MIRI MRS Recommended Strategies

Recommendations for planning MIRI MRS science observations, based on pre-launch knowledge of the instrument, are provided in this article.

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The MIRI medium resolution spectrometer (MRS) offers integral field spectroscopy between 4.9 and 28.1 μm using 4 different nested integral field units (IFUs). Full spectral coverage is obtained by making 3 exposures using the 3 different 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 will:

  • 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.

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 (2 for the MRS, one for the imager), so it is important to consider MIRI data volume limits.

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 more detailed advice on detector usage, observers are encouraged to review MIRI Generic Recommended Strategies (Detectors).


See also: MIRI MRS Dithering

MRS dithering is strongly recommended 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 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. 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 filters, the system will issue a warning because the dither points are separated by less than one FWHM. However, at these longer wavelengths, the 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.

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" × 7.6" (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 MIRI MRS Dedicated Sky Observations. Dedicated backgrounds (flagged as such in APT) will be automatically subtracted from science observations in the pipeline. 


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

  • Added additional links

    Updated to reflect changes in APT 25.4.2
Originally published