NIRCam Wide Field Slitless Spectroscopy
JWST NIRCam's wide field slitless spectroscopy observing mode uses grisms and filters spanning 2.4–5.0 µm to obtain R ~ 1,600 at 4 µm spectra of objects in a field of view of up to 9 arcmin².
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See also: NIRCam Wide Field Slitless Spectroscopy APT Template
NIRCam wide field slitless spectroscopy (WFSS) observations will produce R ~ 1,600 at 4 µm 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 2 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/pixel, and the LW pixel scale is 0.063". Consequently, some 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 and location of the pickoff mirrors, and is illustrated in Figure 3. When using grism C, the coronagraph mount 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.
JWST sensitivity update
Preliminary results from observatory commissioning indicate that system throughput is higher than pre-mission expectations, by amounts that vary with wavelength. Until detailed results are available and updated in Pandeia, results calculated by the ETC and JIST tools will underestimate S/N. ETC updates needed for Cycle 2 proposal preparation will be available in early December 2022.
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.
Slitless observations disperse the zodiacal and thermal background light just like they do any source in the field. This results in a higher level of background than in imaging observations. Different grism and imaging filter combination result in both a different background level as well as a different structure of the dispersed background.
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Greene, T. et al. 2017, JATIS, 035001
λ = 2.4 to 5 μm spectroscopy with the James Webb Space Telescope Near-Infrared Camera