The JWST NIRSpec MOS, IFU, and FS observing modes support several options for dithers and nods.
Dither and nod options for NIRSpec
Parent article: NIRSpec Operations
NIRSpec dithers and nods are telescope offsets to acquire multiple exposures within a single visit. The general goal is to improve data quality. Because the NIRSpec PSF is under sampled at most wavelengths, most observations with JWST require dithering. Some observing modes also support nods. Both dithers and nods can also help to mitigate the effects of light leakage through the MSA.
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.
Below is a table of terminology used for different types of dithering in different NIRSpec instrument modes. These are described more comprehensivley in NIRSpec Dithering Recommended Strategies.
In the Fixed Slit APT template, "Primary" dithers move the target to a different location along the slit. These moves are equivalent to the IFU in-scene nods and the MOS nods within the slitlet. The purpose is to perform in-scene background subtraction.
In the Fixed Slit APT template, "Secondary" dithers apply a small offset in either the dispersion, spatial, or both directions from the nominal position of the primary dither. The purpose is to better sample the source spectrum, and to mitigate detector artifacts.
|IFU||In the IFU APT template, the source is moved to different locations in the IFU aperture. Nods will specifically be used to subtract in-field background flux in the pipeline. Nods should only be used on point-like and compact sources (with less than 0.2"–0.3" extents).||In the IFU APT template , the source is moved to a different location within the IFU aperture. There will be some overlap of data from dithered positions in the IFU. These are small sub-slice offsets to allow for improved sampling of the spatial point spread function (PSF).|
|MOS||In MPT, MSA sources will be moved to different positions within each of their slits. No re-configuration of the MSA is needed. Data from nodded positions will be used to measure/subtract background from the source spectrum.||In MPT, MSA sources are moved to different locations in the MSA shutter plane requiring a reconfiguration of the MSA shutters. The purpose is to place the spectra at different locations on the detectors to be able to recover lost wavelengths, or mitigate detector artifacts.|
The fixed slits (FS), integral field unit (IFU), and multi-object spectroscopy (MOS) observing modes support a variety of dither and nod options:
- fixed slits dithers and nods
- integral field unit dithers and nods
- multi-object spectroscopy dithers and nods
The bright object time-series (BOTS) mode does not use dithers or nods.
Near Infrared Spectrograph (NIRSpec)
NIRSpec Observing Guidance
Science Use Cases