NIRCam WFSS Background Variation
JWST NIRCam Wide Field Slitless Spectroscopy observations yield spectra from sources within a field of view that varies with the filter and grism selected
The effective field of view of the NIRCam wide field slitless spectroscopy (WFSS) mode varies with the cross filter used in combination with the grism. At 3.95 µm, light is undeviated by the grisms, appearing in the same location on the detector as it would for imaging observations. At other wavelengths, the grisms disperse light by 1 nm/pixel along detector rows for Grism R and columns for Grism C. Thus, depending on the location of the source and the grism used, some (or all) of the spectrum may fall on the detector pixels, while the rest (or all) may be lost.
The following areas on the sky are defined for an WFSS observation with a given filter and grism:
- effective field of view—sources yield either partial or complete spectra on a detector
- full spectrum field of view—sources yield complete spectra on a detector
- optimal field of view—sources yield complete spectra on a detector when using either Grism R or Grism C
The extents of all of these areas are provided below for all filters available for WFSS observations: the 12 medium and wide filters in the long wavelength (LW) channel (2.4–5.0 µm).
Note that Grism C observations may be affected by the coronagraph masks and substrate. This is not an issue for F444W, F460M, or F480M. For all other filters, the sky background and any sources present in the coronagraph area will complicate estimates of the background and contamination.
Full spectrum field of view tables
Table 1 contains the extent of the full spectrum field of view when Grism R is paired with each available filter for module A and B. The coordinates are given in pixels in each detector's ideal coordinate system.
Table 1. Full spectrum field of view for NIRCam WFSS Grism R observations in each available filter
|GRISMR + FILTER||Module A xλmin||Module A xλmax||Module B xλmin||Module B xλmax|
Table 2 contains the extent of the full spectrum field of view when Grism C is paired with each available filter for module A and B. The coordinates are given in pixels in each detector's ideal coordinate system.
Table 2. Full spectrum field of view for NIRCam WFSS Grism C observations in each available filter
|GRISMC + FILTER||Module A yλmin||Module A yλmax||Module B yλmin||Module B yλmax|
Effective and full spectrum field of view figures
The following plots illustrate the effective and full spectrum field of view for every available combination of grism and filters. Examples are shown of sources yielding spectra that fall outside, partially within, and fully within the LW detectors. Note the wider filters (e.g., F322W2) yield longer spectra and thus smaller full spectrum fields of view.
Observations with both grisms are useful to mitigate confusion from overlapping spectra. Complete spectra from both grisms are only obtained for sources within the optimal fields.
Table 3 and Figure 13 report the optimal fields in modules A and B for the 12 NIRCam filters supported for WFSS. Each optimal field is a square with center coordinates and side length given in pixels. The area of each optimal field is expressed both as a fraction of the full detector area and in arcmin2. The wavelength range of each filter is given by λmin to λmax.
Table 3. Optimal fields for NIRCam WFSS observations
Greene, T. et al. 2017, JATIS, 035001
λ = 2.4 to 5 μm spectroscopy with the James Webb Space Telescope Near-Infrared Camera
Robberto, M. 2017, JWST-STScI-005995,
An Analysis of the Sky Areas Mapped by NIRCam LW Grisms