NIRCam WFSS Backgrounds

Estimated backgrounds are available for JWST NIRCam wide field slitless spectroscopy (WFSS) observations with each filter, grism, and module.

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The effective field of view in the NIRCam wide field slitless spectroscopy (WFSS) observing mode varies with the cross filter used in combination with each grism. The shape and background (which is dominated by the zodiacal light component) levels can be expected to be different for different pairing of the NIRCam grisms and cross filter. The following plots show these backgrounds for every possible combination of grism and filter available for WFSS observations. For simplicity, the default background level and components as predicted by the JWST ETC for the coordinates of 0:00:00, 0:00:00 are adopted. The background is expected to vary as a function of time and position and this is not included in these examples. An increase in the zodiacal background level of a factor of up to 2 can be expected, as discussed in JWST Background Variability. Note that in some cases, with very blue or very red filters, there are areas in the detector that receive no light, and hence the background drops smoothly to zero.


Estimated maximum dispersed background levels

Table 1 lists the expected background levels (based on current Exposure Time Calculator background spectra and pre-launch grism dispersion relations) for all of the possible combinations of filters and grisms in each module. This is also plotted in the following figures. Module B numbers are expected to be ~20% lower than module A due to the lower throughput of the module B grisms. The background is expected to be slighly higher for some filter choices with Grism C as some extra background light makes it past the coronagraph that is located above the detectors in both modules.


Table 1. Estimated maximum dispersed background level in each filter, grism, and module combination

FilterModule A
GRISM R
(e–/s) 		Module A
GRISM C
(e–/s)		Module B
GRISM R
(e–/s)		Module B
GRISM C
(e–/s)
F250M0.070.130.050.10
F277W0.300.370.220.27
F300M0.140.140.100.10
F322W20.730.810.540.60
F335M0.160.160.120.12
F356W0.410.410.310.31
F360M0.190.190.140.14
F410M0.250.250.190.19
F430M0.140.140.110.11
F444W0.780.780.580.58
F460M0.160.160.120.12
F480M0.230.230.170.17


Estimated background along the dispersion axis

Figure 1. Estimated NIRCam WFSS background levels versus position along dispersion axis

Estimated background levels (e–/s) in WFSS observations for each available filter along the dispersion direction for Grism R (left) and Grism C (right).

Estimated background in each filter

F250W

Figure 2. Estimated NIRCam WFSS backgrounds with the F250M filter

Estimated WFSS background levels (e–/s) as a function of position within each long wavelength detector when using the F250M filter with each module and grism as labeled.

F277W

Figure 3. Estimated NIRCam WFSS backgrounds with the F277W filter

Estimated WFSS background levels (e–/s) as a function of position within each long wavelength detector when using the F277W filter with each module and grism as labeled.

F300M

Figure 4. Estimated NIRCam WFSS backgrounds with the F300M filter

Estimated WFSS background levels (e–/s) as a function of position within each long wavelength detector when using the F300M filter with each module and grism as labeled.

F322W2

Figure 5. Estimated NIRCam WFSS backgrounds with the F322W2 filter

Estimated WFSS background levels (e–/s) as a function of position within each long wavelength detector when using the F322W2 filter with each module and grism as labeled.

F335M

Figure 6. Estimated NIRCam WFSS backgrounds with the F335M filter

Estimated WFSS background levels (e–/s) as a function of position within each long wavelength detector when using the F335M filter with each module and grism as labeled.

F356W

Figure 7. Estimated NIRCam WFSS backgrounds with the F356W filter

Estimated WFSS background levels (e–/s) as a function of position within each long wavelength detector when using the F356W filter with each module and grism as labeled.

F360M

Figure 8. Estimated NIRCam WFSS backgrounds with the F360M filter

Estimated WFSS background levels (e–/s) as a function of position within each long wavelength detector when using the F360M filter with each module and grism as labeled.

F410M

Figure 9. Estimated NIRCam WFSS backgrounds with the F410M filter

Estimated WFSS background levels (e–/s) as a function of position within each long wavelength detector when using the F410M filter with each module and grism as labeled.

F430M

Figure 10. Estimated NIRCam WFSS backgrounds with the F430M filter

Estimated WFSS background levels (e–/s) as a function of position within each long wavelength detector when using the F430M filter with each module and grism as labeled.

F444W

Figure 11. Estimated NIRCam WFSS backgrounds with the F444W filter

Estimated WFSS background levels (e–/s) as a function of position within each long wavelength detector when using the F444W filter with each module and grism as labeled.

F460M

Figure 12. Estimated NIRCam WFSS backgrounds with the F460M filter

Estimated WFSS background levels (e–/s) as a function of position within each long wavelength detector when using the F460M filter with each module and grism as labeled.

F480M

Figure 13. Estimated NIRCam WFSS backgrounds with the F480M filter

Estimated WFSS background levels (e–/s) as a function of position within each long wavelength detector when using the F480M filter with each module and grism as labeled.


Latest updates

  • Updated estimates of backgrounds to include effect of the pick-off mirror (POM) and coronagraph assembly. 
Originally published