MIRI LRS Recommended Strategies

Recommendations for planning MIRI LRS science observations, based on pre-launch knowledge of the instrument. 

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The MIRI low resolution spectrometer (LRS) offers slit and slitless spectroscopy from 5 to 12 μm. This page givesrecommendations that, together with the MIRI Cross-Mode Recommended Strategies article, should help observers plan MIRI LRS observations. Note that guidance in these pages are pre-launch recommendations that will be updated with results from post-launch commissioning. 

For LRS slitless observations, please refer to the dedicated MIRI TSO Recommended Strategies page. 

Detector readout mode

See also: Understanding Exposure TimesMIRI Cross-Mode Recommended Strategies

The recommended detector readout mode for both slit and slitless observations is FAST* mode. For slitless observations (which are by default time-series observations), FAST mode is the only option. For slit observations, SLOW mode is also available but users are advised to choose FAST mode unless APT specifically reports problems with data volume.

* Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.


See also: MIRI LRS Dithering

In slit mode, LRS dithering is required for science observations (see MIRI Low Resolution Spectroscopy Template in APT). Two options are available:

  • The ALONG SLIT NOD represents a 2-point dither, which is recommended for point sources. This mode allows for both redundancy and background subtraction. The user should verify that there are no sources close to the target that could occupy the other dither position, which would defeat the purpose of the dither strategy—such 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, in the Extended parameter field, 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.

Dithering is not allowed for the slitless mode as this option has been specifically designed to carry out time-series observations. Selecting the SLITLESSPRISM subarray in the APT LRS template will automatically select the Time Series Observation and No Parallelspecial requirements, which disables dithering. 

Dwell time limit

See also: JWST Communications Subsystem

Dwell time defines how long you can stay at a single dither position (i.e., your exposure time, not your integration time). Ground-based detector tests indicate that long-term drifts in the detector are too weak to require restrictions on the length of an exposure per dither position for the LRS. However, 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). Please refer to the MIRI Cross-Mode Recommended Strategies for further guidance on this on MIRI-specific guidelines for the length of integrations within an exposure.

An exception to the time limitation is made for time series observations, which require lengthy continuous exposures of exoplanetary transits or similar phenomena. When using LRS in slitless mode for such observations, the 10,000 s exposure time limit is therefore waived

Target acquisition considerations

See Main Article: MIRI Cross-Mode Recommended StrategiesMIRI LRS Slit Target AcquisitionMIRI LRS Slitless Target AcquisitionJWST Pointing Performance

For slitless TSO observations, target acquisition (TA) is required. Accurate target placement is especially important if multi-epoch transit observations will be combined. 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). 

For slit observations, TA is highly recommended for point or compact sources given the size of the slit (length 4.7", 3.18 mm, 42.7 pix; width 0.51", 0.33 mm, 4.6 pix) 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 for dedicated background exposures if these is required. When observations are carried out without a TA, the target placement accuracy is determined by the JWST pointing performance and the accuracy of the target coordinates (including proper motion).

Users can obtain a TA verification image, which will be obtained after the science target has been moved into the slit. This image will verify the placement of the target in the slit. For further information, see LRS Slit Target Acquisition - Verification Image.

For observations of point sources in the LRS slit, an ALONG THE SLIT NOD should be specified. This will enable pair-wise subtraction of the spectral images from the two nod positions.

For extended sources in the LRS slit, observers are strongly encouraged to obtain background observations by defining a separate background target.  The coordinates should point to a suitable region nearby, preferably within 20", and the observation should otherwise be identical to that for the science target. These two observations should then be linked as a non-interruptible sequence and then, for the science target, the background target should be specified as such (in the Background Target section). This method will obtain two images of the science target, one in each nod position, and two corresponding background images. 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 the background exposure from the target exposure.  The background exposures are co-added into a single background image, which is subtracted from each nod separately. This method is preferred, because it mitigates for bad pixels on the array (whether permanent or due to a recent cosmic ray hit).

A MAPPING dither pattern with either two spatial positions or two spectral positions could also be used, but this method is strongly discouraged, because the pipeline will have no means of identifying the background position and will not subtract it.  In addition, this approach would not mitigate for bad pixels. If the observer wishes to pursue this option, the extended science target will be observed in the center of one of the two slit positions. The background position will rotate with the position angle of the slit (at roughly 1 degree per day) and can only be constrained by constraining the observing time.

For slitless TSO observations, which by default are not dithered and often very long in exposure time, separate background exposures are not recommended. The current background removal strategy removes the background by estimating it from the detector columns 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



Latest updates

  • Updated to align with changes to APT.