MIRI Low Resolution Spectroscopy Template Parameters

JWST's MIRI Low Resolution Spectroscopy template, available in the Astronomer’s Proposal Tool (APT), has parameters for specifying and scheduling observations.


 Format definitions...

Purple text indicates the parameter is Limited Access.

Boldface italics type indicates the name of an APT parameter or a value for a parameter.

(warning)Red text indicates restrictions on a parameter.

(red star) Black text indicates an important note.

Brown text indicates notes for the developers.

Green text indicates the name of the parameter used by Commanding.

Items in brackets - <value> - are required values.

Items in square brackets - [<value>] - are optional.

The MIRI Low Resolution Spectroscopy (LRS) template is to be used for all observations using the low resolution spectroscopy mode of MIRI, and this article describes allowed values for all parameters. Advice on how to choose values which optimize your science can be found in the MIRI LRS APT Template article.

The MIRI Low Resolution Spectroscopy template consists of the following parameters:

FieldDetailsValues Notes
General Information   
Observation Numberassign observation number


Observation Label provide observation labeltextoptional
Observation Commentsprovide observation commentstextoptional
Target Nameselect Target Namechoose from listfrom Target List

ETC Wkbk. Calc. ID

specify ETC Workbook Calculation IDnumberfor science exposures (optional) and target acquisition exposures
Mosaic Propertiesspecify mosaic parametersnumberif needed
Special Requirementsspecify special requirementschoose from listif needed
Template Specific Information   
Target Acquisition Exposure
Acquisition Targetselect acquisition targetchoose from list
Acquisition Filterselect filter namechoose from list
Acquisition Readout Patternselect acquisition readout patternchoose from list
Acquisition Number of Groups/Integrationselect number of groups per integrationchoose from list
Acquisition Number of Integrations/Exposureselect number of integrations per exposure1only allowed value
LRS Parameters
Obtain verification image?select option for verification imageYES, NOoptions available only when TACQ_TARGET is the science target and SUBARRAY = FULL.
Science Exposures
Subarrayselect subarraychoose FULL or SLITLESSPRISM
Dither Specifications
Dither Typeselect dither typechoose from list
Number of Spectral Stepsenter number of spectral steps1, 2, 3, ...for DITHER TYPE=MAPPING only
Spectral Step Offsetenter offset per spectral stepany positive numberfor DITHER TYPE=MAPPING only
Number of Spatial Stepsenter number of spatial steps1, 2, 3, ...for DITHER TYPE=MAPPING only
Spatial Step Offsetenter offset per spatial stepany positive numberfor DITHER TYPE=MAPPING only
Readout Patternselect readout patternchoose from list
Number of Groups/Integration

specify number of groups per integration

Number of Integrations/Exposure

specify number of integrations per exposure

Number of Exposures/Ditherspecify number of times to repeat the exposure for each dither positionnumber

General Information 

 The following parameters are generic to all templates, and are not discussed in this article: Observation Number,  Observation Label, Observations Comments, Target Name, ETC Wkbk. Calc. ID, Mosaic Properties, and Special Requirements. 

Target Acquisition Parameters

The following parameters are used to define MIRI Target Acquisition observations. Note that for Low Resolution Spectroscopy, the science target is generally used for the acquisition, although when necessary, an offset star may be used. For more information, see MIRI LRS Slit Target Acquisition.

Acquisition Target 

If an offset star is to be used for the target acquisition observation, select the ACQUISITION TARGET NAME [TACQ TARGET] from the list of targets previously entered. When the science target is a Solar System target, the acquisition target MUST be the science target. Or, if no target acquisition is needed, select ACQUISITION TARGET to beNONE”.

Note for developers: For APT, the ACQUISITION TARGET field should always be present, but should be defaulted to the prime target for the Observation. 

Note for developers: If the acquisition target is different from the science target, APT should provide an error if the distance between the ACQ and Science targets is greater than the Visit splitting distance found in the PRD. Also, if there are multiple pointings within a visit, then the rules have to apply to the greatest separation between acq target and pointing within the visit.

Note for developers: LRS target acquisition will use the FULL array.

Note for developers: If the observation target is a fixed target, remove solar system targets from the acquisition target options. If the observation target is a Solar system target, remove all options from the acquisition target list except for the observation target and NONE

Acquisition Target Filter 


This parameter specifies the TARGET ACQUISITION FILTER to be used for the target acquisition. 

Acquisition Readout Pattern   


This field specifies the readout pattern to be used to obtain the acquisition data. 

Acquisition Number of Groups/Integration  

NUMBER OF GROUPS/INTEGRATION [TACQ NGROUPS] = 4, 6, 8, 10, 12, 22, 36, 44, 66, 86, 98

This field specifies the number of groups in an integration. (warning) When SUBARRAY=FULL, the only values available are 4, 6, 8, 10.

Acquisition Number of Integrations/Exposure 

The NUMBER OF INTEGRATIONS/EXPOSURE [TACQ NINTS]  is set to 1 for target acquisition observations.

LRS Verification Image

Obtain Verification Image


This field specifies whether a verification image will be obtained. If OBTAIN VERIFICATION IMAGE = YES, an additional image is obtained using the Target Acquisition parameters.

Table 1. Selection of LRS Verification Image

SubarrayTarget AcquisitionYes/No buttons editable?DefaultLRS Verification Image
SLITLESSPRISMscience targetnoYESLRS Verification image cannot be un-selected.
FULLscience targetyesYESLRS Verification image is selected and may be un-selected.
FULLNONEnoNONo LRS Verification image

Science Exposures   



This parameter specifies the region of the detector to be read out.  For more details about these subarrays, see Subarray in the MIRI LRS APT Template article.

