Wide field slitless spectroscopy (WFSS) is one of five observing modes available for the Near-Infrared Camera (NIRCam). The WFSS mode uses grisms to obtain multi-object spectroscopy from 2.4–5.0 μm with R ~ 1500. WFSS can be obtained in one module (2.2′ × 2.2′), or with both modules (note the module B grism throughputs are 25% lower than module A). There are two grisms available on the pupil wheels that disperse in orthogonal directions, along detector rows (GRISMR) and columns (GRISMC). Use of both grisms mitigates confusion from overlapping spectra.
WFSS observations include simultaneous short-wavelength (SW; 0.6–2.3 µm) imaging over roughly the same field of view via a dichroic. These SW images aid in wavelength calibration by precisely tracking the position of each dither. Direct imaging in the long-wavelength (LW) channel is also supported, including coverage of out-of-field sources, which disperse light onto the detector despite being outside the imaging field of view. LW direct images and out-of-field dithers occur at the final dither position in a dither sequence.
The observer will have control over five primary parameters for NIRCam WFSS:
Allowed values are documented and maintained in the NIRCam Wide Field Slitless Spectroscopy Template parameters and described below.
Step-by-step APT instructions
Observers can select A or ALL. Module A covers one 2.2′ × 2.2′ field of view, while ALL includes both modules A and B to double the field of view. The Module B grism throughputs are 25% lower than those on Module A.
Grism (long wavelength)
Main article: NIRCam Grisms
Specify GRISMR, GRISMC, or BOTH (for the long-wavelength observations). GRISMR disperses along the detector rows, and GRISMC disperses along the detector columns. Use BOTH to help disentangle overlapping spectra in the data. When using both grisms, a FULL dither sequence is executed for GRISMR, then executed again for GRISMC.
Science (GRISM) exposures
Main article: NIRCam Wide Field Slitless Spectroscopy Dithers
See also: NIRCam Dithers and Mosaics
Choose a NIRCam WFSS dither pattern. Larger primary dithers cover the 4″–5″ gaps between the short-wavelength detectors. Smaller subpixel dithers improve the spatial resolution of the final combined image, which is especially important at wavelengths shorter than the Nyquist wavelengths: 2 μm in the SW channel, and 4 μm in the LW channel.
Specify the grism exposure sequences. Multiple exposures may be defined here. For each, specify:
Users should consult the Exposure Time Calculator, ETC, to achieve sufficient signal-to-noise for their science without saturating during each integration. Approximate sensitivity curves for 10,000-second exposures are available at NIRCam Wide Field Slitless Spectroscopy.
Direct image exposures
Main article: NIRCam Wide Field Slitless Spectroscopy
For each requested direct imaging, specify:
The available options for all of these parameters are the same as those above in Science (Grism) Exposures.
Exposure sequence display
The Exposure Sequence Display dialog box shows the sequence of observations. The exposure sequence is:
Parameters cannot be edited in this display box, but they can be changed by editing parameters in the boxes above: Science (Grism) Exposures and Direct Image Exposures.
Robberto, M., 2009, JWST-STScI-001721, SM-12
Robberto, M., 2010, JWST-STScI-002100, SM-12