NIRISS WFSS Dithers
The JWST NIRISS wide field slitless spectroscopy (WFSS) mode has a predefined number of dither steps and sizes. Dithering is required to mitigate NIRISS PSF undersampling at wavelengths < 2 μm.
See also: NIRISS Wide Field Slitless Spectroscopy, NIRISS WFSS Recommended Strategies, NIRISS WFSS Template APT Guide
The NIRISS wide field slitless spectroscopy (WFSS) observing sequence can be constructed using the GR150R grism, GR150C grism, or a combination of both grisms. Dithering is required for WFSS observations because the JWST point spread function (PSF) is heavily undersampled by NIRISS when observing at the wavelengths covered by WFSS. Dithers can significantly improve the effective sampling of the PSF and mitigate the impacts of ghosts. The LARGE dither pattern works best to minimize the impacts of ghosts on the final combined spectral product.
Dither pattern steps for WFSS are chosen to have sufficient spacing (typically ≥3 pixels) along both rows and columns to accommodate exposures with GR150R and GR150C grisms. The dither patterns are chosen to achieve the best possible pixel-phase sampling for a given number of dither positions.
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The selection of dither patterns is made by specifying 2 parameters:
- Number of dither steps (Image Dithers): 2, 3 ,4 ,6, 8, 12, and 16 steps. The first step corresponds to the initial pointing. A minimum of 4 dither steps is recommended.
- Dither step amplitude (Pattern Size): SMALL (~0.3"), MEDIUM ( ~0.6"), or LARGE (~1.2"). Note that the average pixel scale for NIRISS is 0.066"/pixel.
As to the latter parameter, 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 and imaging modes) was chosen to be appropriate for extragalactic studies at moderate to high redshifts. The SMALL patterns are thought to be 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).
The dither step sizes for each pattern are the same for the WFSS and and imaging modes.
The spatial offsets with respect to the initial pointing (step #1 in all patterns listed below) are given in Tables 1-3 for the NIRISS WFSS dither patterns. Note: when the option GRISM = BOTH is selected in the APT template for NIRISS WFSS, the initial pointing depends on the grism (GR150R or GR150C) and the filter. This is described in the NIRISS WFSS Recommended Strategies article.
The NIRISS WFSS APT template also supports optional 3-position dither patterns for the direct image(s) that are taken after a dithered set of grism images (see the NIRISS WFSS Recommended Strategies article for details on when to use these). The spatial offsets of these patterns are given in Table 4 below.
