NIRISS Wide Field Slitless Spectroscopy

The wide field slitless spectroscopy (WFSS) mode of JWST’s Near Infrared Imager and Slitless Spectrograph (NIRISS) enables low-resolution (R ≈ 150) spectroscopy between 0.8–2.2 μm, over the 2.2’ × 2.2’ FOV.

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See also: NIRISS Wide Field Slitless Spectroscopy APT Template, NIRISS WFSS Recommended Strategies, NIRISS WFSS Science Use Case

The wide field slitless spectroscopy (WFSS) mode of NIRISS enables low-resolution (R ≈ 150) spectroscopy over the wavelength range 0.8–2.2 μm for all objects within the 2.2’ × 2.2’ field of view (FOV) of the NIRISS detector.

The NIRISS WFSS mode is particularly useful for detecting high-redshift emission-line galaxies and can be used very efficiently as a parallel observing mode with other instruments.

The WFSS mode uses a pair of identical grisms (GR150R and GR150C) that are mounted in the filter wheel so that their respective dispersion directions are perpendicular to each other on the detector.

  • The GR150R grism disperses along the direction of fast readout.
  • The GR150C grism disperses along the direction of slow readout.  

Data acquired with both dispersion directions helps to disentangle blended spectra in crowded fields. The NIRISS WFSS Recommended Strategies article discusses using both grisms in more detail.

WFSS observations are obtained by using one or both of the grisms in combination with a wide- or medium-band blocking filter located in the pupil wheel (PW). The blocking filters limit the wavelength coverage and therefore the extent of spectra on the detector, which reduces the blending of spectral traces from objects distributed throughout the FOV. 

Figure 1. Elements in the NIRISS pupil and filter wheels used by the WFSS mode

Elements in the NIRISS pupil and filter wheels used by the WFSS mode

Optical elements available to the WFSS mode are marked by green dashed circles. The GR150R and GR150C grisms located in the filter wheel (FW) can be used in combination with any of the blue filters located in the pupil wheel (PW) to obtain dispersed images. Direct images are obtained by selecting one of the blue filters from the PW, which is used in combination with the CLEAR aperture in the FW.
Figure 2. WFSS simulations of direct and dispersed images through the F115W filter

Simulated image of the massive lensing cluster MACS J0416.1-2403 observed with the NIRISS F115W filter

Simulated image of the massive lensing cluster MACS J0416.1-2403 observed with the GR150C grism and F115W blocking filter.

Simulated images of the massive lensing cluster MACS J0416.1-2403 observed with the GR150R grism and F115W blocking filter

Simulated images of the massive lensing cluster MACS J0416.1-2403 observed with the NIRISS F115W filter (top panel); the GR150C grism and F115W blocking filter (middle panel); and the GR150R grism and F115W blocking filter (bottom panel). The orthogonal dispersion directions of the GR150R and GR150C grisms help to disentangle blended spectra.

Layout of spectral orders

 Figure 3. Relative positions of WFSS spectral orders for all blocking filters

Relative positions of spectral orders for all the filters that can be used in conjunction with the GR150R and GR150C grisms in the WFSS mode. The PW filter names are indicated on the left side of the figure. Spectral order numbers are -1, 0, 1, and 2 from left to right, respectively. 

WFSS observation sequence

See also: NIRISS WFSS Template APT Guide, NIRISS GR150 Grisms, NIRISS Filters, NIRISS WFSS Dithers, NIRISS Mosaics

An observation in the WFSS mode proceeds as follows:

  1. A blocking filter is selected in the PW; the FW is set to CLEAR.
  2. A single (i.e., non-dithered) image of the scene is taken, called the the "pre-image."
  3. One of the GR150 grisms is selected, either GR150C or GR150R.
  4. Dithered grism images are taken.
  5. At the last dither position, the CLEAR element is selected again and a single image of the scene is taken, called the "post-image." As of APT 2020.2, this "post-image" can optionally be executed with 2 extra dithers, if desired.
  6. Optional: select a different blocking filter and repeat steps 1–5 with the same grism.
  7. Optional: select the other GR150 grism and repeat steps 1–6.

Exercising optional step 6 will increase the wavelength coverage for spectra of the objects in the scene, while exercising step 7 will produce observations of the scene with spectra dispersed in orthogonal directions on the detector.

The pre- and post-image through the relevant blocking filter is required:

  • to enable proper identification of object(s) seen in the grism image(s), and
  • to determine the origin of the absolute wavelength scale for the grism spectra for each object in the field.  

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

Users also have the option in the Astronomer's Proposal Tool (APT) to specify BOTH grisms in an observation sequence. If this option is selected in APT, the sequence will first be executed with the GR150R grism followed by the GR150C grism, in combination with the selected blocking filter. This option also increases the fraction of the NIRISS field of view for which both direct images and first-order spectra are captured (see NIRISS WFSS Recommended Strategies for more detail on this).

WFSS blocking filters

See also: NIRISS Filters

The NIRISS WFSS mode uses 6 blocking filters in the PW. These are the 4 wideband filters (F090W, F115W, F150W, and F200W), and 2 medium-band filters (F140M, and F158M). Figures 3 and 4 provide different visualizations of the wavelength coverage and spatial extent of the traces isolated by the blocking filters.

Figure 4. Filters for use with the WFSS mode grisms

Filters for use with the WFSS mode grisms

WFSS subarrays

See also: NIRISS Detector Subarrays

Subarrays are not available for general observations in the WFSS mode. However, to support calibration observations of brighter standard stars, subarrays are available in the external calibration template for use by STScI instrument scientists. More details are provided in the NIRISS subarrays article.

WFSS dither patterns

See also: NIRISS WFSS Dithers, NIRISS Dithers

The NIRISS PSF is undersampled in the wavelength range (0.8–2.2 μm) covered by the WFSS mode. For compact sources, a significant fraction of the flux in WFSS spectra is concentrated in the central few pixels of the cross-dispersion profile. Dithering is therefore required for WFSS observations in order to mitigate the impact of hot pixels, cosmic rays, and residual flat field errors, and also to improve the effective spatial resolution.

Since WFSS observing programs are likely to use both the GR150R and GR150C grisms in an observing sequence, optimized dither patterns have been defined for this mode. Details on the available dither patterns are provided on the NIRISS WFSS dithers page.

Simulating WFSS observations

See also: Mirage JWST Data Simulator

The software for simulating NIRISS WFSS data and the necessary grism configuration files can be obtained from the MIRAGE (Multi-Instrument RAmp GEnerator) github repository.


Dixon, V., Willott, C., 2013, STScI Newsletter, Volume 30, issue 2
Wide-Field Slitless Spectroscopy with Webb's NIRISS


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    Added optional three-point dither for "post-images" implemented in APT 2020.2.

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