NIRISS WFSS Recommended Strategies
The JWST NIRISS wide field slitless spectroscopy (WFSS) mode enables low resolution (R ≈ 150) spectroscopy between 0.8–2.2 μm, over the 2.2’ × 2.2’ FOV. Recommendations for crafting a NIRISS WFSS observing program are given.
The wide field slitless spectroscopy (WFSS) mode of NIRISS enables low-resolution (R ≈ 150) spectroscopy over the wavelength range 0.8–2.2 μm for all objects within the 2.2’ × 2.2’ field of view (FOV) of the NIRISS detector. 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. 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. A direct image is taken before and after each grism exposure.
Since the NIRISS point spread function (PSF) is undersampled at the wavelengths covered by the WFSS mode, dithering is essential. Dithering also helps remove detector artifacts and cosmic rays.
Advice is offered below to guide the user in choosing observing parameters and to discuss considerations that impact a WFSS program.
Recommended dither size and number of dither steps
Main article: NIRISS WFSS Dithers
The selection of dither patterns for WFSS is done by specifying two parameters:
- Dither Size (or amplitude): Small (~0.3"), Medium ( ~0.6"), or Large (~1.2").
- Number of Dithers: 2, 3, 4, 6, 8, 12, or 16 steps. In case NIRISS WFSS is used as the prime instrument mode in a coordinated parallel combination, there are additional custom dither patterns available with 2, 3, 4, or 9 steps. (The latter patterns are recommended for science cases where achieving optimal pixel phase sampling is important for both the prime and parallel instrument modes.) In all cases, the first dither step corresponds to the initial pointing.
As to the Dither Size parameter (#1 above), consider that smaller dither amplitudes yield the benefit of higher conservation of pixel phase sampling across the detector (due to varying geometric distortion), while larger dither amplitudes allow one to step over larger targets and hence offer a better mitigation of flat fielding errors for astronomical scenes involving such targets. The user should choose the best compromise for a given science case. The “Medium” patterns (the default choice in the APT template for the WFSS mode) was chosen to be appropriate for extragalactic studies at moderate-to-high redshift. The "Small" patterns are useful for sparse fields of compact sources, while the "Large" patterns should be useful for observing astronomical scenes involving relatively large objects (e.g., galaxies at relatively low redshift).
As to the selection of the number of dithers, the following questions are relevant to consider:
- Efficiency considerations. If the science goals require rather long exposure times with the grisms (e.g., ≥ 10,000 s per filter) whereas that is not the case for the associated direct images, it makes sense to split up the total exposure time per filter into several dithered exposures, since each WFSS exposure specification in APT produces two direct images (one at the first dither and one at the last dither position).
- PSF sampling vs. depth considerations. Science programs whose main aim is to achieve the best possible astrometric and photometric precision in the output image (especially for strongly undersampled images) will prefer to have several dither positions, so that each object will be placed at a wide variety of locations with respect to the pixel grid. On the other hand, users mostly interested in reaching the maximum photometric depth within a relatively short time may prefer fewer dithers, so that individual exposures will reach as deep as possible for a given total exposure time. Many programs, of course, will seek to compromise between those two extremes. As an aid in making such decisions, Figure 1 depicts measures of the relative quality of combined images for the different WFSS filters as function of the number of dithers, from simulated data. Note that for practical purposes, the main benefit of executing more than 8–9 dithers in WFSS mode is observing efficiency (cf. item #1 above) rather than photometric or astrometric quality.
More detailed advice and metrics for selecting the most appropriate dither pattern for a particular science goal is given in a report "NIRISS Dither Patterns for the WFSS and Imaging Observing modes" (see References section below).
Target density at which both grisms are recommended
It is not possible to provide rigorous general guidelines for source crowding in WFSS observations because whether overlap will occur depends on source sizes, the source distribution in the field of view, filter that is to be used, and the on-sky rotation angle at the time the observations are to be taken. The best way to estimate crowding is to simulate the WFSS spectra for a range of field rotations to see which orientations may cause problems for the science targets. However, some general comments about crowding can be made.
