The NIRSpec integral field unit (IFU) spectroscopy mode has several dither and nod options available. of the target position that will be used to detector effects, help remove cosmic rays, and improve spatial sampling. In IFU mode, the primary difference between dithers and nods is that nodded exposures will be subtracted in data processing to remove in-field background flux. As a result, the nod options are best used for targets that are not The IFU dither and nod pattern positions are designed to optimize the sub-spatial element sampling in both dimensions.
The IFU nod and dither options can provide between one to sixty exposures: one exposure results from the no dithering option, and sixty offset exposures will be acquired if the full IFU cycling pattern is used. Table 1 defines the dither and nod options for NIRSpec IFU Spectroscopy. are also illustrated in Figure 1.
Table 1. NIRSpec IFU dither and nod options
|No dither or nod|
No dithering or nodding is performed. The target is positioned at the IFU aperture center.
Two points separated by ~ in both X (dispersion) and Y (spatial) directions (see Figure 1). Both points lie within less than one IFU aperture of each other, so there will be some overlap in th
Four points constituting a box that is ~1.6" 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 1 IFU aperture of each other, so there will be some overlap in the fields.
Same positions are observed as the 4-POINT-DITHER (see Figure 1), but nods will specifically be used to subtract in-field background flux in the pipeline. Nods should only be used on and compact sources (less than 0.2"–0.3" extents).
The CYCLING and SPARSE-CYCLING 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). U
|CYCLING||SIZE||Available in three 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 three 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/dither positions span approximately twice the area covered by the largest cycling pattern. The positions in the IFU dither and nod options are presented in Tables 2 and 3 below.
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
Figure 1. IFU nod and dither patterns
|Options for nod and dither pattern field positions with the NIRSpec IFU. The 2-point or 4-point nods can be used to move compact sources in the aperture for in-field background subtraction. The 4-point dither uses the same pattern as the 4-point nod, but is intended for extended sources and will not execute in-field background subtraction. In the 60-point cycling patterns, as many as 60 dithers can be selected with pattern extents of 0.25", 0.5" or 1" . The light blue points highlight the first 9 positions in the 60-point cycling patterns. These|
Tables 2 and 3 give the offsets in arcseconds relative to the IFU aperture center of positions in each OFFSET TYPE.
Table 2. IFU dither and nod positions
|OFFSET TYPE||Position index||X (dispersion)||Y (spatial)|
Table 3. IFU dither cycling positions for the small, medium and large patterns
Scroll across to see all columns
|OFFSET TYPE||Position index||Small||Medium||Large|
| || ||X (dispersion)||Y (spatial)||X (dispersion)||Y (spatial)||X (dispersion)||Y (spatial)|
|CYCLING or SPARSE-CYCLING||1||-0.125||-0.075||-0.225||-0.175||-0.425||-0.375|
Notes for Tables 2 and 3: X and Y positions above are expressed as offsets in arcseconds relative to the IFU aperture center. For all OFFSET 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 values that are representative of the dithers that the user will select. Bear in mind that values might change.