NIRISS Wide Field Slitless Spectroscopy APT Template

Instructions for filling out the JWST NIRISS wide field slitless spectroscopy template, using the Astronomers' Proposal Tool (APT), are provided in this article.

Wide field slitless spectroscopy (WFSS), one of 4 observing modes available in NIRISS, is used for low-resolution (R ≈ 150) multi-object spectroscopy of sources over the full 2.2' × 2.2' NIRISS field of view.

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See also: NIRISS Wide Field Slitless Spectroscopy, JWST Wide Field Slitless Spectroscopy RoadmapNIRISS WFSS Recommended StrategiesNIRISS WFSS with NIRCam Parallel Imaging of Galaxies in Lensing Clusters

This mode uses a pair of identical grisms (GR150R, GR150C) that are mounted orthogonally to each other in the filter wheel and cover the wavelength range from 0.8 to 2.2 μm. The relative dispersion directions are perpendicular to each other, allowing blended spectra to be disentangled in crowded fields. Individual WFSS grism exposures are performed with a filter in the pupil wheel (PW) to (1) select the wavelength range of interest, and (2) reduce blending spectra in different wavelength regions from objects throughout the field. Obtaining spectral coverage across the full 0.8–2.2 μm bandpass requires separate exposures in the adjacent filters.

Each WFSS observation will consist of one or more exposures in APT that include: (1) direct imaging and (2) dithered grism exposures using the same PW filter as the direct image. The direct imaging enables identification of objects in the grism image and calibration of the absolute wavelength zeropoint for grism spectra of each object in the field. One direct image will be taken at the first dither position and a final one at the last dither position. Since the NIRISS point spread function (PSF) is undersampled over the WFSS wavelength range, dithering will be required for WFSS observations. 

The observer will have control over 3 primary parameters for NIRISS WFSS: (1) filter for direct and grism images, (2) dithering pattern, and (3) detector readout mode and exposure time (via the number of groups and integrations). Allowed values are documented in the NIRISS WFSS Template parameters article.

Step-by-step APT instructions are provided below.

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.

Coordinated Parallel observations

See also: JWST Parallel Observations

NIRISS WFSS supports coordinated parallel observations with NIRCam imaging or MIRI imaging. When the Coordinated Parallel box (visible when the Template parameter is set to NIRISS Wide Field Slitless Spectroscopy) is checked, one of these 2 parameter values can be selected:

  2. NIRISS WFSS-NIRCam Imaging

Note that the default option is None Selected.

NIRISS Wide Field Slitless Spectroscopy tab

Science Observation


See also: NIRISS WFSS Dithers, NIRISS Dithers 

Choose the NIRISS WFSS dither pattern. The dither offsets are large enough to ensure that a compact source does not fall on the same pixel twice, but not so large as to be affected by distortion across the detector.


  • Image Dithers: number of steps, where the first step corresponds to initial pointing; allowed values are 2, 3, 4, 6, 8, 12, 16
    • Note: if the NIRISS WFSS mode is used as prime in coordinated parallel mode, additional customized dither patterns become available through the Image Dithers pull-down menu. (These customized dither patterns have been designed to work well for both the primary and the selected parallel instrument mode.) If such a customized dither pattern is selected, the next selector (Pattern Size) disappears.
  • Pattern Size: amplitude of dither step; allowed values are SMALL (≈0.3")MEDIUM (≈0.6")and LARGE (≈1.2")


See also: NIRISS Wide Field Slitless Spectroscopy, NIRISS GR150 Grisms, NIRISS Filters, NIRISS Detector Readout Patterns 


  • Grism: GR150RGR150CBOTH. Note that APT will issue a warning that spectral overlap from multiple sources may occur if GRISM=BOTH is not selected. Users are advised to address this issue in the text of their proposal.

