JWST NIRISS operations include telescope pointing maneuvers (dithers and target acquisitions) using targetable areas called apertures.
- Dithers are performed between exposures to improve sky coverage and image quality. Dithers are required for the wide field slitless spectroscopy (WFSS) mode to mitigate the poor sampling of the point spread function in that mode. Dithering is also supported (albeit discouraged) for the aperture masking interferometry (AMI) mode. Dithering is not supported for the single object slitless spectroscopy (SOSS) mode.
- Target Acquisition (TA) is required when using a subarray in SOSS and AMI observing modes, and it is strongly encouraged in those modes even when using full frame readout. TA is not implemented for WFSS.
- All targeting is performed using apertures, which define the target's position and extent in the field of view of the detector.
- Mosaics can be used in the WFSS mode to cover an area larger than the 2.2' x 2.2' NIRISS field of view.