrations).

For current information on performance of the MIRI instrument as well as calibration, features, and caveats specific to MIRI MRS data processing in the JWST Science Calibration Pipeline see: MIRI MRS Known Issues

*Click on the figure for a larger view.*
Left: The effective rectangular footprint of each of the four MRS channels is projected on the sky in the spacecraft (V2, V3) coordinate frame. Channels 1, 2, 3, and 4 are shown in blue, green, yellow, and red respectively. Individual slice locations are shown for illustrative purposes for the twelve slices in channel 4.

Middle: MRS spectra from all four channels are dispersed simultaneously onto two detectors for a single exposure. Each color-coded stripe represents the "long-slit" spectrum from a single slice. Slices that are adjacent on the sky are interleaved on the detector (as indicated in the channel 4 illustration).

Right: The JWST calibration pipeline rectifies MRS data into a regularly-sampled 3-D cube format and can combine information from all four channels and three grating settings (**SHORT** , **MEDIUM**, and **LONG**). Note the larger footprint of channel 4 compared to channel 1.

Figure credit: Law et al. 2023

MRS wavelength coverage and spectral resolution

The MRS has 4 separate IFUs (channels 1 through 4), each covering a separate wavelength range between 4.9 to 27.9 μm. All 4 channels are observed simultaneously, but each exposure can only cover one-third of the available wavelength range of each channel in a single configuration. A single MRS exposure thus covers 4 discontinuous wavelength intervals (the first, middle, or last thirds of each channel); for complete spectral coverage, 3 different grating settings must be observed; SHORT (A), MEDIUM (B), and LONG (C). Combining the 4 channels and 3 grating settings, there are 12 different wavelength bands increasing in wavelength from "1A" to "4C." These bands are summarized in Table 1 and Figure 2. While channel 4C technically extends to 28.7 μm, the quantum efficiency is 5% around 27.9 μm and drops quickly at longer wavelengths.

The spectral resolving power changes between each MRS band, and ranges from (R = λ/Δλ) ~3,500 at 5 μm to ~1,500 at 28 μm as indicated by Figure 3. In-flight measurements are generally consistent with pre-flight expectations for point sources, although some uncertainties remain (particularly at longer wavelengths).

Table 1. Characteristics of the 4 IFU channels (flight-measured)

FOV name λ-range (μm)	FOV (arcsec)	Number of slices	Slice width (arcsec)	Pixel size (arcsec)	Sub-band name	λ-range (μm)	Resolving power (λ/Δλ)
Channel 1 4.9–7.65	3.2 × 3.7	21	0.176	0.196	SHORT (A) MEDIUM (B) LONG (C)	4.90–5.74 5.66–6.63 6.53–7.65	3,320–3,710 3,190–3,750 3,100–3,610
Channel 2 7.51–11.7	4.0 × 4.8	17	0.277	0.196	SHORT (A) MEDIUM (B) LONG (C)	7.51–8.77 8.67–10.13 10.01–11.70	2,990–3,110 2,750–3,170 2,860–3,300
Channel 3 11.55–17.98	5.2 × 6.2	16	0.387	0.245	SHORT (A) MEDIUM (B) LONG (C)	11.55–13.47 13.34–15.57 15.41–17.98	2,530–2,880 1,790–2,640 1,980–2,790
Channel 4 17.7–27.9	6.6 × 7.7	12	0.645	0.273	SHORT (A) MEDIUM (B) LONG (C)	17.70–20.95 20.69–24.48 24.40–27.90	1,460–1,930 1,680–1,770 1,630–1,330

Wavelength coverage of the MIRI MRS channels. A single MRS exposure will simultaneously obtain data in one-third of Channels 1, 2, 3, and 4 (either the SHORT, MEDIUM, or LONG wavelength ranges).
Plot generated for Cycle 4. Figure credit: STScI MIRI Team.

Comparison between pre-flight estimated MRS spectral resolution (grey regions with 1σ confidence level; Labiano, A. et al. 2021) and values derived from in-flight observations of bright emission lines in the compact planetary nebula SMP LMC 058 (Jones, O. et al. 2023). The solid red curve represents an approximate functional fit R(λ) = 4603 - 128 λ(μm) + 10^(-7.4 λ(μm)). Note that extended sources that fill a slice may obtain slightly lower effective spectral resolution. © Jones, O. et al. 2023

MRS dither pattern

The point spread function (PSF) produced by JWST and delivered to the MRS imager slicers and detector pixels is undersampled by design, as is the spectral line spread function (LSF). Optimal sampling in both spatial and spectral dimensions therefore requires that objects be observed in four dither positions that include a half-integer offset in both the along-slice and across-slice directions. Assuming that such dithered observations are obtained, MRS data products provide a PSF FWHM of about 0.3" at 5 μm and 1" at 25 μm (see MIRI MRS PSF and Dithering, Figure 2).

