NIRSpec Dithers and Nods

The JWST NIRSpec MOS, IFU, and FS observing modes support several options for dithers and nods.

See Also:  NIRSpec Dithering Recommended StrategiesNIRSpec FS Dither and Nod PatternsNIRSpec IFU Dither and Nod PatternsNIRSpec MOS Dither and Nod Patterns

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. Alternatively, nods can be used in place of dithers in observations of extended sources.  But, in this case, custom post-processing will be required to avoid combining pair subtracted data, and extra care may be needed to measure the background.  This approach may be advantageous for some MOS observations, where multiplexing efficiency is improved by using nods in place of dithers.

Below is a table of terminology used for different types of dithering in different NIRSpec instrument modes.  These are described more comprehensively in NIRSpec Dithering Recommended Strategies.


Table 1.   Nods and Dithers Terminology

Operating ModeNodsDithers
Fixed Slits

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. FS dither and nod patterns describes the specifics of each type of offset.

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. FS dither and nod patterns describes the specifics of each type of offset.

IFUIn 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.  With the possible exception noted in the box above, nods should only be used on point-like and compact sources (with less than 0.2"–0.3" extents). IFU dither and nod patterns describes the specifics of each type of offset.In the IFU APT template , the source is moved to a different location within the IFU aperture. These are small slice plus sub-slice offsets to allow for improved sampling of the spatial point spread function (PSF). IFU dither and nod patterns describes the specifics of each type of offset.
MOSIn 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. MOS dither and nod patterns describes the specifics of each type of offset.

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. MOS dither and nod patterns describes the specifics of each type of offset.

For users wishing to dither in MOS observations created outside of MPT (e.g. the long slit at the Q4 field point), the mosaic capability should suffice.  Users wishing to pursue this option should contact the help desk for support in designing their observations.

BOTSThe bright object time-series (BOTS) mode does not use dithers or nods.

The bright object time-series (BOTS) mode does not use dithers or nods.





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