The JWST NIRSpec MOS, IFU, and FS observing modes support several options for dithers and nods.
NIRSpec dithers and nods are telescope offsets to acquire multiple exposures within a single visit. The general goal is to improve data quality. Most observations with JWST require dithering, and some observing modes also support nods.
Dithers are offsets of the target position over multiple exposures, to even out or mitigate detector effects, help remove cosmic rays, improve spatial sampling, and increase signal-to-noise and flux accuracy. Nods are offsets that produce data to be subtracted in pipeline processing, in order to cancel in-field background flux. Nod options are typically best used for targets that are not significantly spatially extended. The fixed slits (FS), integral field unit (IFU), and multi-object spectroscopy (MOS) observing modes support a variety of dither and nod options:
- multi-object spectroscopy dithers and nods
- integral field unit dithers and nods
- fixed slits dithers and nods
The bright object time-series mode does not use dithers or nods.