NIRCam WFSS Recommended Strategies

Guidance is provided for astronomers preparing JWST NIRCam WFSS observations using the Astronomers' Proposal Tool (APT).

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In this page we provide guidance for preparing NIRCam wide field slitless spectroscopy (WFSS) observations in APT. This complements the step-by-step instructions given in the NIRCam Wide Field Slitless Spectroscopy APT Template.

In particular, we give advice on how to choose amongst the available grisms, cross filters (for use with the grisms), imaging filters (for the mandatory accompanying imaging of the field), and mosaicking strategies.

Similarity with NIRCam imaging

Several NIRCam imaging recommended strategies are relevant and applicable to NIRCam WFSS observations. In particular, one should follow the same practices described in the NIRCam Imaging Recommended Strategies article for the dither pattern, readout pattern, data volume, and overheads.

Additional WFSS-specific recommended strategies are provided below.

Cross filter

The choice of cross filter affects the size of the dispersed spectra and hence the effective field of view (FOV). Depending on the specific cross filter that is used, full dispersed spectra will be available for different parts of the field of view of the pick-off mirror (POM), and not just the field of view of the detectors themselves. For some filters and grism combinations, even though the spectra can be short (e.g., F250M) objects in the POM FOV but not the detector FOV can result in a full spectrum on the detector. The use of out-of-field imaging is therefore typically used to obtain a full census of all the objects likely to produce a full or partial spectra on the NIRCam LW detectors. 

Figure 1 shows an example for the F250M filter combined with the GrismR (spectra dispersed along the row direction) and GrismC (spectra dispersed along the column direction). The NIRCam WFSS Field of View article shows all the possible cases.

Figure 1. 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.

The choice of a very wide filter will result in many truncated spectra. For example, using F322W2 only provides full wavelength coverage for a narrow region near the center of the bandpass.



Observations with both GrismsR and GrismC are useful to mitigate confusion from overlapping spectra. Observations obtained using multiple orientations may also be needed to unambiguously identify emission lines. However, complete spectra from both grisms are only obtained for sources within the optimal fields (see NIRCam WFSS Field of View).

When using GrismC, the coronagraph substrate will affect the observations in 2 ways: (1) objects occulted by the neutral density filter or coronagraph features will not produce a spectrum. (2) Other objects within the coronagraph will be affected by the reduced transparency of the coronagraph substrate. This will make modeling and extracting these spectra more complicated. Note however that the redder filters (F444W, F460M, and F480M) are not affected by the coronagraph when used with the GrismC. This can be see in the NIRCam WFSS Field of View article.

The GrismC mode will, in general, result in a higher background level than the GrismR mode. This is due to additional sky area that is picked up by the pick-off mirror around the coronagraph. Using medium band filters can result in a significantly lower dispersed background, which may be essential to reach faint emission lines or continuum light. The NIRCam WFSS Backgrounds article shows the structure of the background when GrismR and GrismC are used with different filters and lists the expected level of dispersed background in each case.


Imaging filter

In principle, APT allows to choose, for the mandatory pre-imaging, a filter that is different from the one chosen for the following grism observation(s). It is generally preferable to use the same filter, e.g., F356W coupled with F356W/GrismR. There are 2 reasons for this: The first is that modeling of the field (to compute contamination) will be more accurate if the broadband photometry matches the bandpass of the grism observations. The second is that there might be "wedge offsets" between different filters. As of now, this is believed to be a small effect, but the use of a different filter for imaging might result in a small offset between the grism observations and the broadband observations. This will affect the assumed location of the trace and for small unresolved objects can be important even if these offsets are small (<0.1 pixel). These offsets would also affect the wavelength calibration of the spectra. 


Designing a mosaic using WFSS is not a very efficient process. It necessarily involves choosing between continuity of the field of view and possible overlap of grism observations taken using diffent grisms and/or orientation of the sky. Several mosaic examples are shown in the NIRcam WFSS Deep Galaxy Observations article.

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