NIRSpec IFU Dither and Nod Patterns
The JWST NIRSpec integral field unit (IFU) spectroscopy mode has simple nodding and dithering patterns available in with 2- or 4-point positions. Alternatively, the cycling pattern options can each use up to 60 unique offset positions at small (<0.25"), medium (0.5") or large (1") spatial scales.
The NIRSpec integral field unit (IFU) spectroscopy mode has several dither and nod options available. The IFU dither and nod pattern positions are designed to optimize the sub-spatial element sampling in both dimensions. Additionally dithers and nods can help mitigate the effects of light leakage through the MSA.
Dithers are smaller offsets of the target position that will be used to mitigate detector effects and help remove cosmic rays.
In IFU mode, nods are larger offsets that enable subtraction of the exposures for background removal. The nod options are best used for targets that are not significantly spatially extended on the scale of the offset.
The primary difference between dithers and nods is that dithered exposures will not be used for background subtraction by the calibration pipeline. The user can add another exposure specification for a separate background observation that can be subtracted from the dithered science exposures. This strategy is described as an off-scene nod option in the article NIRSpec Dithering Recommended Strategies.
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In addition to using dither patterns to improve sampling, remove cosmic rays, and remove detector effects, IFU users can also
- use mosaic tools to tile multiple IFU positions to map larger spatial regions;
- take advantage of target groups for multiple discontinuous pointings—the use of target groups to provide an off-scene background measurement is described in the article NIRspec Dithering Recommended Strategies.
Table 1. NIRSpec IFU dither and nod options
APT Dither Type values
|No dither or nod|
No dithering or nodding is performed. The target is positioned at the IFU aperture center.
Two points separated by ~1.6" in both X and Y directions (see Figure 1). Both points lie within less than one IFU aperture of each other, so there will be some overlap in their fields. 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).
Four points constituting a box that is ~0.4" on a side (see Figure 1). The 4 points in the dither pattern have 0.025" sub-slice offsets to allow for improved sampling of the spatial point spread function (PSF). All 4 points lie within less than one IFU aperture of each other, so there will be some overlap in the fields.
Four points constituting a box that is ~1.6" on a side (see Figure 1), but 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 (less than 0.2"–0.3" extents).
The CYCLING and SPARSE-CYCLING Dither Type options allow access to dither positions in 60-point cycling patterns. If one of these is selected, the user must specify the pattern size, Size, which is available in 3 spatial scales: SMALL (0.25" extent), MEDIUM (0.5" extent), and LARGE (1.0" extent). For the CYCLING option, users must also choose the Starting Point and Number of Points parameter values. For SPARSE-CYCLING, values should be entered for the Points parameter.
|CYCLING||Size||Available in 3 spatial scales: SMALL (0.25" extent), MEDIUM (0.5" extent), and LARGE (1.0" extent).|
|Starting Point||An integer from 1 to 60. It specifies the index of the starting point. If Starting Point is defined, Number of Points must also be defined.|
Number of Points
|An integer from 1 to 60, and less than or equal to 60 minus Starting Point. This specifies the number of points of the cycling pattern to execute in sequence.|
|SPARSE-CYCLING||Size||Available in 3 spatial scales: SMALL (0.25" extent), MEDIUM (0.5" extent), and LARGE (1.0" extent).|
|Points||A list of the specific indices from the cycle table, that represent the desired dithers (see Table 3). This is an alternative way (compared to Starting Point plus Number of Points) for the user to specify a list of selected points from the cycling table.|
Note that the 2-point nod and 4-point nod positions span approximately twice the area covered by the largest cycling pattern. The positions in the IFU dither and nod options are presented in Table 2 and Table 3 below. Because the source is closer to the IFU corners in these cases, use of these patterns without target acquisition (i.e., TA Method of Verify_Only or None) is not recommended. (For more information see NIRSpec Dithering Recommendations.)
The first 9 positions in each of the small, medium and large cycling patterns were designed to provide N-point dither options (N from 2 to 9). For example, selecting CYCLING, Size = SMALL, Starting Point = 1 and Number of Points = 4 will give a 4-point dither with 0.25" spatial extent with spatial PSF optimizing subpixel offsets.
|Dither Type||Position index||X (dispersion)||Y (spatial)|
X and Y positions above are expressed as offsets in arcseconds relative to the IFU reference point. For Dither Type values other than the cycling options, all of the pointings in the table will be executed. For cycling, the user will additionally need to select a starting point and number of points or a set of index points from this table that represent the dithers to be executed. The tables present on-orbit determined values that are representative of the dithers that the user will select. Bear in mind that values might change.