The wide field slitless spectroscopy (WFSS) mode of NIRISS enables low-resolution (R ≈ 150) spectroscopy over the wavelength range 0.8–2.2μm for all objects within the 2.2’ × 2.2’ field of view (FOV) of the NIRISS detector.
The NIRISS WFSS mode is particularly useful for detecting high-redshift emission-line galaxies and can be used very efficiently as a parallel observing mode with other instruments.
The WFSS mode uses a pair of identical grisms (GR150R and GR150C) that are mounted in the filter wheel so that their respective dispersion directions are perpendicular to each other on the detector.
- The GR150R grism disperses along the direction of fast readout.
- The GR150C grism disperses along the direction of slow readout.
Data acquired with both dispersion directions helps to disentangle blended spectra in crowded fields.
WFSS observations are obtained by using one or both of the grisms in combination with a wide- or medium-band blocking filter located in the pupil wheel (PW). The blocking filters limit the wavelength coverage and therefore the extent of spectra on the detector, which reduces the blending of spectral traces from objects distributed throughout the FOV.
Figure 1. Elements in the NIRISS pupil and filter wheels used by the WFSS mode
Optical elements available to the WFSS mode are marked by green dashed circles. The GR150R and GR150C grisms located in the filter wheel (FW) can be used in combination with any of the blue filters located in the pupil wheel (PW) to obtain dispersed images. Direct images are obtained by selecting CLEAR in the FW and one of the blue filters from the PW.
Figure 2. WFSS simulations of direct and dispersed images through the F115W filter
Simulated images of the massive lensing cluster MACS J0416.1-2403 observed with the NIRISS F115W filter (top panel); the GR150C grism and F115W blocking filter (middle panel); and the GR150R grism and F115W blocking filter (bottom panel). The orthogonal dispersion directions of the GR150R and GR150C grisms help to disentangle blended spectra.
Layout of spectral orders
Figure 3. Relative positions of WFSS spectral orders for all blocking filters
Relative positions of spectral orders for all the filters that can be used in conjunction with the GR150R and GR150C grisms in the WFSS mode. The PW filter names are indicated on the left side of the figure. Spectral order numbers are -1, 0, 1, and 2 from left to right, respectively.
WFSS observation sequence
An observation in the WFSS mode proceeds as follows
- A blocking filter is selected in the PW; the FW is set to "CLEAR"
- A single (i.e., non-dithered) image of the scene is taken: the "pre-image"
- One of the GR150 grisms is selected, either GR150C or GR150R
- Dithered grism images are taken
- The CLEAR element is selected again and a single image of the scene is taken: the "post-image"
- OPTION: Select a different blocking filter and repeat steps 1–5 with the same grism
- OPTION: Select the other GR150 grism and repeat steps 1–6
Exercising optional step 6 will increase the wavelength coverage for spectra of the objects in the scene, while exercising step 7 will produce observations of the scene with spectra dispersed in orthogonal directions on the detector.
The pre- and post-image through the relevant blocking filter is required:
- to enable proper identification of object(s) seen in the grism image(s); and
- to determine the origin of the absolute wavelength scale for the grism spectra for each object in the field.
More details about the observation sequences are provided in the NIRISS operations page.
WFSS blocking filters
The NIRISS WFSS mode uses 6 blocking filters in the PW. These are the 4 wide-band filters (F090W, F115W, F150W, and F200W), and 2 medium-band filters (F140M, and F158M). Figures 3 and 4 provide different visualizations of the wavelength coverage and spatial extent of the traces isolated by the blocking filters.
Figure 4. Filters for use with the WFSS mode grisms
Subarrays are not available for general observations in the WFSS mode. However, to support calibration observations of brighter standard stars, subarrays are available in the external calibration template for use by STScI instrument scientists. More details are provided in the NIRISS subarrays article.
WFSS dither patterns
The NIRISS PSF is undersampled in the wavelength range (0.8–2.2 μm) covered by the WFSS mode. For compact sources, a significant fraction of the flux in WFSS spectra is concentrated in the central few pixels of the cross-dispersion profile. Dithering is therefore required for WFSS observations in order to mitigate the impact of hot pixels, cosmic rays, and residual flat-field errors, and also to improve the effective spatial resolution.
Since WFSS observing programs are likely to use both the GR150R and GR150C grisms in an observing sequence, optimized dither patterns have been defined for this mode. Details on the available dither patterns are provided on the NIRISS dithers page.