Medium-resolution spectroscopy is an observing mode for JWST's Mid-Infrared Instrument (MIRI) for obtaining spatially resolved spectroscopic data between 4.9 and 28.8 μm over a FOV up to 7.2" × 7.9".
The JWST MIRI medium-resolution spectrometer (MRS; Wells et al. 2015) will observe simultaneous spatial and spectral information between 4.9 and 28.8 μm over a contiguous field of view up to 7.2" × 7.9" in size. This is the only JWST configuration offering medium-resolution spectroscopy (with R from 1500 to 3500) 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 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 3 primary variables: (1) wavelength coverage, (2) dithering pattern, and (3) detector read out mode and exposure time (via the number of frames and integrations).
MRS wavelength coverage
The MRS has 4 separate IFUs (channels 1 through 4), each covering a separate wavelength range between 4.9 and 28.8 μm. All 4 channels are observed simultaneously, but each exposure can only cover one-third of the available wavelength range in a single configuration. For complete spectral coverage, 3 different spectral settings must be observed; SHORT (A), MEDIUM (B), and LONG (C). There are therefore 12 different wavelength bands, increasing in wavelength from 1A to 4C. These settings are summarized in Table 1 and Figure 2.
The spectral resolving power changes between each MRS band, and also varies spatially across the field of view as indicated by Figure 3.
Table 1. Characteristics of the four IFU channels
|Number of Slices|
Slice Width (arcsec)
|Pixel Size (arcsec)|
|3.3 × 3.7||21||0.176||0.196|
|4.2 × 4.8||17||0.277||0.196|
|5.6 × 6.2||16||0.387||0.245|
|7.2 × 7.9||12||0.645||0.273|
The 4 channels of the MRS each cover an overlapping but distinct region of the JWST focal plane (see details on the MRS field of view, coordinate systems, and pointing origin). The spatial point spread function (PSF) seen by the imager slicers is undersampled by design, as is the spectral line spread function (LSF) sampled by the detector pixels. Full sampling in both spatial and spectral dimensions therefore requires that objects be observed in at least 2 (and ideally 4) dither positions that include an offset in both the along-slice and across-slice directions. 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
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:
The JWST exposure time calculator 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 borne 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/background signal
- A suitable target should be chosen that is adequate for target acquisition
All MRS articles
JWST Integral Field Spectroscopy: Introduction to Integral Field Spectroscopy with JWST
MIRI Medium Resolution Spectroscopy: Main overview of the MIRI MRS (this page)
- MIRI MRS Hardware: Overview of the MRS imager slicer hardware
- MIRI MRS Field: Overview of the MRS field of view, coordinate systems, and pointing origins
- MIRI MRS Dithering: Detailed information on MRS dithering strategies
- MIRI MRS Mosaics: Information on MRS mosaicing strategies
- MIRI MRS Dedicated Sky Observations: Information on MRS Dedicated Background Exposures
- MIRI MRS Target Acquisition: Target acquisition procedures for the MRS
JWST User Documentation Home
Mid-Infrared Instrument, MIRI
MIRI Spectroscopic Elements
MIRI Filters and Dispersers
MIRI MRS Dithering
MIRI Detector Readout Overview
MIRI Detector Readout Slow
MIRI Detector Readout Fast
JWST Astronomers Proposal Tool, APT
JWST APT website
JWST Exposure Time Calculator, ETC
JWST ETC website