MIRI MRS Recommended Strategies

Recommendations for planning MIRI MRS science observations are provided in this article.

On this page

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

The MIRI medium resolution spectrometer (MRS) offers integral field spectroscopy between 4.9 to 27.9 μm using 4 different nested integral field units (IFUs). Full spectral coverage is obtained by taking exposures using the 3 spectral settings: SHORT (A), MEDIUM (B), and LONG (C)

This article gives recommendations that, together with the MIRI Cross-Mode Recommended Strategies, should help the observer to plan MIRI MRS observationsNote that these are pre-launch recommendations that will be updated with results from on-orbit commissioning.

MIRI simultaneous observations (MRS and imaging)

See also: MRS Simultaneous Imaging

The default set-up for the MRS template in the Astronomer's Proposal Tool (APT) is to make use of the adjacent MIRI imaging detector for simultaneous imaging. Simultaneous imaging is a highly recommended procedure that can:

  • Improve the MRS data astrometric accuracy and result in better-combined data cubes, not only within an observing program but across different ones.
  • Provide scientific images of the field adjacent to the MRS, which are especially useful when observing extended objects.
  • Image the primary science target while the MRS is observing a dedicated background.

If taking simultaneous imaging primarily to improve the astrometric accuracy of MRS, the default recommended setting for the simultaneous imaging can be FASTR1 with the F770W filter and Ngroups = 50, if the length of the MRS observation allows. These settings should be adjusted to best fit within the specifications of the MRS exposures as the simultaneous imaging integration time should not be greater than the MRS integration time. 

There are cases in which the user should not consider this option, e.g., a very bright extended target that will heavily saturate the imager field of view in all filters, even in subarray mode.

This observing mode makes use of all 3 MIRI detector arrays (two for the MRS, one for the imager), so it is important to consider MIRI data volume limits. For large data volume programs using SLOWR1 below 15µm would also be acceptable.

Detector readout mode

See also: Understanding Exposure TimesMIRI Generic Recommended Strategies (Detectors)

The default readout mode for the MRS is FASTR1, primarily because it provides more samples of the ramp with which to reject cosmic rays. This is not mandatory and there are scenarios, such as the need for reducing data volume, where the user could select SLOWR1 mode. For the MRS, it is recommended that FASTR1 be used for observations where Ngroups is 100 or less. For more detailed advice on detector usage, observers are encouraged to review MIRI Generic Recommended Strategies (Detectors).


See also: MIRI MRS PSF and Dithering

MRS 4-pt dithering is strongly recommended for all science observations with the exception of dedicated backgrounds to obtain better sampling of the spatial point spread function and spectral line spread function (both are undersampled by the MRS optics).

However, a "staring" option is offered and may be desired in certain science cases (e.g., time-series observations) or as a strategy to obtain some dedicated background observations. When making use of simultaneous imaging, the dither patterns used are those of the primary mode, which in this case is the MRS (although see Background dither pattern available in Cycle 3). In order to keep targets in the MRS field of view, the dithers are small; as a result, if the observer is using long wavelength imaging filters, the system will issue a warning because the dither points are separated by a smaller distance than ideal for typical imaging observations. However, at these longer wavelengths, the imaging PSF is well-sampled and this dithered data will be suitable for combination.

When defining a target in APT, users should specify, in the Extended parameter field, if the target is spatially extended; the options are YES, NO, and Unknown. The dither pattern should be consistent with this choice. Sources defined as extended or unknown should use an extended source dither pattern and point sources should use a point source dither pattern. Deviations from these choices must be justified in the proposal.

Dwell time limit

Dwell time defines how long you can stay at a single dither position (i.e., your exposure time, not your integration time). All the ground-based detector testing data carried out so far indicates that, based on the presence of detector long term drifts, there are no restrictions on the length of an exposure per dither position for MRS. However, it is recommended that total exposure time be kept to ~700 s or less in order to limit cosmic ray effects. 

Guidelines on the exposure length can also be found in the MIRI Cross-Mode Recommended Strategies. Users should also note that the observatory imposes a limit of 10,000 s on the length of an individual exposure to allow for moves of the high gain antenna (HGA). This is only waived for TSO observations.

Target acquisition considerations

See also: MIRI Cross-Mode Recommended Strategies (TA)  MIRI MRS Target Acquisition

Target acquisition (TA) for the MRS is performed with the MIRI imager (see Figure 1). TA for MRS observations is not mandatory, but proposers should take caution when deciding against using TA. Most importantly, proposers should consider the JWST pointing performance and the impact on their science case. The MRS field of view ranges from 3.2'' × 3.7'' (slice width 0.176") to 6.6'' × 7.7'' (0.645" slice width). Therefore, a large dither pattern coupled with a pointing error could be problematic. Even for a point source, the dither pattern results in overlapping fields of view in each channel that are smaller than the smallest field of view (~ 1" × 3"), highlighting the importance in ensuring that the target is properly placed in the MRS aperture.

Maximum separation between MRS science and TA targets

There are circumstances where TA should not be performed on the science target, either because they are extended and/or too faint. In this case, a nearby object within a visit-splitting distance (preferably a bright point source) can be used for TA. Some restrictions apply to this TA procedure; observers should check the TA section of the MIRI Cross-Mode Recommended Strategies for details.

Background observations

See also: JWST Background ModelJWST Background-Limited ObservationsMIRI Cross-Mode Recommended Strategies

The need for dedicated background exposures is discussed in detail in MIRI MRS Dedicated Sky Observations. Dedicated MRS background observation are strongly recommended to provide good data quality and should be taken with detector readout parameters (i.e., Ngroups) equal to that used for the science exposures. For most science cases, a 2-pt dithered background observation is sufficient to help mitigate the effects of bad pixels and cosmic rays (see MIRI MRS PSF and Dithering); a single background position (i.e., no dithers) is acceptable in some situations. Dedicated backgrounds (flagged as such in APT) will be automatically subtracted from science observations in the pipeline. Dedicated backgrounds for extended sources and moving targets are highly recommended. Faint extended science sources may require background observations with a total exposure depth comparable to that used for the science observation. Isolated, bright point sources may avoid the need for dedicated backgrounds through use of an in-field background annulus, but proposers are encouraged to add dedicated backgrounds for these targets if they have any concerns about potential data quality issues. Please reach out to the JWST Help Desk if you have any questions prior to proposal submission. Requests for adding backgrounds after proposal acceptance are unlikely to be approved.



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

Glasse et al. 2015, PASP, 127, 686G
The Mid-Infrared Instrument for the James Webb Space Telescope, IX: Predicted Sensitivity

Latest updates
    Dedicated background recommendations updated for Cycle 3

    Updated to reflect changes in APT 25.4.2
Originally published