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".

See also: MIRI MRS APT TemplateJWST Integral Field Spectroscopy

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 four 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 (two spatial and one spectral dimension) data cube combining the information from each of these spatial slices. This process is illustrated schematically in Figure 1.

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

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Figure 1. Schematic overview of the MIRI MRS


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 12 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 represent 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 all three grating settings (SHORT 1, MEDIUM, and LONG). Note the larger footprint of channel 4 compared to channel 1. 


MRS wavelength coverage

See also: MIRI Bright Source LimitsMIRI Sensitivity

The MRS has four separate IFUs (channels 1 through 4), each covering a separate wavelength range between 4.9 and 28.3 μm. All four 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 four discontinuous wavelength intervals (the first, middle, or last thirds of each channel); for complete spectral coverage, three different grating settings must be observed; SHORT (A), MEDIUM (B), and LONG (C).  Combining the 4 channels and three 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) ~ 3000 at 5 μm to ~ 1500 at 28 μm as indicated by Figure 3.


Table 1. Characteristics of the four IFU channels (Pre-Flight, Nov 2019)

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
4.88–7.52

 3.2 × 3.7210.1760.196

SHORT (A)

MEDIUM (B)

LONG (C)

4.88–5.75

5.63–6.63

6.41–7.52

3,320–3,710

3,190–3,750

3,100–3,610 

Channel 2
7.48–11.75

 4.0 × 4.8170.2770.196

SHORT (A)

MEDIUM (B)

LONG (C)

7.48–8.76

8.71–10.23

10.02–11.75

2,990–3,110

2,750–3,170

2,860–3,300 

Channel 3
11.52–18.08

5.5 × 6.2160.3870.245

SHORT (A)

MEDIUM (B)

LONG (C)

11.52–13.49

13.36–15.65

15.43–18.08

2,530–2,880

1,790–2,640

1,980–2,790 

Channel 4
17.65–28.34

6.9 × 7.9120.6450.273

SHORT (A)

MEDIUM (B)

LONG (C)

17.65–20.94

20.41–24.22

23.88–28.34

1,460–1,930

1,680–1,770

1,630–1,330

Figure 2. MRS filter bandpasses

Graphical representation of IFU channels' sensitivity

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).

Figure 3. Resolving power as a function of wavelength for MIRI MRS

The plot shows the range of spectral resolving power of the MRS across the FOV.  The red lines indicate spatially averaged values across each sub-band. The wavelength ranges of each sub-band are indicated, as well as some relevant mid-infrared lines. Note, however, that the resolving powers shown here may be underestimated by as much as 10% compared to the values given in Table 1. Refined measurements of the spectral resolving power will be obtained during commissioning.  Figure credit: Wells et al. 2015



MRS dither pattern

Main article: MIRI MRS Dithering
See also: MIRI Dithering, MIRI MRS Mosaics, MIRI MRS Coordinate Systems

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 two (and ideally 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 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:

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

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



MRS exposure time

Main article: 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: 

  1. SLOW mode
  2. FAST 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

A few additional considerations should be kept in mind:

  1. 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.
  2. A suitable target should be chosen that is adequate for target acquisition.
  3. The MIRI imager can be used at the same time as the MRS for simultaneous imaging.
  4. 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


References


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



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Latest updates

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    Revised all numbers, updated all sections
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    Added links to MRS best practices, simultaneous imaging, APT guide
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    Added figures and table, updated text

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