Instructions for filling out the JWST Astronomer's Proposal Tool (APT) MIRI MRS template, which is used for integral field unit (IFU) spectroscopy above 5 μm, are provided in this article.

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See also: MIRI Medium Resolution SpectroscopyJWST Integral Field Spectroscopy RoadmapMIRI MRS Recommended StrategiesMIRI MRS Spectroscopy of a Late M StarMIRI MRS and NIRSpec IFU Observations of Cassiopeia AMIRI MRS and NIRSpec IFU Observations of SN1987AProposal Planning Video Tutorials

Medium-resolution spectroscopy is one of 4 observing modes available with the the Mid-Infrared Instrument (MIRI). The MIRI medium resolution spectrometer (MRS) will be used to obtain 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.

The MRS is the only JWST observing mode offering spectroscopy (R = 1,500–3,500) longward of ~ 14 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. The MRS IFUs use slicers to split the field of view into spatial slices. Each slice produces a separate dispersed "long slit" spectrum. Post-processing produces a 3-dimensional (2 spatial and one spectral dimension) data cube. 

MRS operations have been designed to allow for efficient observations of point sources, compact sources, and extended sources. The observer will have control over 3 primary variables for MRS spectroscopy:

  1. wavelength coverage 
  2. dithering pattern 
  3. detector readout mode and exposure time (via the number of frames and integrations)

Allowed values are documented and maintained in MIRI MRS template parameters (an article in the JWST Proposal Parameters documentation), but are also described below.

Step-by-step APT instructions are provided below.

Generic parameters

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

The following parameters are generic to all templates, and are not discussed in this article: observation Number, observation Labelobservations CommentsTarget name, ETC Wkbk. Calc ID (in the Filters dialog box), Mosaic Properties, and Special Requirements.

MIRI Medium Resolution Spectroscopy tab

Target Acquisition Parameters

The target acquisition parameters Acq TargetAcq Filter, and Acq ExposureTime are found under the MIRI Medium Resolution Spectroscopy tab, in the Target Acquisition Parameters panel.

Target ACQ 

See also: MIRI MRS Target Acquisition

For MRS spectroscopy, choose Acq Target from the pull down menu. Filters available are F560WF1000WF1500W, and FND. As of APT 25.4.2, NONE may also be selected for target acquisition, which will disable target acquisition for the observation.

Acq Exposure Time 

The Acq Exposure Time fields must be completed by selecting a MULTIACCUM exposure configuration. Each exposure is specified by setting the readout pattern and characteristics parameters: Acq Readout Pattern and Acq Groups/Int

Users should use the Exposure Time Calculator (ETC) to determine the best exposure configuration to optimize the signal to noise.

Acq Readout Pattern

See also: MIRI Detector Readout Overview

Choices for the Acq Readout Pattern are 

  • FAST (the default)

Number of groups and integrations

See also: Understanding JWST Exposure Times

Groups/Int = 2 requires permission from STScI, which can be obtained with a request through the Help Desk.

Acq Groups/Int

The MIRI readout timing pattern for the TA exposure is defined by only one of the MULTIACCUM parameters: Acq Groups/Int, the number of groups per integration, where a group is the product of cycling through all the pixels.

Acq Integrations/Exp, Acq Total Integrations and Acq Total Exposure Time cannot be changed by the user.

MRS Parameters

The Primary Channel parameter, as well as Dithers, Simultaneous Imaging, and Exposure Parameter dialog boxes are found under the MIRI Medium Resolution Spectroscopy tab, in the MRS Parameters panel.

Primary Channel 

See also: MRS Channels

The Primary Channel defines the pointing origin (i.e., where the science target would be centered in the absence of dithering), available dither patterns, and field boundaries (for mosaic overlap calculations). Options are:

  • ALL MRS*

The IMAGER option should only be used to place the science target in the simultaneous imaging field while the MRS is observing a dedicated background.

* The default choice should be ALL MRS, unless the science case wishes to prioritize data quality in a particular wavelength channel at the expense of other wavelengths (e.g., maximizing long wavelength source separation at the possible expense of the target not being in the short wavelength field of view).


See also: MIRI MRS PSF and Dithering

The Dithers dialog box handles the creation of dither patterns, which must be created before defining your exposure sequence. Each DITHER is specified by setting several parameters: Dither TypeOptimized For, and Direction.

Dither Type

  • 2-Point dithering allows for basic image separation in order to use one exposure as the "background image" for another, and to detect the source with redundant detector pixels. It does not achieve optimal half-integer spatial sampling throughout much of the field of view.
  • 4-Point adds an additional offset compared to 2-Point dithering and achieves optimal half-integer spatial sampling throughout the entire field of view of all 4 channels.

