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.

On this page

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. 

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

Grism spectra will be dispersed across detector rows (R) or columns (C) as indicated by the large arrows pointing in the direction of increasing wavelength. Ideal coordinate axes (Xideal, Yideal) are shown in each NIRCam module (A and B). Adapted from Figure 4 of Greene et al. (2017).

Direct imaging

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.    

 

Out-of-field sources

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.

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.

Sensitivity

Approximate continuum and line sensitivities are shown in Figure 4 for a 10 ks integration on module A. Module B grisms have 30% lower transmission. These figures qualitatively show the change in sensitivity as function of wavelength and module only. Please consult the Exposure Time Calculator (ETC) for the proposed observations.

Figure 4. Grism sensitivities in module A

Sensitivity of the NIRCam LW grism on module A, expressed as the flux required to reach SNR ~ 10 in a ~10 ks observation. These plots were obtained using results from the ETC v4.0.

Top panel: results for a continuum source.

Bottom panel: results for individual, unresolved emission lines. For clarity, the medium filters are shown only in the top panel. In this background-dominated SNR regime, the effect of the wavelength-integrated, slitless background as a function of filter width is clearly seen as the sensitivity gets better (smaller flux) for narrower passbands. In both cases, the 10 ks observation was configured as 10 exposures of one integration each, using the MEDIUM8 readout pattern with 10 groups. This choice keeps each exposure around the maximum recommended NIRCam exposure time of 1,000 s, which avoids excessive cosmic rays contamination. The sky background level was set to medium. The cross-dispersion extraction area was set to 3 pixels and the background extraction annulus to 10–25 pixels from the source. For the continuum case, the target SNR is expressed per spectral pixel (as the ETC provides results per detector pixel). For the emission line case, the SNR is obtained after integrating the line flux over 3 spectral pixels. 

Users should refer to the ETC for all their estimation as the ETC contains continuously updated information.

Background levels

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 2 simulated backgrounds when combining the GRISMR+F356W (left) or GRISMR+F410M (right) filters.

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

Estimates for every combination of grism and filter can be found on the NIRCam Slitless Background page.




References

Greene, T. et al. 2017, JATIS, 035001
λ = 2.4 to 5 μm spectroscopy with the James Webb Space Telescope Near-Infrared Camera



Notable updates
  •  
    Sensitivity plots were updated with ETC v4

  •  
    Edited information to match recent paper (Greene, T. et al. 2017, JATIS, 035001)

  •   
    New sections on out-of-field sources and direct imaging
Originally published