MIRI Low Resolution Spectroscopy

Low-resolution spectroscopy is an observing mode for JWST’s Mid-Infrared Instrument (MIRI) that offers slit and slitless spectroscopy from 5 to 14 μm.

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See also: MIRI LRS APT Template

MIRI's low-resolution spectrometer (LRS; Kendrew et al. 2015) offers both slit and slitless spectroscopy from 5 to 14 μm using a double prism mounted in the MIRI filter wheel. These optics provide a spectral resolving power (R = λ/Δλ) of ~100 at 7.5 µm, varying from R ~ 40 at 5 μm to R ~ 160 at 10 μm. The LRS forms part of the MIRI imager, and spectra are imaged onto the imager detector array.

The LRS can be operated to obtain spectra in the slit or in slitless mode. Slitless spectroscopy is only available for time-series observations of time variable phenomena, such as transiting exoplanets or eclipsing binaries. 

When preparing observations, the observer has control of 3 primary variables: (1) choice of slit vs. slitless, (2) dithering requirements (for slit only), and (3) detector readout mode and exposure settings. 

Users should use the Exposure Time Calculator for all sensitivity calculations.

Slit vs. slitless spectroscopy

See also: JWST Slit SpectroscopyJWST Wide Field Slitless Spectroscopy, MIRI LRS TSOs

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

The LRS can be operated in slit or slitless mode. Incoming light travels the same path for both modes and use the same double prism as the disperser. The modes are very similar apart from the target placement in the field.  For slit spectroscopy, the target is placed at the location of the slit, and for slitless it is positioned at a specific pointing location in a dedicated detector subarray. Figure 1 shows the relevant detector locations in the imager focal plane layout. The choice between slit and slitless operation for LRS is dictated solely by the need for high precision spectrophotometric performance and the conditions

The slit and slitless modes have some operational differences.

  • The LRS slit mode is suited to a broad range of spectroscopic observations and therefore supports a wider range of operational choices.

    The single slit is 4.7" long (3.18 mm; 42.7 pixels) and 0.51" wide (0.33 mm; 4.6 pixels). The projected location of the slit on the focal plane array lies between the imager field of view and the coronagraphy regions on the imager detector; its location is fixed. There is no subarray choice for LRS slit: for these observations, the entire imager array is read out (APT Subarray parameter set to FULL). The structure containing the slit blocks a large portion of the background close to the source from the detector, providing better sensitivity than the slitless mode. 

  • Slitless LRS is a mode dedicated to time-series observations (TSOs), and operation is optimized for high precision spectrophotometry, e.g. of exoplanet transits.

    This mode uses a dedicated subarray region on the detector (APT Subarray parameter set to SLITLESSPRISM); its location is shown in Figure 1. The use of a smaller subarray provides faster read times and thus a greater dynamical range. The saturation limit is several magnitudes brighter than for LRS slit. The absence of the slit, however, allows more background radiation to be dispersed over the science spectrum, reducing the sensitivity by around an order of magnitude approximately. While LRS slitless is ideal for high-precision spectrophotometric observations of bright point-source targets, the slit is expected to give better performance for faint targets.

As both modes use the same dispersing element, the dispersion profile is, in principle, the same for both. The nominal spectral range of 5 to 14 μm is dispersed over approximately 390 pixels. The dispersion profile however folds over below 4.5 µm (where the prism throughput is very low), superimposing two parts of the spectrum onto each other. A dedicated filter is mounted over the slit to block radiation shortward of 4.5 µm to avoid this contamination in the slit. This effect is not mitigated for LRS in slitless mode, causing some spectral contamination at the shortest wavelengths.  The calibration at wavelengths past 12 µm is still a work in progress, due to limited signal/noise in the available calibration data, but this situation will improve with time.

Table 1 below shows the main differences between the slit and slitless modes of LRS.

Figure 1. LRS slit and slitless spectra positions on MIRI imager focal plane

Locations of the LRS slit and slitless spectra positions on MIRI imager focal plane. (The background image is an imager test exposure and has not been processed through the reduction pipeline, and does not reflect the performance of the imager.)