The default choice should be 4-Point. Dithering may be turned off in order to create zero-offset exposures in the Exposure Parameters table.

Optimized For


The POINT SOURCE dithering schemes provide the maximum possible separation between the dither locations. The EXTENDED SOURCE dithering scheme provides the minimum possible separation that still achieves ideal spatial sampling and use of redundant detector locations. When observing point sources or otherwise small objects, the POINT SOURCE dithering scheme should generally be selected. When observing extended sources that fill much of the field of view, or when mosaicing together multiple pointings, some science cases may wish to use the EXTENDED SOURCE scheme. Note that programs using the EXTENDED SOURCE pattern should also include a separate dedicated background exposure at an off-source location since the image separation achieved in this dither scheme will be insufficient to achieve proper background subtraction.  The BACKGROUND dithering scheme provides large offsets suitable for optimizing MIRI simultaneous imaging data while the MRS is observing an empty dedicated background field.



This choice defines the orientation of the dither offsets in the JWST focal plane; they are mirror symmetric with respect to the IFU slice orientation and achieve identical sampling. This option is provided to allow for flexibility in dithering direction if the telescope orientation is to be fixed for a given observation. The default is NEGATIVE, and is unimportant for the majority of science cases.

Simultaneous Imaging 

See also: MRS Simultaneous Imaging

MIRI offers the ability to obtain simultaneous imaging with the MIRI imager while obtaining MRS spectroscopy.  Choose:

  • YES
  • NO

Grating Wheel Direction

For engineering purposes only, the direction of the grating wheels can be controlled with this limited access parameter.

Imager Subarray 

See also: MIRI Detector Subarrays

If you choose simultaneous imaging, MIRI imaging supports the use of a pre-defined set of Imager Subarrays for observing targets bright enough to saturate the image in full frame readout. Each subarray is associated with a brightness limit, depending on the filter used, above which a point source will saturate in the shortest 2-group integration. A MIRI simultaneous imaging observation can only support a single subarray. If the target must be imaged using another subarray, it is necessary to create another MIRI simultaneous imaging observation.


Size (pixels)

Usable size (arcsec)

Frame time


1024 × 1032

74" × 113"

2.775 s


512 × 512

56.3" × 56.3"

0.865 s


256 × 256

28.2" × 28.2"

0.300 s


128 × 136

14.1" × 14.1"

0.119 s


64 × 72

7" × 7"

0.085 s

Exposure Parameters 

Users should use the Exposure Time Calculator (ETC) to determine the best exposure configuration to optimize the signal to noise.

An MRS sequence is specified by first setting the Wavelength Range desired for each exposure. There are 3 available settings: SHORT (A), MEDIUM (B), and LONG (C). Each setting covers one-third of the wavelength range of each channel; a full observation from 4.9 to 27.9 μm  would thus require an exposure at each of these 3 settings.

Each sequence must then be completed by selecting a MULTIACCUM exposure configuration for each detector being used (in the Exposure Parameters dialog box). Each exposure is configured by setting the readout pattern and characteristics parameters: Readout PatternGroups/Int, and Integrations/Exp. If you choose simultaneous imaging, you must select an imaging Filter for each MRS sequence. Each MRS sequence can support only a single imaging Filter. Finally select a Dither pattern from the list of Dithers already defined for the observation (or None for no dithering). The exposures will be repeated at each dither position following an offset of the telescope to the new position. The number of exposures at each dither position is set by Exposures/Dith, but this input must be the same for all detectors.  

Readout Pattern

See also: MIRI Detector Readout Overview

MIRI offers 2 readout modes: 

Number of groups and Integrations

See also: Understanding JWST Exposure Times

The MIRI timing pattern per exposure is defined by only 2 MULTIACCUM parameters: 

  • Groups/Int: the number of groups during an integration, where a group is the product of cycling through all the pixels
  • Integrations/Expthe number of integrations during an exposure, where integration is defined as the time between resets. 

Other tabs

Mosaic Properties

See also: JWST Mosaic OverviewMIRI MRS Mosaics

See also:  Specifying Mosaics in APT 

The MIRI MRS may be used to obtain data for a region larger than their size by creating a MIRI MRS mosaic pattern under the Mosaic Properties tab.

Special Requirements

A variety of observatory level Special Requirements may be chosen under the Special Requirements tab.


The Comments field (under the Comments tab) should be used for observing notes.

Latest updates
    Updated to add description of the MRS background dither and Imager pointing.

  • Updated to explain the existence of the Grating Wheel Direction pull down menu

    Minor updates for Cycle 2

  • Revised proposal guide for APT 25.4.2 to include no-TA and new specification of MRS wavelength range in the exposure parameters box.
Originally published