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 Strategies, Dither Positions for the NIRSpec Slits
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Table 1. Options for the NIRSpec FS offsets
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.
|2||There will be 2 primary positions along the slit.|
|3||There will be 3 primary positions along the slit.|
|5||There 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).
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.|
|SPECTRAL||Three spectral dither positions are defined for each primary nod position along the slit.|
|SPATIAL||Two spatial dither positions are defined for each primary nod position along the slit.|
|BOTH||Four 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.
The spectra from the "A" fixed slits (S200A1, S200A2, S400A1, 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
|Slit||Primary 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 exposure||Three exposures||Two exposures||Four 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.