NIRCam WFSS Field of View

JWST NIRCam wide field slitless spectroscopy observations yield spectra from sources within a field of view that varies with the selected filter and grism.

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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 a 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.

These fields of view are shown in APT Aladin. By default, Aladin shows the full spectrum field of view. The larger effective field of view may be shown by selecting the option to show partial spectra region. Note these regions are most easily viewed before adding the direct image and out-of-field images to the observations in APT.


Update from commissioning data

The grism dispersions (nm/pix) measured during commissioning are about 2-4% smaller than predicted pre-flight, resulting in slightly longer spectra on the detectors. The longer spectra cause a decrease in the size of the full spectrum field of view (FOV). The figures below (Figures 2–14) show the old and new FOVs for F322W2 for both Grism C and Grism R.

Figure 1. NIRCam WFSS effective field of view with the F277W filter

Depending on where a source is placed, the resulting spectrum may not fall entirely within the detector. In these figures, the blue hatched region marks the effective FOV where at least some of a source's spectrum will fall within the detector. The red outline marks the detector edges. The black rectangle outlines the pre-flight full spectrum FOV, and the blue/orange rectangles mark the post-commissioing full spectrum FOVs. Here we show F322W2 for both grisms C and R.

The remainder of this article below presents the pre-flight FOVs.


Full spectrum field of view tables

Grism R

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 + FILTERModule A xλminModule A xλmaxModule B xλminModule B xλmax
F250M15302164509-130
F277W15602164479-130
F300M11722164877-130
F322W215501827499202
F335M83321641216-130
F356W88318671166162
F360M63420461415-17
F410M18615481863481
F430M-13015192164510
F444W16684118831188
F460M-13011502164878
F480M-13088121641148

Grism C

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 + FILTERModule A yλminModule A yλmaxModule B yλminModule B yλmax
F250M1530269815302698
F277W1560269815602698
F300M1172269811622698
F322W21550182715401837
F335M83322958232305
F356W88318678731877
F360M63420466242057
F410M18615481761558
F430M-1721519-1821529
F444W166841156851
F460M-1941150-1981161
F480M-194881-198891


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.

F250M

Figure 2. NIRCam WFSS effective field of view with F250M filter

Effective field of view when the F250M filter is combined with Grism R (top) and Grism C (bottom). Each star represents a source yielding a spectrum illustrated by a corresponding arrow. The spectrum is dispersed along the length of the arrow with wavelength increasing toward the arrow head. Sources are within the full spectrum field of view if their spectra fall completely within a detector's field of view (black square outlines). The full spectrum field of view is shaded blue for module A (left) and orange for module B (right). However, note that for Grism C, the coronagraph masks (illustrated at top) may impact some spectra. Sources within the regions with diagonal stripes yield partially truncated spectra. The effective field of view includes both the striped and shaded regions. The outer black rectangles indicate the full regions that receive incoming light from NIRCam's pick off mirrors. Sources outside those regions do not yield spectra.

F277W

Figure 3. NIRCam WFSS effective field of view with the F277W filter

Effective field of view when the F277W filter is combined with Grism R (top) and Grism C (bottom), as described in Figure 2.

F300M

Figure 4. NIRCam WFSS effective field of view with the F300M filter

Effective field of view when the F300M filter is combined with Grism R (top) and Grism C (bottom), as described in Figure 2.

F322W2

Figure 5. NIRCam WFSS effective field of view with the F322W2 filter

Effective field of view when the F322W2 filter is combined with Grism R (top) and Grism C (bottom), as described in Figure 2.

F335M

Figure 6. NIRCam WFSS effective field of view with the F335M filter

Effective field of view when the F335M filter is combined with Grism R (top) and Grism C (bottom), as described in Figure 2.

F356W

Figure 7. NIRCam WFSS effective field of view with the F356W filter

Effective field of view when the F356W filter is combined with Grism R (top) and Grism C (bottom), as described in Figure 2.

F360M

Figure 8. NIRCam WFSS effective field of view with the F360M filter

Effective field of view when the F360M filter is combined with Grism R (top) and Grism C (bottom), as described in Figure 2.

F410M

Figure 9. NIRCam WFSS effective field of view with the F410M filter

Effective field of view when the F410M filter is combined with Grism R (top) and Grism C (bottom), as described in Figure 2.

F430M

Figure 10. NIRCam WFSS effective field of view with the F430M filter

Effective field of view when the F430M filter is combined with Grism R (top) and Grism C (bottom), as described in Figure 2.

F444W

Figure 11. NIRCam WFSS effective field of view with the F444W filter

Effective field of view when the F444W filter is combined with Grism R (top) and Grism C (bottom), as described in Figure 2.

F460M

Figure 12. NIRCam WFSS effective field of view with the F460M filter

Effective field of view when the F460M filter is combined with Grism R (top) and Grism C (bottom), as described in Figure 2.

F480M

Figure 13. NIRCam WFSS effective field of view with the F480M filter

Effective field of view when the F480M filter is combined with Grism R (top) and Grism C (bottom), as described in Figure 2.

Optimal fields

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 14 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
Filter
λmin
(µm)
λmax
(µm)
X center
Module A
(pixels) 
X center
Module B
(pixels) 
Y center
(pixels) 

Length on 
each side
(pixels) 

Fraction of 
detector area
Area 
(arcmin2)
 F250M2.4122.5951790296217895020.06060.278
 F277W2.4163.1271787296517875060.06150.282
 F300M2.8293.1551579317315809190.2030.931
 F322W22.4304.0131749300317494570.0500.23
 F335M3.1773.53814073345140712670.3861.77
 F356W3.1403.98014093343141012000.3461.59
 F360M3.4273.81412803472128115170.5532.54
 F410M3.8644.301887386588716030.6172.83
 F430M4.1674.398796395679715920.6092.80
 F444W3.8804.98653742155389340.2100.962
 F460M4.5154.747622413062212430.3711.70
 F480M4.6624.977507424550710130.2471.13

Figure 14. NIRCam WFSS optimal fields

Optimal fields for NIRCam WFSS observations in each filter (color-coded). Module A is on the left, and module B is on the right. Sources within these areas will yield complete spectra on the NIRCam detectors for both Grism R and Grism C observations.

References

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 



Latest updates

  • Added commissioning update

  •  
    Aladin visualizations of FOVs added in APT 2021.2 
Originally published