NIRCam Wide Field Slitless Spectroscopy APT Template

Instructions for designing JWST NIRCam wide field slitless spectroscopy observations using the Astronomer's Proposal Tool (APT).

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See also: NIRCam Wide Field Slitless SpectroscopyJWST Wide Field Slitless Spectroscopy RoadmapNIRCam WFSS Recommended StrategiesNIRCam WFSS Deep Galaxy Observations

Wide field slitless spectroscopy (WFSS) is one of 5 observing modes available for the Near-Infrared Camera (NIRCam). The WFSS mode uses grisms to obtain multi-object spectroscopy from 2.4–5.0 μm with R ~ 1,500. WFSS can be obtained in one module (2.2′ × 2.2′), or with both modules (note the module B grism throughputs are 25% lower than module A). There are 2 grisms available on the pupil wheels that disperse in orthogonal directions, along detector rows (GRISMR) and columns (GRISMC). Use of both grisms mitigates confusion from overlapping spectra. 

WFSS observations include simultaneous short wavelength (SW; 0.6–2.3 µm) imaging over roughly the same field of view via a dichroic. These SW images aid in wavelength calibration by precisely tracking the position of each dither. Direct imaging in the long wavelength (LW) channel is also supported, including coverage of out-of-field sources, which disperse light onto the detector despite being outside the imaging field of view. LW direct images and out-of-field dithers occur at the final dither position in a dither sequence.

The observer will have control over 5 primary parameters for NIRCam WFSS:

  1. NIRCam module(s)
  2. Dither pattern
  3. Grism choice
  4. Filter choice for direct and grism images
  5. Exposure parameters (detector readout pattern, number of groups, and integrations).

Allowed values are documented and maintained in the NIRCam Wide Field Slitless Spectroscopy Template parameters and described below.

Step-by-step APT instructions 


Words in bold italics are buttons 
or parameters in GUI tools. Bold 
style represents GUI menus/
panels & data software packages.

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.


See also: NIRCam Modules

Observers can select A or ALL. Module A covers one 2.2′ × 2.2′ field of view, while ALL includes both modules A and B to double the field of view. The Module grism throughputs are 25% lower than those on Module A


See also: NIRCam Subarrays

The only available option is FULL. Subarrays are not currently available for NIRCam WFSS mode, but are available in NIRCam Grism Time Series mode.

Grism (long wavelength)

See also: NIRCam Grisms

Specify GRISMR, GRISMC, or BOTH (for the long wavelength observations). GRISMR disperses along the detector rows, and GRISMC disperses along the detector columns. Use BOTH to help disentangle overlapping spectra in the data. When using both grisms, a FULL dither sequence is executed for GRISMR, then executed again for GRISMC.

Science (GRISM) exposures

Dither parameters

See also: NIRCam Wide Field Slitless Spectroscopy Dithers, NIRCam Dithers and Mosaics

Choose a NIRCam WFSS dither pattern. Larger primary dithers cover the 4–5″ gaps between the short-wavelength detectors. Smaller subpixel dithers improve the spatial resolution of the final combined image, which is especially important at wavelengths shorter than the Nyquist wavelengths: 2 μm in the SW channel, and 4 μm in the LW channel.


  • PRIMARY DITHER TYPE: choose whether to use one of the INTRAMODULE dithers or NONE. 
  • PRIMARY DITHERS: number of primary dither positions; allowed values are different for INTRAMODULE, INTRAMODULEBOX, and INTRAMODULEX.
  • SUBPIXEL POSITIONS:  number of subpixel dithers; allowed values are 2-POINT, 4-POINT, 9-POINT, and NONE.

When NIRCam WFSS is used as prime mode in a coordinated parallel combination, additional customized subpixel dither patterns become available which work well for both NIRCam WFSS and the parallel instrument mode. The NIRCam-specific subpixel dither patterns can still be selected in that case by specifying “NIRCam Only”, which results in an additional pull-down selector “NIRCam Positions” that can be used to specify the number of subpixel positions.


See also: NIRCam Filters, NIRCam Wide Field Slitless Spectroscopy, NIRCam Detector Readout Patterns

Specify the grism exposure sequences. Multiple exposures may be defined here. For each, specify:

  • DIRECT IMAGE: check the box to obtain "direct" LW and SW images after each LW grism + SW image exposure (required for the final exposure).
  • LONG FILTER: choose the long wavelength medium or wide filter to be used with the grism.
  • SHORT FILTER: choose the short wavelength filter used for simultaneous imaging.
  • READOUT PATTERN: The NIRCam detectors are read out continuously, 10.7 s per frame. Groups of frames are averaged according to readout patterns to reduce data volume for long exposures. Of the 9 readout patternsRAPID, BRIGHT2, SHALLOW4, MEDIUM8, and DEEP8 are currently recommended as yielding higher signal to noise for faint sources (Robberto 2009, 2010; and more recent tests with the ETC).
  • GROUPS/INT: The number of groups to save per integration. Multiple groups are desirable to enable "up-the-ramp" fitting to observed count rates. The resulting integration time is relevant for saturation. Each integration is preceded and followed by detector resets.
  • INTEGRATIONS/EXP: The number of integrations per exposure. Multiple dithers are preferred over multiple integrations to improve data quality, though dithers will increase overheads somewhat.
  • ETC Wkbk.Calc ID: The ETC workbook and calculation ID used to determine the exposure setup can be entered here.

Users should consult the Exposure Time Calculator, ETC, to achieve sufficient signal to noise for their science without saturating during each integration. Approximate sensitivity curves for 10,000 s exposures are available at NIRCam Wide Field Slitless Spectroscopy

Direct image exposures

Direct images

See also: NIRCam Wide Field Slitless SpectroscopyNIRCam Detector Readout PatternsNIRCam FiltersUnderstanding JWST Exposure Times

For each requested direct imaging, specify:

  • LONG FILTER: choose the long wavelength filter to be used for direct imaging.
  • SHORT FILTER: choose the short wavelength filter used for simultaneous imaging. This can be a different filter than what was used simultaneously with the grism observations.
  • ETC Wkbk.Calc ID

The available options for all of these parameters are the same as those above in science (grism) exposures.

Exposure sequence display


The Exposure Sequence Display dialog box shows the sequence of observations. The exposure sequence is:

  • (DIRECT LW + DIRECT SW)1 (optional)
  • (DIRECT LW + DIRECT SW)2 (optional)
  • ...

Parameters cannot be edited in this display box, but they can be changed by editing parameters in the boxes above: science (grism) exposures and direct image exposures.


Robberto, M., 2009, JWST-STScI-001721, SM-12
NIRCAM Optimal Readout Modes

Robberto, M., 2010, JWST-STScI-002100, SM-12
NIRCAM Optimal Readout II: General Case (Including Photon Noise)

Latest updates
    Updated to be consistent with APT 2020.2

  • Updated to be consistent with APT 25.4.2
Originally published