Parent page: NIRCam Observing Modes
See also: NIRCam Wide Field Slitless Spectroscopy APT Template
NIRCam wide field slitless spectroscopy (WFSS) observations will produce R ~ 1,600 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.
Figure 1a. Simulation of dispersed grism data
Simulated single module NIRCam observations of a section of the GOODS-S ERS field (RA 214.967560°, Dec. 52.932961°). Left: Simulated 2-hour F356W image based on deep HST mosaics of this field. Right: 2-hour grism R + F356W module B simulation with all known emission lines in the fields. All spectra were simulated using the information from multiple broadband imaging to determine the spectral energy distribution of each object individually. Known [OIII] 3729 Å, Hβ 4861 Å, [OIII] 4959 Å, [OIII] 5007 Å, and Hα 6564 Å lines of z > 3 sources were added. Wavelength of the dispersed spectra increases from left to right with grism R in module A. The F356W background has been subtracted from this simulated grism data.
Figure 1b. Simulation of dispersed grism data with different cross filters
Simulated single module NIRCam observations of a section of the GOODS-S ERS field. Top panel: full simulations that include estimated dispersed grism backgrounds. Bottom panel: same as top panel but the backgrounds have been subtracted.
Figure 2. Grism dispersion layout
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.
Figure 3. Grism out-of-field sources
NIRCam WFSS boundaries for out-of-field sources. The gray shaded areas are the imaging fields of view, the blue boxes mark the grism R boundaries, and the red boxes mark the grism C boundaries. The coronagraph substrates are transparent, but contain occulting masks and neutral density squares. Additional dithers allow for direct images at ΔV2 = ±12″ for grism R and ΔV3 = −12″, +35″ for grism C to cover out-of field sources.
Approximate continuum and line sensitivities are shown in Figure 4 for a 10 ks integration using a 2 × 5 pixel extraction aperture (two pixels in the spectral direction by five pixels in the spatial direction). Please consult the Exposure Time Calculator (ETC) for the proposed observations.
Figure 4. Grism sensitivities in module A and module B
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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.
Figure 5. NIRCAM module A dispersed backgrounds for GRISMR+F356W and GRISMR+F410M
The background of NIRCAM slitless observations are expected to have significant structure caused by the final shape of the pick up mirror illuminating the grism. This Figure shows two simulated backgrounds when combining the GRISMR+F356W1(left) or GRISMR+F410M (right) filters.
Estimates for every combination of grism and filter can be found on the NIRCam Slitless Background page.
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