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.