A variety of different dither patterns are offered that optimize observations for a variety of different considerations:

Point source or extended source observations (prioritizing PSF separation between successive exposures, or large common field across all exposures)
Spatial sampling at specific wavelengths or at all wavelengths.
Number of dither locations (2 or 4)
Standard or inverted dither orientation
Dedicated background observations

Details on the available patterns can be found at the MIRI MRS PSF and Dithering article. Information about mosaicing options can be found on the MIRI MRS Mosaics article.

MRS exposure time

MIRI MRS exposure times are not specified directly. Rather, the detectors are read using up-the-ramp sampling tied to specific timing readout patterns. Two detector readout patterns are supported for MRS spectroscopy:

SLOWR1 mode
FASTR1 mode (default)

General guidance is provided for both the expected sensitivity and saturation limits of the MRS; however, the JWST Exposure Time Calculator (ETC) should be used to determine which mode is best for a given set of observations, and how many frames and integrations are required in order to reach the target depth.

Additional considerations for program design

A few additional considerations should be kept in mind when designing an MRS observing program:

Depending on the dither pattern selected, it may be necessary to include a dedicated sky observation in order to measure the astronomical foreground and background signal.
Pointing accuracy can be improved using target acquisition (for suitable targets).
The MIRI imager can be used at the same time as the MRS for simultaneous imaging.
A variety of questions on usage are answered in the MIRI MRS Recommended Strategies article.

Working with MRS data products

The primary data products provided for MIRI MRS by the JWST data reduction pipeline are three-dimensional spectral cubes in which the individual dithered exposures have been resampled and combined onto a single rectified grid with regular voxel sizes (see discussion by Law et al. 2023). In most cases the pipeline will also produce 1-D spectra extracted from these data cubes; either summing the entire field of view for EXTENDED sources or applying traditional aperture photometry for point sources. A guide to assorted features and caveats in these data products can be found at MIRI MRS Known Issues.

If reprocessing MRS data through the JWST pipeline, an example notebook to run the pipeline and modify steps that users may desire to modify can be found on Github at MRS_FlightNB1.

All MRS articles

JWST Integral Field Spectroscopy provides an introduction to integral field spectroscopy with JWST

MIRI Medium Resolution Spectroscopy provides a main overview of the MIRI MRS (this page)

MIRI MRS APT Guide: step-by-step instructions on how to fill out APT

MIRI MRS Recommended Strategies: frequently asked questions on best practices for specifying observations

MIRI MRS Dedicated Sky Observations: information on MRS dedicated background exposures

MIRI MRS PSF and Dithering: detailed information on MRS dithering strategies

MIRI MRS Field and Coordinates: overview of the MRS field of view, coordinate systems, and pointing origins

MIRI MRS Hardware: overview of the MRS imager slicer hardware

MIRI MRS Mosaics: information on MRS mosaicing strategies

MIRI MRS Simultaneous Imaging: information on using the MIRI Imager during MRS observations

MIRI MRS Target Acquisition: target acquisition procedures for the MRS

MIRI MRS Known Issues: features and caveats specific to MIRI MRS data processing in the JWST Science Calibration Pipeline

References

Argyriou, I. et al. 2023, A&A, 675, 111
JWST MIRI flight performance: The Medium-Resolution Spectrometer

Jones, O. et al. 2023, MNRAS, 523, 2519
Observations of the planetary nebula SMP LMC 058 with the JWST MIRI medium resolution spectrometer

Labiano, A. et al. 2021, A&A, 656, A57
Wavelength calibration and resolving power of the JWST MIRI Medium Resolution Spectrometer

Law, D. et al. 2023, AJ, 166, 45L
A 3D Drizzle Algorithm for JWST and Practical Application to the MIRI MRS

Wells, M. et al. 2015, PASP, 127, 646
The Mid-Infrared Instrument for the James Webb Space Telescope, VI: The Medium Resolution Spectrometer

Notable update	15 Jul 2024 Updated PCE plot (Figure 2) for Cycle 4. 09 Aug 2023 Updated resolving power with additional in-flight measurements and added information on working with MRS data products. Updated PCE plot (Fig 2). 12 Jul 2022 Updated Wavelength range and FOV of MRS to in-flight numbers
Originally published	22 Dec 2016