The lengths of the first-order grism spectra correspond to angular intervals that range from 2.6 arc-seconds for the F140M filter to 7.9 arc-seconds for the F200W filter. For any given target of interest, if there are several nearby sources that are of similar brightness (say at least 10% as bright as the target under consideration) within a radius of 4 arc-seconds then the overlap of different spectra is likely to occur. In such circumstances use of both grisms is recommended. If one is most interested in the F090W or F200W filter spectra then the crowding threshold is lower because the spectra are longer, and the density at which one needs to be concerned about possible overlap is somewhat lower than this estimate: having several sources within a radius of 8 arc-seconds indicates that use of both grisms should be considered.
To efficiently set up an observation that uses both grisms for a chosen filter in the NIRISS WFSS observation template in the Astronomer's Proposal Tool, users can select the option BOTH for GRISM.
Ensuring that all dispersed spectra are captured on FOV
Main article: NIRISS Mosaics
Because WFSS spectra are extended along the dispersion axis and offset from the target position in the direct image, complete spectra across the entire filter bandpass will only be available within a portion of the full NIRISS FOV that depends on the filter used. By taking a set of Mosaic exposures, complete spectra can be obtained over the entire 2.2' × 2.2' NIRISS FOV.
Table 1 indicates the Mosaic overlap percentage values that should be inputted into the Astronomer's Proposal Tool (APT) for each filter to obtain complete spectra over the entire FOV. Whether or not Row or Column overlaps should be specified depends on the grism, as indicated. If "BOTH" grisms are used then a 2 × 2 mosaic will be required to ensure complete spectra in both of them.
Table 1. APT Mosaic tool overlap percentages that can be used to ensure that complete spectra are available for all targets within the nominal 2.2' × 2.2' NIRISS FOV at the center of the mosaic.
|GR150C||Column Overlap %|
|GR150R||Row Overlap %|
Mitigating effects of bright sources
For a given target position, any bright star with a radius of about 1.6 arc-minutes may be within the field of view for some orientations of JWST. Stars brighter than about 10.5 (Vega) in J-band or about 9 (Vega) in K-band will produce strong saturated GR150 spectra even for the first frame of an observation. Any such stars within 1.6 arc-minutes of the center of the field-of-view will produce saturated spectra in the dispersed images. Even if such a star is slightly outside the imaging field of view for a given orientation, it may produce a spectrum if it is within about 9 arc-seconds of the edge of the imaging field. If a star is very bright even the higher spectral orders may be visible in the spectral images. For the first order spectra, only stars just below the edge of the imaging field or stars just to the right of the imaging field can produce spectra for the GR150R and GR150C grisms respectively. If a star that is brighter than the limits given above is within the field of view or close to the edge of the field of view the user may need to restrict the roll angle of the observation to avoid seeing the spectra from the bright star. In some cases, a small offset of the target position may also be used to avoid this type of problem, in those cases where the field is at low ecliptic latitude and thus the range of roll angles is restricted.
Recommended Aperture Strategy Choices for Exposure Time Calculator
The Exposure Time Calculator (ETC) is used to calculate signal-to-noise ratios (SNRs) for an observation based on input exposure parameters. When determining exposure parameters in the ETC, users can select the aperture radius from which the flux is extracted and the background subtraction method. We recommended the following filter-dependent source extraction aperture half-height values, which are based on the mean 80% encircled energy radii for a point source calculated from a WebbPSF grid. The recommended sky sample region for extracting the background has a start region of 2x the aperture half-height value and an end region of 4x the aperture half-height value.
Background Sky Sample Region
|Start Region (")||End Region (")|
Note: These extraction values are for point sources. If the source is extended, it is up to the user to define the region of interest for calculating the SNR. It is also up to the user to ensure that other sources from the ETC scene are not included in the background extraction area, unless this effect is intended.
Designing an efficient science program with multiple blocking filters
Main article: NIRISS Pupil and Filter Wheels
When using both the GR150R and GR150C grisms for observations, grism exposures should be obtained through the same filter before switching to another filter. Overheads are associated with each mechanism move. To design an efficient WFSS observing program, it is recommended to select filters in sequential order in the Pupil Wheel, as illustrated in Figure 2.
Goudfrooij, P., 2015, JWST-STScI-004466
NIRISS Dither Patterns for the WFSS and Imaging Observing modes