Regular NIRCam imaging observations generally produce images with a field of 2x2.2x2.2 arcminutes. While NIRCam offers grism wide-field slitless spectroscopy (WFSS) in the long-wave (LW) channel, it is generally not possible to obtain full spectra of every source in the direct imaging field in a single pointing.

But don't worry! We have done the calculations, and the optimal FOV for NIRCam WFSS observations, as a function of filter bandpass, is available in JDox.

There are multiple reasons for this:

• The grisms are wedge-shaped and, therefore, can project spectra from sources outside of the imaging field of view (FOV) on the detector.
• Similarly, some areas inside the imaging FOV will fall outside the detector in the corresponding grism observation.
• Spectral traces can be long. All, some, or none of the spectrum may fall on the detector to be recorded, depending on the location of the source in the FOV.
• When grism observations use both orthogonal dispersion directions (along-rows and along-columns to mitigate source confusion), only sources within a smaller field of view will produce full spectra in both directions.
• All of this depends on the selected filter bandpass.

Reference

“An Analysis of the Sky Areas Mapped by NIRCam LW Grisms,” 2017, M. Robberto, JWST-STScI-005995, SM-12.