NIRSpec Multi-Object Spectroscopy APT Template

NIRSpec's multi-object spectroscopy (MOS) observing mode provides the means to simultaneously obtain the spectra of multiple objects within its 3.4 × 3.6 arcmin overall field of view. Because of the complexity of this mode, the MSA Planning Tool (MPT) within APT was developed to help users create MOS observations.

 Caution: MOS observations are created differently!
The MOS APT Template should not normally be filled out by the user. Instead, the MSA Planning Tool (MPT) should be used.

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Introduction: Start preparing your MOS observation with the MSA Planning Tool

For most MOS science, users should not begin by filing out the MOS APT template. Start with the MSA Planning Tool (MPT) instead.  MPT will automatically populate the template when an observation is created. Follow the instructions in the MSA Planning Tool, MPT page.

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

Long slit observations of moving targets and/or extended targets are an exception to the rule. In these specific cases, the user should start with the MOS Observation template following the steps shown in the Long Slit Observations article.

When an observation is defined using the MPT, the user should then complete a few remaining fields in the observation template:

MOS APT template parameters

The NIRSpec multi-object spectroscopy template parameters are listed in Table 1.

Table 1. NIRSpec MOS APT template parameters

FieldDetailsValues Notes

General observation information

Observation NumberAssign an observation number


During program implementation, the observation number should not be changed.

 Observation LabelProvide an observation labelTextOptional

Observation Instrument

Observation Template
NIRSpec Multi-Object Spectroscopy
Coordinated ParallelCheckboxChoose NIRCam or MIRI imaging from pull-down listCheck the box if adding NIRCam or MIRI parallels, then select from the pull-down menu 
TargetSelect a target name from the Targets folder
Can be a Moving Target, Fixed Target, or an MSA Catalog Target
Special RequirementsSpecify special requirementsChoose from listIf needed
Observation CommentsProvide observation commentsTextOptional

Target acquisition parameters

TA MethodSelect TA method from pull-down listNONE, MSATA, WATA, VERIFY_ONLY

 MSATA is required for an MSA Catalog Target

WATA is the default for moving target or long slit observation of extended fixed target

MSATA visit-level selections

Reference Star BinList of reference stars and MSA configurationChoose from listBins of candidate reference stars are constructed for each TA filter and readout pattern when MSATA is used. The user must select one bin. This is typically done for the program update (after an Aperture PA is assigned by StScI).

Acq Filter

Filter name
Will be determined from the user-selected reference star bin

Acq Readout Pattern

Readout pattern
Will be determined from the user-selected reference star bin

Acq MSA Configuration

Select MSA configuration Choose Auto Acq MSA Config or ALLOPENDefault is an auto-generated protective MSA configuration for selected reference star bin

WATA target acquisition exposure

Acq TargetSelect acquisition targetChoose from list
Acq SubarraySelect subarraySUB32, SUB2048, FULL
Acq FilterSelect filterF110W, F140X, or CLEAR
Acq Readout PatternSelect readout patternNRSRAPID, NRSRAPIDD6

Acq ETC Wkbk. Calc. ID

Specify ETC Workbook Calculation IDNumberOptional ETC workbook ID

Pointing verification (PV) image

PV Filter

Select filter name

Choose from list F110W, F140X, or CLEAR

PV images are offered when TA Method = VERIFY_ONLY.  These images are taken at the end of each visit.

