NIRCam wide field slitless spectroscopy (WFSS) observations will produce R ~ 1500 spectra of all objects within (or just outside) the field of view in the long wavelength channel. Two grisms with perpendicular dispersion directions are available; use of both can mitigate overlapping spectra. Each grism must be used in combination with a wide or medium filter (2.4–5.0 µm) and observations with both modules will be obtained with identical optical elements. Dithers and mosaics will be defined and supported. Short wavelength imaging data occurs simultaneously with the long wavelength grism observations.
The grisms deflect the light parallel to the dispersion direction for all wavelengths except 3.95 μm. The size of the deflection is 1 nm/0.065″, where 0.065″ is the size of a pixel on the long wavelength detectors. Consequently, sources that fall outside of the imaging field-of-view produce spectral streaks on the detector. The extent of the out-of-field coverage is limited by the size of the pickoff mirrors, as illustrated in Figure 3. When using GRISMC, the coronagraph mounts also block some out-of-field sources. The coronagraph substrates themselves are transparent, but are populated by neutral density squares and coronagraphic occulting masks that will complicate the interpretation of grism spectra for sources that are located behind the substrate. Dithers parallel to the dispersion axes provide direct images of the out-of-field sources. The short wavelength imaging field of view remains unchanged when the grisms are in use.
Direct imaging with the long wavelength channel is required to identify the sources that are responsible for the spectra. Direct images occur automatically at the end of the dither sequence and three positions are imaged: one at the final dither position, and two additional fields in each direction parallel to the dispersion axis to cover the out-of-field sources.
Simultaneous direct imaging with the short wavelength channel is required during grism observations to precisely measure the dither offsets, which is critical for wavelength calibration. Short wavelength direct images of the out-of-field sources can also be obtained simultaneously with the long wavelength images. Saturation in the short wavelength imaging data is a concern, but can be mitigated by selecting narrower filters or by using a sequence of short integrations.
Approximate continuum and line sensitivities are shown in Figure 4 for a 10 ks integration using a 2 × 5 pixel extraction aperture (2 pixels in the spectral direction by 5 pixels in the spatial direction). Please consult the Exposure Time Calculator (ETC) for the proposed observations.
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Near Infrared Camera, NIRCAM
NIRCam Grism Time Series
NIRISS Wide Field Slitless Spectroscopy
Near Infrared Imager and Slitless Spectrograph, NIRISS