NIRCam WFSS Backgrounds
Estimated backgrounds are available for JWST NIRCam wide field slitless spectroscopy (WFSS) observations with each filter, grism, and module.
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 two can be expected, as discussed in 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
The following table 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
Estimated background along the dispersion axis
Estimated background in each filter