PV Readout Pattern

Select readout pattern


PV Groups/Int

Specify number of groups/integration

PV MSA Configuration
Specify filename or value

Choose from list that includes ALLOPEN, ALLCLOSED, Q4 Field Point 1 Long SlitQ4 Field Point 2 Long Slit

Science parameters

Primary Candidate ListSelect from available candidate sets defined in the MSA Catalog Target
MPT will populate this
Filler Candidate ListSelect from available candidate sets defined in the MSA Catalog Target
MPT  will populate this
Aperture PAPlanned and Assigned angle fields are shown.  Assigned is blank until an angle is assigned by STScI.Values from 0 to 360 degreesPlanned aperture position angle is the angle used to run MPT, and is greyed out in the template.  Assigned is the Aperture position angle given to the observation for making flight updates.
Dither Type (if Coordinated Parallels checkbox is enabled and NIRSpec MOS-NIRCam imaging is selected.)Select option for dithers








This option is only available when NIRCam imaging coordinated parallels are included. Additional small dithers in the cross-dispersion direction improve PSF sampling in the NIRCam imaging.
Science ApertureSelect apertureMSA_Center, Q4 Field Point 1, Q4 Field Point 2Select MSA_Center for MOS observations planned with MPT.  Other selection options are for use with the pre-defined MSA long slit configurations.

Science exposure specification table

Grating/FilterSelect grating/filter nameChoose from listSelect one grating/filter pair per exposure line
Readout PatternSelect readout patternNRS, NRSRAPID, NRSIRS2, NRSIRS2RAPIDSelect from one of the traditional or IRS2 readout modes.
Groups/IntSpecify number of groups per integrationNumber
Integrations/ExpSpecify number of integrations per exposureNumber
AutocalSelect auto calibration optionChoose from NONE, WAVCAL, FLAT, BOTHNONE is recommended

ETC Wkbk. Calc. ID

Specify ETC Workbook Calculation IDNumberOptional

Science configurations/pointings table

MSA ConfigurationSelect MSA configurationChoose from listCan be a custom MSA configuration, one created using the MSA Planning Tool, or a pre-defined long slit.
Exposure SpecificationSelect Grating/Filter combinationChoose from list of grating/filter specification in the Exposure Specification table

Indicates the line from the Exposure Specification table that is used for a row in the Configurations/Pointings table.

If generated from an MPT Plan, these will be filled in.

PointingRA and Dec of the PointingDefaults to MSA Catalog Target Reference point if not specifiedThe base pointing; if the observation was generated from an MPT Plan, this is the pointing of the exposure.
Dispersion Offset
Number in (fractional) shuttersOffset from base pointing in dispersion direction (X)
Cross-Dispersion Offset
Number in (fractional) shuttersOffset from base pointing in spatial, or cross-dispersion, direction (Y) 
Nod Pattern

Choose from list when building MOS observation manually

2 shutter slitlet, 3 shutter slitlet, 5 shutter slitlet, 5 shutter slitlet with gaps

Can be specified directly in the template, or selected by MPT. If generated from an MPT Plan, this is automatically populated.
Total Integrations
NumberValue depends on the number of nods selected
Total Exposure Time
Number of secondsExposure duration for all integrations in the exposure specification
Edit Configclick "Edit" buttonClick button to edit configurationOption to alter the MSA configuration of one or more exposures. Clicking this will bring up the MSA configuration editor.

Confirmation images

Obtain Confirmation ImagesSpecify if confirmation images are desiredChoose After Target Acq or After Target Acq and New MSA ConfigOption to obtain confirmation images of the science sources in the science slits with the science dispersers. (Selecting After Target Acq will not work for  TA Method = VERIFY_ONLY or NONE)
Confirmation Type  Reflects the selection of Obtain Confirmation Images parameter
Conf. Readout PatternSelect readout patternNRS, NRSRAPID, NRSIRS2, NRSIRS2RAPIDReadout pattern choices will be a subset depending on the science mode (traditional or IRS2).
Confirmation Groups/IntSpecify number of groups per integrationnumber


Karakla, D. et al. 2014, Proc. SPIE 9149
The NIRSpec MSA Planning Tool for multi-object spectroscopy with JWST

Notable updates
    Added warning to Table 1 row 1 that observation number should not be changed during program implementation

    Clarified some template parameters

    Added info about "Planned" and "Assigned" Aperture PA

    Updated confirmation image readout patterns
Originally published