The NIRISS WFSS mode can also be used in coordinated parallel mode. When this option is selected, additional customized dither patterns become available through the primary instrument's APT template. These customized dither patterns have been designed to work well for both the primary and the selected parallel instrument mode.
Table 1. WFSS mode offsets for the SMALL dither patterns
| # of Dithers (Image Dithers) | Step # | ΔX pixels | ΔY pixels | ΔX arcsec | ΔY arcsec |
|---|---|---|---|---|---|
| 2 | 1 | 0.000 | 0.000 | 0.0000 | 0.0000 |
| 2 | 3.500 | 3.500 | 0.2289 | 0.2296 | |
| 3 | |||||
| 1 | 0.000 | 0.000 | 0.000 | 0.000 | |
| 2 | 4.333 | 4.333 | 0.2833 | 0.2842 | |
| 3 | 8.667 | 8.667 | 0.5668 | 0.5685 | |
| 4 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 5.000 | 2.500 | 0.3270 | 0.1632 | |
| 3 | 2.500 | 6.000 | 0.1635 | 0.3946 | |
| 4 | -2.500 | 4.500 | -0.1635 | 0.2972 | |
| 6 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | -2.000 | 3.500 | -0.1308 | 0.2312 | |
| 3 | -4.667 | 8.500 | -0.3052 | 0.5614 | |
| 4 | 2.667 | 6.000 | 0.1744 | 0.3945 | |
| 5 | 4.333 | 2.000 | 0.2833 | 0.1305 | |
| 6 | 6.667 | -2.500 | 0.4360 | -0.1667 | |
| 8 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 5.000 | 2.500 | 0.3270 | 0.1632 | |
| 3 | 3.500 | 7.000 | 0.2289 | 0.4602 | |
| 4 | -1.500 | 4.500 | -0.0981 | 0.2969 | |
| 5 | -2.500 | -1.000 | -0.1635 | -0.0651 | |
| 6 | 2.500 | 1.500 | 0.1635 | 0.0981 | |
| 7 | 1.000 | 6.000 | 0.0654 | 0.3950 | |
| 8 | -4.000 | 3.500 | -0.2616 | 0.2318 | |
| 12 | |||||
1 | 0.000 | 0.000 | 0.000 | 0.000 | |
| 2 | 5.000 | 2.500 | 0.3270 | 0.1632 | |
| 3 | 3.500 | 7.000 | 0.2289 | 0.4602 | |
| 4 | -1.500 | 5.500 | -0.0981 | 0.3628 | |
| 5 | -2.500 | -1.000 | -0.1635 | -0.0651 | |
| 6 | 2.000 | 1.500 | 0.1308 | 0.0982 | |
| 7 | 1.000 | 6.000 | 0.0654 | 0.3950 | |
| 8 | -4.500 | 3.500 | -0.2943 | 0.2320 | |
| 9 | -2.000 | 2.000 | -0.1308 | 0.1324 | |
| 10 | 2.500 | 4.500 | 0.1635 | 0.2958 | |
| 11 | 1.500 | 9.000 | 0.0981 | 0.5925 | |
| 12 | -4.000 | 6.500 | -0.2616 | 0.4295 | |
| 16 | |||||
| 1 | 0.000 | 0.000 | 0.000 | 0.000 | |
| 2 | 5.000 | 2.500 | 0.3270 | 0.1632 | |
| 3 | 4.500 | 7.000 | 0.2943 | 0.4599 | |
| 4 | -1.500 | 5.500 | -0.0981 | 0.3628 | |
| 5 | -2.500 | -1.500 | -0.1635 | -0.0981 | |
| 6 | 2.500 | -1.000 | 0.1635 | -0.0666 | |
| 7 | 1.000 | 6.000 | 0.0654 | 0.3950 | |
| 8 | -5.000 | 3.500 | -0.3270 | 0.2321 | |
| 9 | -3.500 | 1.000 | -0.2289 | 0.0669 | |
| 10 | 3.500 | 4.500 | 0.2289 | 0.2955 | |
| 11 | 2.000 | 9.500 | 0.1308 | 0.6253 | |
| 12 | -4.000 | 5.000 | -0.2616 | 0.3306 | |
| 13 | -6.000 | 1.500 | -0.3924 | 0.1006 | |
| 14 | -1.000 | 3.000 | -0.0654 | 0.1980 | |
| 15 | -0.500 | 8.000 | -0.0327 | 0.5273 | |
| 16 | -6.500 | 7.500 | -0.4250 | 0.4961 |
Table 2. WFSS mode offsets for the MEDIUM dither patterns
| # of Dithers (Image Dithers) | Step # | ΔX pixels | ΔY pixels | ΔX arcsec | ΔY arcsec |
|---|---|---|---|---|---|
