The JWST NIRISS imaging mode has a predefined number of dither steps and sizes. Dithering is required for NIRISS imaging for various reasons (see below). When used as a coordinated parallel mode, NIRISS imaging will follow the dither pattern of the prime observing mode.

See also: NIRISS Imaging, NIRISS Imaging APT Template, JWST Parallel Observations, APT Coordinated Parallel Observations

NIRISS imaging is severely affected by undersampling at λ ≤ 4 μm, requiring dithers to improve sampling of the PSF. Dithering can also mitigate the impact of ghosts on the final combined image, flat-fielding errors, and detector artifacts like bad pixels and occulting spots. The LARGE dither pattern works best to minimize the impacts of ghosts on the final combined spectral product.

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.
• Dither step amplitude (Pattern Size): SMALL (~0.3"), MEDIUM ( ~0.6"), or LARGE (~1.2"). 

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.

When NIRISS imaging is used as a coordinated parallel observing mode, the prime instrument's template will allow the user to select customized dither patterns that work well for both the prime and the parallel instrument modes. 

Table 1. Imaging mode offsets for the SMALL dither patterns

Table 2. Imaging mode offsets for the MEDIUM dither patterns

Table 3. Imaging mode offsets for the LARGE dither patterns