NIRCam WFSS Backgrounds
Estimated backgrounds are available for JWST NIRCam wide field slitless spectroscopy (WFSS) observations with each filter, grism, and module. Additionally, master sky images have been measured using flight data to improve background subtraction in the pipeline for the most common wide-band filters for each module and grism.
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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. Additionally, master sky images for the most common wideband filters (F277W, F322W2, F356W, and F444W) have now been measured using flight data to provide the community with improved background subtraction in the pipeline.
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
The table below shows simulated WFSS background levels (e–/s) as a function of position within each long wavelength detector when using the specified filter with each module and grism as labeled.
Table 2. Simulated background levels for each filter
| Filter | Simulated background levels (e-/s) |
|---|---|
F250W |
|
F277W |
|
F300M |
|
F322W2 |
|
F335M
|
|
F356W
|
|
F360M
|
|
F410M |
|
F430M |
|
F444W |
|
F460M |
|
F480M |
|
Measured background in the most common wideband filters
Using all public observations taken with either grism (GRISMR or GRISMC), new global-sky images for wide field slitless spectroscopy data from both long-channel modules (A and B) and both grisms (GRISMR and GRISMC) are available for the four most common wideband filters: F277W, F322W2, F356W, and F444W. Additional filters will be considered in the future. T
he method to generate the global-sky images is described in Russell et al. 2025 (in review). Figures 2 and 3 below show the smoothed background images and the root-mean-square (RMS) through the collection of images for the four bandpasses (F277W, F322W2, F356W, and F444W) for both modules and both grisms. It is worth noting that the F277W+GRISMR combination has a considerable corruption from unmasked spectral traces, which is largely a direct consequence of the number of independent exposures used for the analysis and short exposure times.
Additionally, GRISMC shows some considerable structure, particularly for the F277W, F322W2, and F356W, most of which comes from the coronograph masks, which have very low throughput just off the V3-axis of the two modules. Therefore, the silhouette of these masks show as dark bands (hence the vertical stripes). Regions of particularly high flux, especially for F277W and F322W2, are regions where a source can produce a full first-order spectral trace and the additional orders can also appear. Lastly, many of the images show regions of very low flux (dark purple) due to the finite size of the pick-off-mirror.
As a final note: the simulated and measured background images have adopted different normalization conditions. The simulated background images were constructed by passing the background spectrum through the optics of the instrument and scaled by the sensitivity curve using Mirage. Therefore, these images are reported in detector units (in this case e–/s) to facilitate direct comparisons with Pandeia.
On the other hand, the measured background images are derived from stacking many observed images, which were individually normalized, resulting in dimensionaless images. Importantly, the background-subtraction step in the calibration pipeline can support both normalization choices, however the derived scaling coefficient will have different units or values to account for the detector gain value in the former case. For example, the scaling coefficient for the simulated background images will have units DN/e–, while in the measured images this will be DN/s.
Figure 2. Measured background images for F277W, F322W2, F356W, and F444W for both modules and grisms
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The final background images for F277W, F322W2, F356W, and F444W for both modules and grisms. As described above, the GRISMC background image shows considerably more structure that arises from the coronograph that is just off the +V3 axis (i.e., just above the edge of the light sensitive area). The images all have a logarithmic stretch around unity and the red rectangles indicate the normalizing regions used to compute the 3σ-clipped median of the valid pixels for each WFSS image to minimize the influence of the background structure.
Figure 3. The RMS for the final measured background images
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The RMS for the final background images. The symbols have the same meaning as in Figure 2 above, but using a different logarithmic stretch for each image to highlight the spatial structure.
Comparison to simulated background
Figure 4 shows the reduced-χ2 from using the simulated backgrounds (blue) and the measured backgrounds (orange) for each grism/filter combination to subtract the background from WFSS images. The distributions from the simulated/measured results are effectively indistinguishable for GRISMR, but this is not surprising given that the derived two-dimensional backgrounds are very similar. However, the GRISMC results show marked improvement. This is equivalent to the GRISMR result, but in reverse, where GRISMC images differ noticeably from the simulated backgrounds (this is not unexpected, as the GRISMC disperses along the axis related to the coronagraphs, which significantly complicates the modeling for simulated backgrounds).
Figure 4. Reduced-χ2 for each grism/filter combination
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Reduced-χ2 after background subtraction for each grism/filter combination using simulated (blue) or measured (orange) backgrounds. The GRISMR distributions are nearly indistinguishable, implying there is little difference between the two derived products. However, the GRISMC distributions show considerable improvement from updated products. The vertical dashed line indicates the nominal value of χ2/ν=1, which is taken as a statistically sound good fit.
Figure 5. Aggregate statistics for images that include the background and sky-subtracted results
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Aggregate statistics for the images that include the background (top panel) and sky-subtracted results (bottom panel). The plot shows the average and standard deviations of the background-subtracted images for the four filters (as indicated in the legend) for both modules and grisms (GRISMC: circle and GRISMR: square). The vertical dashed line indicates the expected background level of zero. By design, the measured products more closely match the observations by reducing the averages both closer to zero and reducing the overall scatter.
References
Ryan, R., Pirzkal, N., Hilbert, B., 2025, (in review)
The Creation of NIRCam Grism Global-Sky Images