NASA's James Webb Space Telescope

How To Discover the Optimal Fields of View for Slitless Spectroscopy with NIRCam

News Feature August 1, 2018

Image showing optimal NIRCam WFSS fields for the broad band filters F322W2 and F444W.
Optimal NIRCam WFSS fields for the broad band filters F322W2 and F444W. The spatial units are in pixels. Grey = areas for which sources are not visible by the WFSS exposure. White = areas for which partial spectra are visible. Yellow = areas for which the full spectrum is visible for the row-dispersed grisms. Orange = areas for which the full spectrum is visible for column-dispersed grisms. Blue = areas for which the full spectrum is visible for both dispersion directions.

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.