NIRISS Mosaics

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) mode when NIRISS is the prime observing instrument.

See also: JWST Mosaic Overview, APT Mosaic Planning, NIRISS WFSS Recommended Strategies

JWST mosaics can be used in the NIRISS wide field slitless spectroscopy (WFSS) mode 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 (note that this will always be the case for NIRISS imaging, which is only allowed in parallel 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. 

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

The NIRISS apertures are aligned to within ~0.5° of the JWST focal plane coordinate system.  When a mosaic is specified, all of the exposures defined in the NIRISS Imaging or NIRISS Wide Field Slitless Spectroscopy tab of the observation template form will be simply replicated at each mosaic position, including the specified dither pattern in the case of WFSS.
Figure 1. Examples of NIRISS mosaic patterns

APT Aladin snapshots showing a single NIRISS pointing (left), a 2 × 2 mosaic with the default 10% overlap in both rows and columns (center), and a 2 × 2 mosaic with 50% overlaps (right). Note that mosaics with relatively small row/column overlap imply rather large telescope offsets (>1') and will be subject to visit splitting for most targets. See APT Mosaic Tile Splitting for more detailed instructions.

Latest updates
    Removed recommendation to use Mosaics to cover dispersed spectra of all sources in NIRISS FOV for WFSS mode 
Originally published