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James Webb Space Telescope User Documentation



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


The MIRI Medium Resolution (Integral Field Unit) Spectroscopy template is to be used for all observations using the medium 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 Medium Resolution Spectroscopy Template APT Guide.

The MIRI Medium Resolution (Integral Field Unit) 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
Science Exposures
Primary Channelselect primary channelchoose from list
Dither Specifications

Dither Typeselect dither type2-POINT,  4-POINT

Optimized Forselect type of science dither optimized forPRIMARY CHANNEL, EXTENDED SOURCE
Directionselect direction of dither patternPOSITIVE, NEGATIVE

Simultaneous Imaging

select if simultaneous imaging requiredYES, NO
Imager Subarrayselect subarray for Imagerchoose from listonly if Simultaneous Imaging = YES
Exposure Specifications

based on selection of Simultaneous Imaging parameter
Wavelength Rangeselect wavelength rangechoose from list
Imager Filterselect filterchoose from listonly if Simultaneous Imaging = YES
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
Ditherselect dither specificationchoose from list

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 

The following parameters are used to define MIRI Target Acquisition observations. Note that for Medium Resolution Spectroscopy, the imager is used for the target acquisition and the science target is generally used for the acquisition, although when necessary, an offset star may be used. For more information on defining a target acquisition specification, see MIRI MRS 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 be NONE.

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: MRS 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] = 5, 7, 9, 11, 13, 19, 27, 39, 57, 85, 99

This field specifies the number of groups in an integration.

Acquisition Number of Integrations/Exposure      

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

Science Exposures   

Primary Channel 


The parameter specifies the channel of interest for this observation. This choice will drive the placement of the target, the size of offsets if a mosaic is request, what size field of view is displayed in Aladin, and the dither pattern. Note the ALL is functionally the same at CHANNEL1.

Dither Specifications    

Most MIRI MRS observations will require dithering; see MIRI MRS Dithering for details.

Dither Type    


This parameter specifies the number of points to be used for dithering.

Optimized For      


Selecting the type of science (extended or not) the dither is optimized for controls the scale of the dither. If a target is chosen before a dither is created the default is PRIMARY CHANNEL unless the target was marked EXTENDED in which case the default is EXTENDED SOURCE

(red star)After OPTIMIZED FOR has a selected value, it will not change if a target is selected, a target's EXTENDED value is changed, or a new target is selected. The user must manually change the value.

Note for developer: If the target is marked EXTENDED, change the default value to EXTENDED SOURCE

Note for developer: Offer the actual value selected for PRIMARY CHANNEL above (e.g. CHANNEL1).



Select the direction of the dither.

Note to developer: When DITHER TYPE is 2-POINT, use only the first two points of the dither pattern for the selected DIRECTION and OPTIMIZED FOR.

Simultaneous MIRI Imaging  


This parameter specifies if simultaneous imaging is required. A value of YES will obtain imaging data at the same time as data is collected on both of the IFU detector arrays.

Note to developer: To store in the database, if SIMULTANEOUS IMAGING  = YES then set DETECTOR = ALL, else set DETECTOR = MRS.

Imager Subarray   


This parameter is only relevant when SIMULTANEOUS IMAGING  = YES. It specifies the region of the detector that is to be read out for the imager data. Note that if a value of IMAGER SUBARRAY other than FULL is used, there may be some data quality issues because of the switch from FULL which is used for the target acquisition. For more details about these subarrays, see Imager subarray.  

Note for developers: Inform the user with a warning if SUBARRAY other than FULL selected.

Note for developers: These tables show what should be displayed in the GUI, along with the appropriate commanding name for the database, for the two values of SIMULTANEOUS IMAGING.


#DetectorNo. of ExposuresReadout PatternNo. of GroupsNo. of IntegrationsDither
1MRSShortcopied from abovecopied from above[NGROUPSSHORT][NINTSSHORT]copied from above




No. of Exposures


Readout Pattern

No. of Groups

No. of Integrations










MRSLongcopied from aboveN/A




copied from above



copied from above





copied from above

Exposure Specifications   

For each exposure to be executed at each position in a dither pattern, specify the NUMBER OF EXPOSURES/DITHER, IMAGER FILTER, READOUT PATTERN, NUMBER OF GROUPS/INTEGRATION, NUMBER OF INTEGRATIONS/EXPOSURE, and DITHER. For the filter bandpasses, see MIRI Filters and Dispersers. For more information on defining an exposure specification, see Exposure Parameters.


This field specifies the detector to be used, and is automatically populated based on the choice of SIMULTANEOUS IMAGING.

Wavelength Range 


Each of the four Integral Field Unit (IFU) image slicers covers a separate wavelength range. Each of these wavelength ranges can be covered by a set of three available gratings. You can request one grating by means of the WAVELENGTH RANGE parameter. Note the same wavelength region will be obtained with each IFU channel. So if you specify SHORT (A) then the short portion of the wavelength range will be obtained simultaneously with each IFU channel.

