Instructions for filling out the Astronomer's Proposal Tool (APT) MIRI LRS templateincluding slit and slitless (time-series) spectroscopy, are provided in this article.

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See also: MIRI Low Resolution SpectroscopyJWST Slit Spectroscopy RoadmapJWST Time-Series Observations RoadmapMIRI LRS Recommended StrategiesMIRI TSO Recommended StrategiesProposal Planning Video Tutorials

Low resolution spectroscopy is one of four observing templates available with the the Mid-Infrared Instrument (MIRI)MIRI's low resolution spectrometer (LRS) offers both slit and slitless spectroscopy from 5 to 14 μm using a double prism mounted in the MIRI filter wheel, and is designed to provide a spectral resolving power of R = 40 at 5 μm, and R = 160 at 10 μm for compact sources (<2"). The slitless LRS mode is dedicated to time-series observations of time variable systems, such as eclipsing binaries or transiting exoplanets — APT parameters for slitless LRS are optimized for this type of observation. However, there may be science cases other than time-series where the slitless mode may be useful.

The APT template requires information on the following aspects of the observation:

  1. Target acquisition
  2. Subarray (which controls whether the observation will be slitless or with slit)
  3. Dither or mapping pattern
  4. Detector read mode and exposure settings
  5. Mosaic settings
  6. Pointing verification image settings and parameters

Allowed values are documented and maintained in MIRI LRS Template parameters (an article in the JWST Proposal Parameters documentation), but are also described below with Step-by-step APT instructions.


Generic parameters

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

The following parameters are generic to all templates, and are not discussed in this article: observation Number, observation Labelobservations CommentsTarget name, ETC Wkbk. Calc ID (in the Filters dialog box), Mosaic Properties, and Special Requirements.

MIRI Low Resolution Spectroscopy 

Target Acquisition Parameters 

Target acquisition (TA) is optional for both fixed slit and slitless LRS observations with MIRI (TA was made optional for slitless LRS as of APT 2024.3.4). The parameters to configure the TA procedure, found under the MIRI Low Resolution Spectroscopy tab in the Target Acquisition Parameters panel, are Acq TargetAcq Filter, and Acq Exposure Time.  MIRI Low Resolution Spectroscopy Target Acquisition Parameters Acq TargetAcq Filter, and Acq Exposure Time

Target ACQ  

See also: LRS Slit Target Acq, LRS Slitless Target AcqMIRI Filters and Dispersers

The target for the TA should be selected from the Acq Target parameter's pull-down list that contains targets defined in the proposal. The filter used for the TA are selected from the Acq Filter's drop-down list: F560WF1000WF1500W, and FND.   

Acq Exposure Time

The Acq Exposure Time fields must be completed by selecting a MULTIACCUM exposure configuration. Each exposure is configured by setting the readout pattern and characteristics parameters: Acq Readout Pattern and Acq Groups/Int

Users should use the Exposure Time Calculator (ETC) to determine the best exposure configuration to optimize signal to noise.

Acq Readout Pattern

See also: MIRI Detector Readout Overview, MIRI Target Acquisition

Choices for the Acq Readout Pattern parameter are:

  • FAST (the default)

Number of groups and integrations

See also: Understanding JWST Exposure Times

The MIRI readout timing pattern for the TA exposure is defined by only one of the MULTIACCUM parameters: Acq Groups/Int, the number of groups per integration. 

Acq Integrations/ExpAcq Total Integrationsand Acq Total Exposure Time cannot be changed by the user. The MIRI TA is always carried out in a single integration and exposure.

The TA saturation limit for each filter depends on the array configuration used for TA, which also determines which subarray should be used for TA. The user should always refer to the ETC for signal-to-noise calculations.

LRS verification image

Obtain Verification Image?

See also: MIRI LRS Slit Target AcquisitionMIRI LRS Slitless Target Acquisition

The MIRI LRS has the capability of taking an additional exposure prior to the the science exposure, in addition to the TA sequence or simply as a pre-image. This allows the user to verify the placement of the target (Pointing Verification, denoted "PV" in the template). 

The exposure can be configured separately from the TA image in the LRS Verification Image block of the template (new as of APT version 2023.1.1). The verification image can use any of the imager science filters, either FASTR1 or SLOWR1 read modes, and custom groups and integration settings for the target + filter combination. The verification image is optional, and can be requested regardless of whether TA is used. 

As of APT2024.3.4, TA is optional for both FULL and SLITLESSPRISM subarray choices. The Obtain verification image? button will initially be set to No, but can be selected for either mode of observation. The verification image has the same options as a science image in terms of filter choices, and number groups and integrations. The array configuration cannot be changed between the verification and science images. 

