MIRI Medium Resolution Spectroscopy
Medium-resolution spectroscopy, an observing mode for JWST's Mid-Infrared Instrument (MIRI), will obtain spatially resolved spectroscopic data between 4.9 and 28.3 μm over a FOV up to 6.9" × 7.9".
The JWST MIRI medium-resolution spectrometer (MRS) (Wells et al. 2015) will observe simultaneous spatial and spectral information between 4.9 and 28.3 μm over a contiguous field of view up to 6.9" × 7.9" in size. This is the only JWST configuration offering medium-resolution spectroscopy (with R from 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 three-dimensional (2 spatial and one spectral dimension) data cube combining the information from each of these spatial slices. This process is illustrated schematically in Figure 1.
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MRS wavelength coverage
The MRS has 4 separate IFUs (channels 1 through 4), each covering a separate wavelength range between 4.9 and 28.3 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 thus 12 different wavelength bands, increasing in wavelength from "1A" to "4C." These bands are summarized in Table 1 and Figure 2.
The spectral resolving power changes between each MRS band, and ranges from (λ/Δλ) ~3,000 at 5 μm to ~1,500 at 28 μm as indicated by Figure 3.
Table 1. Characteristics of the 4 IFU channels (Pre-Flight, Nov 2019)
|Number of slices|
Slice width (arcsec)
|Pixel size (arcsec)|
|3.2 × 3.7||21||0.176||0.196|
|4.0 × 4.8||17||0.277||0.196|
|5.5 × 6.2||16||0.387||0.245|
|6.9 × 7.9||12||0.645||0.273|
MRS dither pattern
The spatial point spread function (PSF) seen by 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 at least 2 (and ideally 4) 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 are nearly diffraction limited longward of 8 μm (see MIRI MRS Dithering, Figure 1).
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
MRS exposure time
See also: MIRI Detector Readout Overview
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:
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.
A few additional considerations should be kept in mind:
- 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.
- A suitable target should be chosen that is adequate for target acquisition.
- 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.
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 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