NIRSpec FS Dither and Nod Patterns

The NIRSpec fixed slits spectroscopy mode has several dither and nod pattern options for improving spatial or spectral sampling and removing astrophysical background flux.  

See also: NIRSpec Dithering Recommended StrategiesDither Positions for the NIRSpec Slits

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The NIRSpec fixed slits (FS) spectroscopy mode has several dither and nod options available. In fixed slit spectroscopy, the Primary Dither Positions in APT refer to nods along the slit that are used to subtract nearby background flux. As a result, the nod options are best used for targets that are not significantly spatially extended on the scale of the offset. In addition, the FS mode offers small scale subpixel (<0.1") offsets of the target position called Sub-Pixel Pattern, which will be used to mitigate detector effects, help remove cosmic rays, and particularly improve spectral and/or spatial sampling. The primary nods/dithers can be combined with subpixel dithers to obtain background subtracted observations with subpixel sampling. Table 1 and Figure 1 show a summary view of these nod and dither options and the ways they can be combined.

Table 1. Options for the NIRSpec FS offsets

Primary nods

2-, 3-, or 5-position nods are available.

The Primary Dither Positions option may take the following values:


There will be one primary position along the slit.

2There will be 2 primary positions along the slit.
3There will be 3 primary positions along the slit.
5There will be 5 primary positions along the slit.

The primary nod positions have been defined to optimize throughput based on the FS hardware characteristics. The precise positions for each nod are slit-dependent (see Table 2).

Subpixel dithers

Small-scale offsets in the spectral (dispersion) direction, spatial (cross-dispersion) direction, or both the spectral and spatial directions are available. These small scale offsets are the same for all slits, and are fixed offsets from the primary nod positions.

The Sub-Pixel Pattern dither option may take the following values:


No subpixel dithering is performed.
SPECTRALThree spectral dither positions are defined for each primary nod position along the slit.
SPATIALTwo spatial dither positions are defined for each primary nod position along the slit.
BOTHFour dither positions are defined for each primary nod position along the slit.

Note that spectral offsets will decenter the source in the slit in the dispersion direction and therefore incur a change in slit losses, which are wavelength dependent. All offered dither options will be calibrated, but this calibration will be more challenging for SPECTRAL and BOTH dither patterns. 

Options selected for dithers and nods determine the number of exposures that are acquired, as shown in Figure 1. Any combination of primary nod positions and subpixel dithers can be selected. Table 2 lists the specific slit-dependent primary nod positions. Table 3 lists the relative offsets with respect to the primary nod positions for the subpixel dithers.

Figure 1. FS dither and nod combinations

FS dither and nod combinations

The approximate locations of the primary nod and dither positions along the specified fixed slit, as indicated in the technical note Dither Positions for the NIRSpec Slits. The Sub-Pixel Pattern options of NONE, SPECTRAL, SPATIAL and BOTH are shown at the top. Under each subpixel dither option, the primary nod position options (Primary Dither Positions) are shown as 4 panels: NONE23, or 5. The number of exposures that result from each selection is shown at the bottom of the figure.

The spectra from the "A"  fixed slits (S200A1S200A2S400A1, and S1600A1) project onto both detectors and will have wavelengths lost to the detector gap in the NIRSpec high spectral resolution mode (R ~ 2,700). However, the wavelength gap can be filled and the full range of high resolution data can be acquired if the S200A1 and S200A2 slits are used together. When the S200A1 and S200A2 option is specified, the nod/dither pattern executes twice. The target is placed in the S200A1 slit and the requested exposures are executed at that position. Then, a move is made to the S200A2 slit and an identical set of exposures are executed.

Table 2.  NIRSpec fixed slit primary nod positions

SlitPrimary Y position (arcsec)























Table note: Primary dither positions ("nods") for the 5 fixed slit apertures in the ideal coordinate frame in cross dispersion (Y) direction in arcseconds. The X position (dispersion direction) is zero arcsec in all cases. All dither positions are relative to the reference point for that aperture. Tabulated values determined from JWST on-orbit data.

Table 3.   NIRSpec fixed slit subpixel dither relative offsets





One exposureThree exposuresTwo exposuresFour exposures

Table note: The relative offset (with respect to the primary dither positions) of the available subpixel dither patterns in NIRSpec ideal coordinates in arcseconds. The total number of exposures resulting from each option are indicated in the last row. The positions for each exposure are represented in Figure 1. Tabulated values determined from JWST on-orbit data.

In addition to using dither patterns to improve sampling and remove detector effects, FS users can also tile multiple FS positions to create "imaging spectroscopy" of larger spatial regions using the mosaic tools in APT.

Notable updates
    Values in Tables 2 and 3 have been updated for on-orbit performance

    Clarified text to differentiate dithers and nods, and to more clearly connect them to APT parameters
Originally published