NIRCam wide field slitless spectroscopy (WFSS) observations will produce R ~ 1600 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 (SW) imaging data occurs simultaneously with the long wavelength (LW) grism observations. Saturation in the SW imaging data is a concern, but can be mitigated by selecting narrower filters or by using a sequence of short integrations.
Direct imaging with the long wavelength channel is necessary to identify the sources that are responsible for the spectra. LW direct images may optionally be requested for each grism+filter combination specified, and will immediately follow completion of the grism exposures (including dithers). The direct image will be taken at the final dither position used for the grism exposures. When direct images are requested, they include two additional exposures, dithered parallel to the dispersion direction, at offsets designed to image any out-of-field sources that may produce spectral streaks in the grism data. Direct LW images are required for at least the last grism+filter combination requested in the observation.
Simultaneous direct imaging with the SW channel is obtained simultaneously with the LW grism exposures. The SW imaging field of view remains unchanged when the grisms are in use. The SW images enable precise determination of dither offsets, and provide extended wavelength coverage as well as supporting production of the source catalog needed for calibration of the LW grism spectra.
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 LW 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, and is 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. SW direct images of the out-of-field sources will also be obtained simultaneously with the LW images.
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