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.

Main article: JWST Mosaic Overview
See also: JWST 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. 

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 1 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.

For NIRISS WFSS, mosaics can be used to fill in areas of the sky where targets fall out of the nominal "direct image" field of view but still disperse spectra onto the detector.  Furthermore, because the 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.

Table 1 indicates the APT mosaic tool (Mosaic Properties tab) overlap percentage values that should be used for each filter if a user requires complete spectra across the entire 2.2' × 2.2' NIRISS FoV. The choice to specify Row Overlap or Column Overlap depends on the grism. 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 to ensure complete spectra for all targets within the nominal 2.2' × 2.2' NIRISS FoV at the center of the mosaic


GR150CColumn Overlap %
F090W97
F115W97
F150W93
F200W87
F140M95
F158M92
GR150RRow Overlap %
F090W97
F115W97
F150W93
F200W87
F140M95
F158M92

 

1  Bold italics font style is used to indicate parameters, parameter values, and/or special requirements that are set in the APT GUI.




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