The TIME SERIES OBSERVATION special requirement is only allowed when SUBARRAY = SLITLESSPRISM. Note that time series observations cannot have parallels attached to them so you must add the  NO PARALLEL special requirement.

Dither Specifications 

Most MIRI LRS observations will require dithering; see MIRI LRS Dithering for details. Note that the dither pattern does NOT apply to the target acquisition image. (warning)To preserve very precise wavelength calibration of the slitless spectrum, dithering is not enabled for the SUBPRISM subarray.

Dither Type 


 This parameter specifies the primary pattern to be used for dithering. There are two primary patterns available for use with the LRS, one for point sources and one for extended targets.

Along Slit Nod 

This option selects a pattern for point sources.  Two exposures are taken, one at each of two points along the slit.  No further parameters are required.


This option selects a grid pattern of dither points.  The following additional parameters are required.

Number of Spectral Steps 


This parameter specifies the number of steps in the spectral direction.

Spectral Step Offset 

This parameter specifies the offset per step in the spectral direction.  The offsets are given in arcseconds, and can be any positive floating point value.

Number of Spatial Steps 


This parameter specifies the number of steps in the spatial direction.

Spatial Step Offset 

This parameter specifies the offset per step in the spatial direction.  The offsets are given in arcseconds, and can be any positive floating point value.

Note for developers: see MIRI Dithering Details for dithering details.

Exposure Specifications    

For each exposure to be executed at each position in a dither pattern, specify the READOUT PATTERN, NUMBER OF GROUPS/INTEGRATION, NUMBER OF INTEGRATIONS/EXPOSURE, and the  NUMBER OF EXPOSURES/DITHER. For more information on defining an exposure specification, see Exposure time in the MIRI LRS APT Template article.

Readout Pattern 


This field specifies the readout pattern to be used to obtain the data.

Note for developer: SLOW is only available when SUBARRAY = FULL.

Number of Groups/Integration 

NUMBER OF GROUPS/INTEGRATION [NGROUPS] specifies the number of groups in an integration.

(warning)  NUMBER OF GROUPS/INTEGRATION must be 2 or greater.

(warning) If READOUT PATTERN = FASTGRPAVG, it is recommended that NUMBER OF GROUPS/INTEGRATION be at least 4. For all other READOUT PATTERNs, a number of 2-4 is allowed but not recommended.

Number of Integrations/Exposure   

NUMBER OF INTEGRATIONS/EXPOSURE [NINTS] field specifies the number of times the integration is repeated/ The default value is 1.

Number of Exposures/Dither 

NUMBER OF EXPOSURES/DITHER [NEXP]  specifies the number of times the exposure is to be repeated at each dither position. The default value is 1.

 (warning) If NUMBER OF EXPOSURES/DITHER > 1, the NO PARALLEL special requirement is required.

Note for developer: If NUMBER OF EXPOSURES/DITHER > 1. provide the following error: " The Special Requirement NO PARALLEL must be used with this Observation." and the following diagnostic explanation: "The NO PARALLEL requirement is required for Coordinated Parallels; Time Series observations; external observations with NUMBER OF EXPOSURES/DITHER > 1 and dithering; NIRSpec observations with Auto Calibrations; NIRCam Grism Time Series; and engineering templates (FGS Internal Flat, MIRI Anneal, MIRI External Flat, NIRSpec Internal Lamp, NIRSpec MSA Masking, NIRSpec MSA Anneal, NIRISS Internal Flat, all WFSC)." 

Change log

 Click here to expand...

Version 12 (November 26, 2019)

  1. PROPINSTJWST-91421  added LRS Parameters section.

Version 11 (September 23, 2019)

  1. PROPINSTJWST-91395 - added new Acq Readout Pattern options.

Version 10 (June 17, 2019)

  1. PROPINSTJWST-91381 - updated Acquisition Number of Groups/Integration section.

Version 9 (April 18, 2019)

  1. Editorial change (moved change log to main article).

Version 8 (December 21, 2018)

  1. PROPINSTJWST-91122 - added developer note  in Acquisition Target section restricting acquisition target options for fixed vs. solar system observations.
  2. PROPINSTJWST-87189 - removed SLOW option from acq readout.

Version 7 (August 10. 2018)

  1. PR 89798 - Updated Acquisition Number of Groups/Integration section.

Version 6 (May 14, 2018)

  1. PR 89879 - NO PARALLEL special requirement required for external observations with NEXP>1, added red note and developer note.

    PR 89323 - Updated red notes in Number of Groups/Integration for FASTGRAVP

Version 5 (December 27, 2017)

  1. PR 89483 - added restriction on Acq Target for SS observations
  2. PR 88411 - added note about TSO requirement and NO PARALLEL
  3. PR 86278 - added option for no target acquisition

Version 4 (November 20, 2017)

  1. PR 87189 - updated readout pattern SLOW to be non-LAP
  2. PR 87895 - updated Exposure field labels to Groups/Integration and Integrations/Exposure
  3. PR 88896 - made TA Number of Groups/Integration be a picklist

Version 3 (May 11, 2017)

  1. PR 85610 - renamed Dither Types

Version 2 (January 16, 2017)

  1. PR 85608 - updated TA parameters
  2.  PR 85922 - renamed SUBPRISM to SLITLESSPRISM
  3. PR 80734 - added Number of Exposures parameter    
  4. PR 85925 - set default Readout Pattern
  5. PR 85925 - added restriction on Number of Groups 
  6. PR 85863 - updated developer note for Acquisition Target
  7. PR 85608 - removed statement about Acq Flux
  8. PR 81292 - removed inconsistency in name of dither parameters

Version 1 (May 6, 2016)

This is the converted Word File from Chapter 11.