| 2 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 6.500 | 6.500 | 0.4251 | 0.4263 | |
| 3 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 6.333 | 6.333 | 0.4141 | 0.4154 | |
| 3 | 12.667 | 12.667 | 0.8283 | 0.8308 | |
| 4 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 8.000 | 4.500 | 0.5231 | 0.2941 | |
| 3 | 3.500 | 11.000 | 0.2289 | 0.7237 | |
| 4 | -3.500 | 7.500 | -0.2289 | 0.4952 | |
| 6 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | -4.000 | 7.500 | -0.2616 | 0.4954 | |
| 3 | -7.667 | 14.500 | -0.5014 | 0.9577 | |
| 4 | 3.667 | 11.000 | 0.2398 | 0.7237 | |
| 5 | 7.333 | 4.000 | 0.4795 | 0.2614 | |
| 6 | 10.667 | -3.500 | 0.6975 | -0.2338 | |
| 8 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 8.000 | 4.500 | 0.5231 | 0.2941 | |
| 3 | 3.500 | 11.000 | 0.2289 | 0.7237 | |
| 4 | -3.500 | 7.500 | -0.2289 | 0.4952 | |
| 5 | -4.000 | -2.000 | -0.2616 | -0.1306 | |
| 6 | 4.000 | 2.500 | 0.2616 | 0.1635 | |
| 7 | -0.500 | 9.000 | -0.0327 | 0.5931 | |
| 8 | -7.500 | 5.500 | -0.4904 | 0.3646 | |
| 12 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 8.000 | 4.500 | 0.5231 | 0.2941 | |
| 3 | 3.500 | 11.000 | 0.2289 | 0.7237 | |
| 4 | -3.000 | 6.500 | -0.1962 | 0.4292 | |
| 5 | -4.500 | -1.500 | -0.2943 | -0.0975 | |
| 6 | 4.000 | 3.000 | 0.2616 | 0.1965 | |
| 7 | -1.000 | 8.500 | -0.0654 | 0.5604 | |
| 8 | -7.500 | 5.000 | -0.4904 | 0.3317 | |
| 9 | -2.500 | 2.000 | -0.1635 | 0.1325 | |
| 10 | 6.500 | 7.500 | 0.4251 | 0.4922 | |
| 11 | 1.000 | 12.000 | 0.0654 | 0.7904 | |
| 12 | -5.500 | 9.5000 | -0.3597 | 0.6276 | |
| 16 | |||||
| 1 | 0.000 | 0.000 | 0.000 | 0.000 | |
| 2 | 8.000 | 3.500 | 0.5231 | 0.2282 | |
| 3 | 3.500 | 11.000 | 0.2289 | 0.7237 | |
| 4 | -3.500 | 7.500 | -0.2289 | 0.4952 | |
| 5 | -4.500 | -2.500 | -0.2943 | -0.1634 | |
| 6 | 5.000 | 2.000 | 0.3270 | 0.1303 | |
| 7 | -1.000 | 9.5000 | -0.0654 | 0.6262 | |
| 8 | -7.500 | 5.000 | -0.4904 | 0.3317 | |
| 9 | -2.000 | 2.500 | -0.1308 | 0.1653 | |
| 10 | 6.500 | 7.000 | 0.4251 | 0.4593 | |
| 11 | 1.500 | 13.500 | 0.0981 | 0.8890 | |
| 12 | -6.000 | 10.000 | -0.3924 | 0.6607 | |
| 13 | -5.000 | 1.500 | -0.3270 | 0.1003 | |
| 14 | 3.000 | 6.000 | 0.1962 | 0.3944 | |
| 15 | -1.500 | 12.000 | -0.0981 | 0.7911 | |
| 16 | -8.500 | 8.500 | -0.5558 | 0.5626 |
Table 3. WFSS mode offsets for the LARGE dither patterns
# of Dithers | Step # | ΔX pixels | ΔY pixels | ΔX arcsec | ΔY arcsec |
|---|---|---|---|---|---|
| 2 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 12.500 | 12.500 | 0.8174 | 0.8199 | |
| 3 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 12.333 | 12.333 | 0.8065 | 0.8089 | |
| 3 | 24.667 | 24.667 | 1.6130 | 1.6179 | |
| 4 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 16.000 | 8.500 | 1.0463 | 0.5553 | |
| 3 | 7.500 | 22.000 | 0.4904 | 1.4473 | |
