JWST Wide Field Slitless Spectroscopy Roadmap

A roadmap to guide users, step-by-step, through the process of designing a JWST slitless spectroscopy observing program using the NIRISS and NIRCam wide field slitless spectroscopy (WFSS) modes is provided in this article.

On this page

Example science cases that use the NIRISS WFSS and NIRCam WFSS modes may be found in these articles:
NIRISS WFSS with NIRCam Parallel Imaging of Galaxies in Lensing Clusters
NIRCam WFSS Deep Galaxy Observations

Each step listed below is followed by a list of articles with additional details.

Preliminary considerations

Spectral wavelength coverage for the science case, and spectral overlap in crowded fields are important considerations when choosing filters and grisms for designing WFSS observations. Make sure to read the recommended strategies to see what options are suitable for the specific science case.

NIRISS WFSS Recommended Strategies
NIRCam WFSS Recommended Strategies

The WFSS modes may also be used for coordinated and pure parallel observations. This roadmap only focuses on the use of WFSS for prime observations.

Step-by-step guidelines

  1. Choose the instrument (NIRISS, NIRCam, or both) to use for the science case, based on the wavelength coverage.
    NIRISS Wide Field Slitless Spectroscopy (0.8–2.2 μm)
    NIRCam Wide Field Slitless Spectroscopy (2.4–5.0 μm)

  2. Choose the blocking filters that cover the wavelengths of interest.
    NIRISS Filters
    NIRCam Filters

  3. Check the the direct image and grism (line and continuum) sensitivities in the WFSS mode(s) and wavelength(s) of interest. 
    NIRISS Sensitivity
    NIRCam Sensitivity

  4. Choose one or both of the orthogonal grisms. Use of both grisms may be needed to disentangle overlapping spectra in crowded fields as discussed in the recommended strategies articles linked above.
    NIRISS GR150 Grisms
    NIRCam Grisms

  5. Decide on the dither pattern required to mitigate detector defects, and achieve the required pixel sampling. NIRISS WFSS mode operates at shorter wavelengths where the PSF is undersampled, so dithering is required.
    NIRISS WFSS Dithers
    NIRCam Wide Field Slitless Spectroscopy Dithers

  6. Decide whether mosaicking is required to cover the target field for the science program.
    NIRISS mosaics
    NIRCam mosaics 

  7. Decide the readout pattern to use.
    NIRISS Detector readout patterns
    NIRCam Detector readout patterns

  8. Use the Exposure Time Calculator (ETC) to determine the exposure parameters for the direct images and for the dispersed images from the grisms.
    JWST ETC Imaging Aperture Photometry Strategy
    JWST ETC Aperture Spectral Extraction Strategy

  9. Fill out the Astronomers Proposal Tool (APT) for NIRISS WFSS or NIRCam WFSS.
    NIRISS Wide Field Slitless Spectroscopy APT template
    NIRCam Wide Field Slitless Spectroscopy APT template

  10. Define mosaic parameters in APT (if needed by science program).
    APT Mosaic Planning

  11. Follow the instructions for coordinated parallels if attaching parallels to the WFSS prime observations.
    APT Coordinated Parallel Observations
    JWST Coordinated Parallels Roadmap

Go to the Getting Started with JWST Proposing to complete the steps for proposal submission.

Notable updates
Originally published


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