Table 1. Comparison between LRS slit and slitless modes 

Not available for time series observationstime series observation setting is mandatory
FULL array onlySLITLESSPRISM subarray only
Better sensitivity, fainter saturation limitWorse sensitivity, brighter saturation limit
Exposures limited to 10,000 sAllows > 10,000 s exposures
Dithering mandatoryNo dithering allowed
No spectral foldover below ~4.5 µmAffected by spectral foldover <4.5 µm
Sensitive to pointing uncertainty, driftNo slit losses
Time Series Observations special requirement not permittedTime Series Observation special requirement is mandatory
Target acquisition recommendedTarget acquisition mandatory

Sources in the imager field of view when performing LRS slit spectroscopy

There is no shutter or other way to block light from entering the imager field of view (see Figure 1) when taking a low-resolution spectrum in the slit. Point sources in the imaging field will therefore appear as slitless spectra on the imager portion of the field of view (FOV).  The broad bandpass of the LRS double prism can easily cause the detector to saturate if bright and extended sources are present in the imager portion of the array. Very bright or saturated sources can cause detector artifacts along rows and columns around the bright source, which could in principle create artifacts in the spectrum of the source in the LRS slit.  If possible, observers should avoid having very bright sources in the imager portion of the detector field of view whilst exposing with the LRS (they can check using the Aladin visualization option in APT). The full detector read out will be available to observers, allowing any such sources, if present, to be identified, which can help with data analysis.

Spectra of sources coincidentally located in the imager field cannot be processed or calibrated. 


See also: MIRI LRS Dithering, MIRI Dithering

LRS Slit

Dithering is available and mandatory for observations in the LRS slit. Dithering mitigates for the effects of bad pixels, provides subpixel sampling, and provides observations of the background for background subtraction purposes.

Two dither options are offered in the MIRI LRS Template in APT:


 The ALONG SLIT NOD option places the target alternately at 30% and 70% along the slit length—a separation of approximately 1.9". This should be the default choice for slit spectroscopy of point or compact sources. 

The MAPPING option can be chosen to map a wider field. Selecting this option then asks the user to define the number of steps in the spatial and spectral directions, and the step size in each direction (in arcseconds). Although the maximum dither size is set by avoiding the need to acquire new guide stars, a useful guideline is that dithers larger than 20" will be much slower than ones smaller than this limit. When choosing the MAPPING option, the user does not control the order in which the exposures will be executed.

LRS Slitless

In LRS slitless mode, dithering is disabled. This mode is optimized for high-precision spectrophotometry in time-series observations; for such observations, dithering is not scientifically useful and therefore not supported.

LRS exposure specifications

See also: MIRI Detector Readout Overview, Understanding Exposure Times

MIRI LRS slit spectroscopy supports 2 different detector readout patterns:

  1. FASTR1 mode (default)
    In slitless mode, only FASTR1 mode is allowed. 
  2. SLOWR1 mode

The maximum exposure duration for a single exposure with the LRS slit is 10,000 s. This limit applies to all JWST instruments and modes that are not time-series observations. However, LRS slitless observations are always marked as time-series observations. Therefore, the 10,000 second limit is waived to allow for lengthy observations of time-variable phenomena.

Users should refer to the MIRI Cross-Mode Recommended Strategies and MIRI LRS Recommended Strategies pages for dedicated advice on LRS exposure settings such as maximum recommended dwell time per dither pointing. 


Additional considerations

A few additional items should be considered:

  • to cover the full extent of extended targets, observers can specify mosaics for slit spectroscopy.
  • for both slit and slitless observations, target acquisition is highly recommended (mandatory for slitless TSOs). 

Further advice for preparing observations with MIRI LRS can be found on the following pages:

MIRI Cross-Mode Recommended Strategies

MIRI LRS Recommended Strategies


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

Latest updates
    Updated for Cycle 2

    minor text fixes & clean up

    updated to align with changes to APT

    removed reference to non-existing figure

    corrected along slit nod position locations

Originally published