| 4 | -7.500 | 15.500 | -0.4904 | 1.0235 | |
| 6 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | -8.000 | 14.500 | -0.5231 | 0.9578 | |
| 3 | -14.667 | 28.500 | -0.9591 | 1.8822 | |
| 4 | 6.667 | 22.000 | 0.4360 | 1.4476 | |
| 5 | 14.333 | 8.000 | 0.9373 | 0.5228 | |
| 6 | 20.667 | -7.500 | 1.3515 | -0.5004 | |
| 8 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 16.000 | 8.500 | 1.0463 | 0.5553 | |
| 3 | 7.500 | 22.000 | 0.4904 | 1.4473 | |
| 4 | -7.500 | 15.500 | -0.4904 | 1.0235 | |
| 5 | -8.000 | -4.000 | -0.5231 | -0.2612 | |
| 6 | 8.000 | 4.500 | 0.5231 | 0.2941 | |
| 7 | -0.500 | 18.000 | -0.0327 | 1.1861 | |
| 8 | -15.500 | 11.500 | -1.0136 | 0.7623 | |
| 12 | |||||
| 1 | 0.000 | 0.000 | 0.000 | 0.000 | |
| 2 | 16.000 | 8.500 | 1.0463 | 0.5553 | |
| 3 | 7.500 | 22.000 | 0.4904 | 1.4473 | |
| 4 | -7.500 | 15.500 | -0.4904 | 1.0235 | |
| 5 | -6.500 | -4.500 | -0.4250 | -0.2946 | |
| 6 | 10.000 | 4.000 | 0.6539 | 0.2606 | |
| 7 | 1.000 | 18.500 | 0.0654 | 1.2186 | |
| 8 | -13.500 | 11.000 | -0.8828 | 0.7288 | |
| 9 | -4.000 | 6.500 | -0.2616 | 0.4295 | |
| 10 | 12.500 | 14.000 | 0.8174 | 0.9187 | |
| 11 | 3.000 | 28.000 | 0.1962 | 1.8440 | |
| 12 | -9.500 | 21.500 | -0.6212 | 1.4194 | |
| 16 | |||||
| 1 | 0.000 | 0.000 | 0.000 | 0.000 | |
| 2 | 16.000 | 8.500 | 1.0463 | 0.5553 | |
| 3 | 7.500 | 22.000 | 0.4904 | 1.4473 | |
| 4 | -7.500 | 15.500 | -0.4904 | 1.0235 | |
| 5 | -6.500 | -3.500 | -0.4250 | -0.2287 | |
| 6 | 10.000 | 4.000 | 0.6539 | 0.2606 | |
| 7 | 1.500 | 18.500 | 0.0981 | 1.2185 | |
| 8 | -13.000 | 11.000 | -0.8501 | 0.7286 | |
| 9 | -4.000 | 6.500 | -0.2616 | 0.4295 | |
| 10 | 12.500 | 14.000 | 0.8174 | 0.9187 | |
| 11 | 3.500 | 27.500 | 0.2289 | 1.8109 | |
| 12 | -9.000 | 21.000 | -0.5885 | 1.3863 | |
| 13 | -11.000 | 2.500 | -0.7193 | 0.1680 | |
| 14 | 5.000 | 10.000 | 0.3270 | 0.6574 | |
| 15 | -3.500 | 24.000 | -0.2289 | 1.5824 | |
| 16 | -16.500 | 17.500 | -1.0790 | 1.1580 |
Table 4. WFSS mode offsets for the 3-step "Direct Imaging" patterns*
Dither Size | Step # | ΔX pixels | ΔY pixels | ΔX arcsec | ΔY arcsec |
|---|---|---|---|---|---|
| SMALL | 1 | 0.000 | 0.000 | 0.0000 | 0.0000 |
| 2 | 7.500 | -2.000 | 0.4904 | -0.1340 | |
| 3 | 5.000 | 2.500 | 0.3270 | 0.1632 | |
| MEDIUM | 1 | 0.000 | 0.000 | 0.0000 | 0.0000 |
| 2 | 11.500 | -3.000 | 0.7520 | -0.2011 | |
| 3 | 7.000 | 3.500 | 0.4577 | 0.2285 | |
| LARGE | 1 | 0.000 | 0.000 | 0.0000 | 0.0000 |
| 2 | 23.500 | -7.000 | 1.5367 | -0.4683 | |
| 3 | 15.000 | 6.500 | 0.9809 | 0.4238 |
* The MEDIUM-size pattern is used for the optional direct imaging dithers when the NIRISS WFSS mode is used as prime mode in a coordinated parallel observation and a customized dither pattern is selected.
References
Goudfrooij, P. 2015 JWST-STScI-004466
NIRISS Dither Patterns for the WFSS and Imaging Observing modes