JWST NIRISS mosaics can be used to cover an area larger than the 2.2' × 2.2' field of view. Mosaics are allowed for the wide field slitless spectroscopy (WFSS) and imaging modes when NIRISS is the prime observing instrument.
JWST mosaics can be used in the NIRISS wide field slitless spectroscopy (WFSS) and imaging modes to cover an area larger than the 2.2' × 2.2' field of view (FOV). Mosaics differ from dithering, which improves sampling of the point spread function, by having larger offsets between exposures to increase the effective FOV. When NIRISS WFSS or imaging is a parallel observing mode while another JWST instrument is prime, NIRISS will follow the mosaic strategy of the prime observing mode.
NIRISS mosaic properties are specified in the Astronomer's Proposal Tool (APT) by indicating the number of rows and columns, the shift (in degrees) between rows and columns, and the percentage of overlap between rows and columns, as described in the documentation for APT's JWST mosaic tool. The resulting mosaic can be viewed in the Aladin visualization tool; an example is shown below.
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