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 are different for different pairings of the NIRCam grisms and cross filter. Additionally, the background is expected to vary as a function of time and position, but the plots and tables below present nominal values for the background from the JWST ETC for (RA, Dec) = (00:00:00, 00:00:00) and day 180. 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
| Filter | Module A GRISM R (e–/s) | Module A GRISM C (e–/s) | Module B GRISM R (e–/s) | Module B GRISM C (e–/s) |
|---|---|---|---|---|
| F250M | 0.07 | 0.13 | 0.05 | 0.10 |
| F277W | 0.30 | 0.37 | 0.22 | 0.27 |
| F300M | 0.14 | 0.14 | 0.10 | 0.10 |
| F322W2 | 0.73 | 0.81 | 0.54 | 0.60 |
| F335M | 0.16 | 0.16 | 0.12 | 0.12 |
| F356W | 0.41 | 0.41 | 0.31 | 0.31 |
| F360M | 0.19 | 0.19 | 0.14 | 0.14 |
| F410M | 0.25 | 0.25 | 0.19 | 0.19 |
| F430M | 0.14 | 0.14 | 0.11 | 0.11 |
| F444W | 0.78 | 0.78 | 0.58 | 0.58 |
| F460M | 0.16 | 0.16 | 0.12 | 0.12 |
| F480M | 0.23 | 0.23 | 0.17 | 0.17 |
Estimated background along the dispersion axis
Figure 1. Estimated NIRCam WFSS background levels versus position along dispersion axis
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Estimated background levels (e–/s) in WFSS observations for each available filter along the dispersion direction for Grism R (left) and Grism C (right).
Simulated background in each filter
F250W
Figure 2. Simulated NIRCam WFSS backgrounds with the F250M filter
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Simulated 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. Simulated NIRCam WFSS backgrounds with the F277W filter
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Simulated 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. Simulated NIRCam WFSS backgrounds with the F300M filter
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Simulated 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. Simulated NIRCam WFSS backgrounds with the F322W2 filter
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Simulated 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. Simulated NIRCam WFSS backgrounds with the F335M filter
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Simulated 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. Simulated NIRCam WFSS backgrounds with the F356W filter
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Simulated 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. Simulated NIRCam WFSS backgrounds with the F360M filter
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Simulated 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. Simulated NIRCam WFSS backgrounds with the F410M filter
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Simulated 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. Simulated NIRCam WFSS backgrounds with the F430M filter
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Simulated 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. Simulated NIRCam WFSS backgrounds with the F444W filter
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Simulated 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. Simulated NIRCam WFSS backgrounds with the F460M filter
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Simulated 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. Simulated NIRCam WFSS backgrounds with the F480M filter
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Simulated 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.