MIRI LRS Recommended Strategies

This page gives recommendations that, together with the MIRI Generic Recommended Strategies  and TSO-specific guidelines for MIRI articles, should help the observer to plan MIRI LRS observationsNote that these are pre-launch recommendations that will be updated with results from on-orbit commissioning

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The MIRI Low-Resolution Spectrograph (LRS) offers slit and slitless spectroscopy from 5 to 12 μm. 

Detector Readout mode

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

The default readout mode is FAST.  For slitless observations only FAST mode is supported, but proposers may consider SLOW mode when using the slit.


See also: MIRI LRS Dithering

In slit mode, LRS dithering is reinforced for science observations (see MIRI Low Resolution Spectroscopy Template in APT). There are two different options:

  • The "along slit mode" represents a two-point dither, and it is recommended for point sources. This mode allows for both redundancy and background subtraction. The user should verify whether sources close to the target can occupy the other dither position and defeat the dither strategy.  If so, unfavorable roll angles should be avoided.
  • The "mapping mode" allows the user to define a certain number of spectral and spatial steps and offsets, and it has been designed to allow for extended source mapping.

When defining a target in APT, users should specify if the target is spatially extended; the options are "Yes," "No," and "Unknown." The selected dither pattern should be consistent with the source definition. Deviations from these default options should be justified in the proposal.

In slitless mode, dithering is not allowed, as this option has been specifically designed to carry out Time Series Observations. Note that in slitless observations the target should be  a point source.

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 LRS.

Target Acquisition Considerations

See Main Article: MIRI Generic Recommended Strategies (Target Acq)MIRI LRS Target AcquisitionMIRI LRS Slitless Target Acquisition

For slitless/TSO observations, Target Acquisition (TA) is highly recommended, particularly if multi-epoch transit observations will be taken. The TA procedure will ensure that the target is placed at the same location for each exposure, with < 10 mas accuracy (corresponding to < 0.1 px). In cases where a dedicated background exposure is required, target acquisition can be disabled for that exposure. 

For slit observations, TA is also highly recommended, especially for point sources given the size of the slit (length 4.7", 3.18 mm, 42.7 px; width 0.51", 0.33 mm, 4.6 px) and the sensitivity of the calibration to the location of the source in the slit.  The no-TA option is intended to be used mostly for extended sources, and as above, for background exposures if this is required. Observers should consider the JWST Pointing Performance.

Background Observations

See also: JWST Background Model, Background-Limited JWST ObservationsMIRI Generic Recommended Strategies (Background)

For slit observations, background subtraction is performed as follows:

  • For point sources, using the along-slit dither option: pair-wise subtraction of the spectral images from the two nod positions.
  • For extended sources, a suitable background position/strategy is expected to be used. This can be either a separate background target or a mosaic.  
  • When a user assigns a background to a science target, that creates a formal association between them. By doing this the pipeline will automatically subtract background exposure from target exposure.

For slitless/TSO observations, which by default are not dithered, separated background exposures are not recommended. They would not be representative or accurate enough to allow for subtraction. The current background-removal strategy removes the background by estimating it from the detector rows adjacent to the source.


Kendrew et al. 2015, PASP, 127, 623K
The Mid-Infrared Instrument for the James Webb Space Telescope, IV: The Low-Resolution Spectrometer

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



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