LRS Parameters


See also: MIRI Detector SubarraysMIRI Low Resolution SpectroscopyMIRI Time-Series Observations

The MIRI LRS supports both slit and slitless configurations, each of which has its own scientific merit. In the LRS Parameters panel, the Subarray parameter choice controls whether the observation will use a slit or slitless setting. The slitless LRS configuration uses a dedicated detector subarray called SLITLESSPRISM—by default, slitless observations are time-series observations (TSO)Low-resolution slit spectroscopy uses the FULL frame readout.


See also: MIRI LRS Dithering

If you chose the SLITLESSPRISM subarray for slitless spectroscopy (a time-series observation), dithers are not permitted.

If you have chosen slit spectroscopy, you must choose one of 2 dither options for the observation from the drop-down menu for Dither Type:

ALONG SLIT NOD is recommended for point or point-like sources. This pattern moves a target between 2 positions along the slit. It allows for mitigation of bad pixels and provides a measurement of the background.

The MAPPING option can be used to define a small pointing grid, which is more suitable for extended targets. Additional parameters are required for this option: the number of steps for the spatial and spectral directions, and their respective step offsets (in units of arcseconds). The number of steps must be greater than 1 in one of the directions, so that the map will have a minimum of 2 positions. If an even number of mapping positions are chosen in either direction, then the targeted coordinates will be centered in one of the slit positions. Users are strongly encouraged to check their observation by visualizing it with the Aladin Viewer.

Exposure Time 

Users should use the Exposure Time Calculator (ETC) to determine the best exposure configuration to optimize the signal to noise.

In the LRS Parameters panel, an LRS sequence must be completed by selecting a MULTIACCUM exposure configuration. Each exposure is configured by setting the readout pattern and characteristics parameters: Readout PatternGroups/Int, Integration/Exp, and Exposures/Dith. Three greyed-out boxes will show calculated parameters based on the exposure dither setting entered; these fields are not editable by the user.

Readout Pattern

See also: MIRI Detector Readout Overview

For the LRS slitless configuration, only the FASTR1 mode readout pattern is available.

For the LRS slit configuration, the following options can be selected:

Number of groups and integrations

See also: Understanding JWST Exposure Times

The MIRI timing pattern per exposure is defined by 3 MULTIACCUM parameters: 

  • Groups/Int: the number of groups during an integration;
  • Integration/Expthe number of integrations during an exposure, where integration is defined as the time between resets;
  • Exposures/Dith: the number of exposures per dither position.

Total Dithers (not user-editable) returns the total number of dither steps for the observations. For ALONG SLIT NOD dithering, the box will show a value of 2. For MAPPING observations, the total number of dithers will be the product of No. Spatial Steps and No. Spectral Steps

Total Integrations (not user-editable) shows the total number of integrations for the observation, taking into account the dither and exposure settings. The formula used is Total Integrations = Total Dithers × Integrations/Exp × Exposures/Dith.

Total Exposure Time (not user-editable) returns the total exposure time calculated from the dither and exposure settings. The formula used is Total Exposure Time = Groups/Int × Total Integrations × "group read time". The group read times for FULL and SLITLESSPRISM array configurations are documented in the MIRI Detector Subarrays article for each subarray. The time is given in seconds.

Groups/Int = 2 requires permission from STScI, which can be obtained with a request through the Help Desk.

Other tabs

Mosaic Properties

See also: JWST Mosaic OverviewMIRI LRS Mosaics

See also:  Specifying Mosaics in APT 

The MIRI LRS may be used to obtain data for a region larger than the slit size by creating a MIRI LRS mosaic pattern under the Mosaic Properties tab. Use of the mosaic pattern is not supported when using the LRS in slitless mode.

Special Requirements

A variety of observatory level Special Requirements may be chosen under the Special Requirements tab. 

When using the LRS in the slitless configuration (SLITLESSPRISM subarray) the Time Series Observation and No Parallel special requirements are automatically selected. For most TSOs of transiting exoplanets (or similar), the user should specify timing constraints to define the phase timing of the observation. The Time Series Observation special requirement cannot be selected for LRS slit observations.


The Comments field (under the Comments tab) should be used for observing notes.

Notable updates
    Updates to reflect cycle 4 changes to LRS slitless TA

    Updates for TA verification image changes

    Updated text on dithers to align with the APT.

    Updated information about what subarrays require TA.

    Added paragraph about verification images.
Originally published