Note for developer: APT will populate the chosen WAVELENGTH RANGE value (with Short/Medium/Long only) into both WAVELENGTH_1_4 and WAVELENGTH_2_3.

Imager Filter Name   

IMAGER FILTER [FILTER] = F560W, F770W, F1000W, F1130W, F1280W, F1500W, F1800W, F2100W, F2550W, OPAQUE, FLENS, F1065C, F1140C, F1550C,F2300C, F2550WR, P750L, FND

This field specifies the filters to be used to obtain the imaging data; this parameter is only relevant when SIMULTANEOUS IMAGING  = YES.

Note to developer: If the PRIMARY CHANNEL = ALL, or CHANNEL1 -or- the dither is OPTIMIZED FOR = EXTENDED, and the IMAGER FILTER =  F1280W,  FND,  F1500W,  F1550C,  F1800W,  F2100W, F2300C, F2550W, or  F2550WR, then warn that "for this filter the MRS dither pattern selected may cause the PSF in the simultaneous MIRI imaging to overlap.  Maximum wavelengths for imager with no PSF overlap is lambda/D >= 6." Same warning if the PRIMARY CHANNEL = CHANNEL2 and the IMAGER FILTER = F2100W, or F2550W.

Readout Pattern  


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

Developer note: see tables above for commanding names.

Developer note: MRS has a default READOUT PATTERN of SLOW and Imager has a default READOUT PATTERN of FAST

(warning)If both SLOW and one of the FAST READOUT PATTERNs are used for the MRS in one observation, a TBD second settling time delay will be incurred each time there is a switch from one to the other.

 (warning)If both SLOW and one of the FAST READOUT PATTERNs are used for the Imager in one observation, no settling time delay will be incurred, but there may be some data quality issues because of the switch.

Note to developer: inform the user with a warning in either of these cases.

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.

 Developer note: see tables above for commanding names.

Number of Integrations/Exposure

NUMBER OF INTEGRATIONS/EXPOSURE [NINTS] specifies the number of times the integration is repeated; default = 1.

While the exposure time must be the same for the Short and Long detectors, the NUMBER OF GROUPS/INTEGRATION and NUMBER OF INTEGRATIONS/EXPOSURE can differ. (warning)Therefore the parameters for groups and integrations must conform to the following restriction:


Developer note: see tables above for commanding names.

Developer note: The MRS exposure determines the total exposure duration. If SIMULTANEOUS IMAGING  = YES, for each triplet, compare the duration for the Imager exposure to the duration for the MRS exposures and give an error if the Imager exposure is longer.

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)." 


Select the number corresponding to one of the dither specification previously specified or NONE.

Note for developers: DITHER should be a required field, and there should be no default value (i.e. do not make NONE the default)

Change log

 Click here to expand...

General Notes


Change Log

Version 10 (April 18, 2019)

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

Version 9 (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 8 (August 14. 2018)

  1. PR 89798 - Updated Acquisition Number of Groups/Integration section.
  2. PR 90970 - revised warning to Wavelength Range section.

Version 7 (May 14, 2018)

  1. PR 89879 - NO PARALLEL special requirement required for external observations with NEXP>1, added red note and developer note.
  2. PR 89323 - Updated red notes in Number of Groups/Integration for FASTGRAVP
  3. PR 86951 - added restricted filters and updated developer note Filter Name section

Version 6 (December 26, 2017)

  1. PR 88480 - revised the WAVELENGTH parameter
  2. PR 89483 - added restriction on Acq Target for SS observations
  3. PR 86278 - added option for no target acquisition

Version 5 (September 27, 2017)

  1. PR 88372 - revised the WAVELENGTH parameter
  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 4 (May 11, 2017)

  1. PR 87747 - revised Readout Pattern for wait period

Version 3 (December 20, 2016)

  1. PR 80734 - added Number of Exposures parameter
  2. PR 85174 - revised Dither Types
  3. PR 85823 - added Primary Channel parameter
  4. PR 85925 - added simultaneous MIRI imaging
  5. PR 85925 - added restrictions on Readout Pattern, Num Groups
  6. PR 85925 - added default for Num Ints
  7. PR 85608 - updated TA parameters
  8. PR 85929 - added Imager Subarray restriction
  9. PR 85863 - updated developer note for Acquisition Target
  10. PR 83015 - updated Optimized For parameter

Version 2 (July 16, 2016)

  1. PR 84810 - updated text for WAVELENGTH REGION to clarify that 3 sets of exposures will be obtained when ALL selected

Version 1 (May 2, 2016)

This is the converted Word File from Chapter 12.


JWST MIRI Medium Resolution (Integral Field Unit) Spectroscopy Template

JWST MIRI Medium Resolution (Integral Field Unit) Spectroscopy template parameters

Format definitions

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.

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Change log