NIRCam Wide Field Slitless Spectroscopy APT Template

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

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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). The use of both grisms improves contamination modeling 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 images occur at the final dither position in a dither sequence.

By default, APT Aladin shows the "full spectrum" field of view; sources in that area yield complete spectra on the detector. The larger effective field of view may be shown by selecting the option to show partial spectra region. Note these regions are most easily viewed before adding the direct image and out-of-field images to the observations in APT.

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 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

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

  1. NIRCam WFSS-MIRI Imaging
  2. NIRCam WFSS-NIRISS Imaging

Note that the default option is None Selected.

NIRCam Wide Field Slitless Spectroscopy tab


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 B 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. 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.

There are 3 parameters to set:

  • Primary Dither Type may be one of the INTRAMODULE dithers (INTRAMODULE, INTRAMODULEBOX, and INTRAMODULEX) or NONE
  • Primary Dithers specifies the number of primary dither positions. Note that there are different allowed values for INTRAMODULE, INTRAMODULEBOX, and INTRAMODULEX.
  • Subpixel Positions specifies the number of subpixel dithers; allowed values are 2-POINT, 4-POINT, 9-POINT, and NONE.

When NIRCam WFSS is used as the 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 at the top of the list.


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

Specify the grism exposure sequences. Multiple exposures may be defined here. The parameters are:

  • 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: for the long wavelength channel, choose a 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, at 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 patterns, RAPID, BRIGHT2, SHALLOW4, MEDIUM8, and DEEP8 are currently recommended as yielding a higher signal-to-noise ratio 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 a sufficient signal-to-noise ratio for their science without saturating during each integration. Approximate sensitivity curves for 10,000 s exposures are available at the NIRCam Wide Field Slitless Spectroscopy article.

Direct Image Exposures

Direct images

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

For each direct imaging request, 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.
  • Readout Pattern
  • Groups/Int
  • Integrations/Exp
  • ETC Wkbk.Calc ID

The available options for these parameters are the same as those listed above for 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 panels above: Science (Grism) Exposures and Direct Image Exposures.

Other tabs

Mosaic Properties

See also: JWST Mosaic OverviewNIRCam Dithers and MosaicsNIRCam Mosaics

See also:  Specifying Mosaics in APT 

Data can be obtained for a region larger than the NIRCam FOV by creating a mosaic pattern under the Mosaic Properties tab.

Special Requirements

A variety of observatory level Special Requirements may be chosen under the Special Requirements tab.


The Comments field (under the Comments tab) should be used for observing notes.


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)

Notable updates
    Added text that APT Aladin shows the "full spectrum" FOV

    Updated to be consistent with APT 2022.1

    Updated to be consistent with APT 2020.2

  • Updated to be consistent with APT 25.4.2
Originally published