  • FilterF090WF115WF140MF150WF158MF200W
  • Readout Pattern
    • NISRAPID: all frames are read and stored. Generally this can be used if the number of frames per integration is small and the data volume is not an issue. There is a limit of 30 on the number of groups per ramp in NISRAPID read out.
    • NIS: four frames are averaged per group—used for producing a lower data rate for longer integrations of faint targets. 
  • Groups/Intthe number of groups during an integration, equal to the number of frames read per integration (for NISRAPID), or the number of frames read out divided by 4 (for NIS).
  • Integrations/Exp: the number of integrations during an exposure, where an integration is the time between detector resets.
  • ETC Wkbk. Calc ID: the accompanying ETC workbook ID number for the calculation (optional).

The Exposure Time Calculator (ETC) should be used to determine the combination of Groups/Int and Integrations/Exp necessary to achieve the required signal-to-noise ratio. Individual integrations longer than 1,000 s are discouraged because of the effect of cosmic ray hits on the detector. For long exposures, a suitable number of integrations and a number of groups less than or equal to 25 is suggested.

To take observations in different filters, Add additional observations and specify the appropriate filter. This option will by default add a corresponding direct image for the new grism observation in the Direct Image Exposure Parameters dialog box.

Direct Image Exposure Parameters

By default, one direct image is taken before and after each set of dithered grism observations. Optionally, the user can choose to take 2 additional dithered direct images after the dithered grism observations. This option is only recommended for the case of where both grisms are used with the same filter (Grism = BOTH in the Science Observation dialog box). See NIRISS WFSS Recommended Strategies for more information on that option.

Direct images

See also: NIRISS Imaging, NIRISS Detector Readout PatternsUnderstanding JWST Exposure Times


  • Readout PatternNIS, NISRAPID (see above)
  • Groups/Int (see above)
  • Integrations/Exp (see above)
  • Two Extra Dithers: The option to take 2 additional dithered direct images after a dithered set of grism observations. The default is NO
  • ETC Wkbk. Calc ID (see above)

The Filter of the direct image will (automatically) match the Filter specified for the corresponding grism image in Science Observation

Note: the Exposure Time Calculator mode for WFSS does not include calculations for direct imaging. To run calculations for direct imaging to determine Groups/Int and Integrations/Exp, use the NIRISS imaging mode in ETC.

All Exposures Display


The All Exposures Display dialog box will show the sequence of observations for each science observation specified in the Science Observation dialog box. If BOTH grisms are selected in the Science Observation dialog box, the exposure sequence for that observation will be DIRECT → N × GRISM → 2 × DIRECT → N × GRISM → DIRECT, where all exposures in this sequence will have the same exposure number and N refers to the number of dithered grism exposures as specified in the Image Dithers parameter. Note: APT only shows one direct image specification in between the grism exposures, when in fact 2 sets of direct images are actually taken. In the NIRISS WFSS observing mode, a direct image is required before and after a set of grism exposures. When switching between grisms, the first (set of) direct image(s) corresponds to the last (set of) direct image(s) after the first set of dithered grism exposures, while the second direct image corresponds to the first direct image prior to the second set of dithered grism exposures.

Note: fields can not be edited in the All Exposures Display dialog box, but must be edited in the Science Observation and Direct Image Exposure Parameters dialog boxes. Changes to these parameters will be propagated to All Exposures Display.

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Mosaic Properties

The NIRISS WFSS mode can be used to obtain data for a region larger than the field-of-view by creating a mosaic pattern. Mosaics are required to ensure that complete spectra are obtained over the full 2.2' × 2.2' NIRISS FOV, as specified on the NIRISS Mosaics article. Visit the Mosaic Parameters article for information on setting up a mosaic.

Special Requirements

A variety of observatory level special requirements may be chosen.

Latest updates
    Updated to include note about the warning APT will show if user selects only one grism.

    Added info on the optional two extra dithers introduced in APT 2020.2 for the direct images taken after the dithered grism observations in the "Direct Image Exposure Parameters" section. Also adjusted the "All Exposures Display" section accordingly.

    Added info on the availability of customized dither patterns for coordinated parallels in the "Dither" section. 

    Added note in "Direct Images" section that to determine number of groups and integrations in ETC, the NIRISS Imaging calculation option has to be selected.

    Updated to have consistent parameter headings as APT v. 27.0
Originally published