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.  

See also: NIRSpec Dithering Recommended StrategiesNIRSpec MSA Leakage Correction for IFU ObservationsNIRSpec MSA Leakage Subtraction Recommended Strategies

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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IFU nod and dither options can provide between one to 60 exposures: one exposure results from the no dithering option, and 60 offset exposures will be acquired if the full IFU cycling pattern is used. The IFU dither and nod pattern positions are designed to optimize the sub-spatial element sampling in both dimensions. Table 1 defines the dither and nod options for NIRSpec IFU spectroscopy. The NIRSpec IFU dither and nod options are also illustrated in Figure 1.

In addition to using dither patterns to improve sampling, remove cosmic rays, and remove detector effects, IFU users can also


Table 1. NIRSpec IFU dither and nod options

Offset option

APT Dither Type values

Description

No dither or nod

NONE

No dithering or nodding is performed. The target is positioned at the IFU aperture center.

2-point nod

2-POINT-NOD

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).

4-point dither

4-POINT-DITHER

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.

4-point nod

4-POINT-NOD

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).

Cycling patterns



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.

CYCLINGSizeAvailable in 3 spatial scales: SMALL (0.25" extent), MEDIUM (0.5" extent), and LARGE (1.0" extent).
Starting PointAn 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-CYCLINGSizeAvailable in 3 spatial scales: SMALL (0.25" extent), MEDIUM (0.5" extent), and LARGE (1.0" extent).
PointsA 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.

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 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 can be selected as 2- to 9-point dither patterns using the first 2 to 9 points in the cycling pattern.
Table 2. IFU dither and nod positions

Dither TypePosition indexX (dispersion)Y (spatial)

NONE

1

0.000

0.000





2-POINT-NOD


1

-0.9077

-0.7635

2

0.7980

0.8718





 

 

4-POINT-DITHER


1

-0.2328

-0.0774

2

0.1292

0.1855

3

0.0259

-0.1333

4

-0.0776

0.2415





 

 

4-POINT-NOD


1

-0.9077

-0.7635

2

0.7980

0.8718

3

0.6437

-0.8357

4

-0.7521

0.9441


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.


Table 3. IFU dither cycling positions for the small, medium, and large patterns

Scroll across to see all columns

Dither TypePosition indexSmall (0.25" extent)Medium (0.5" extent)Large (1.0" extent)


X (dispersion)Y (spatial)X (dispersion)Y (spatial)X (dispersion)Y (spatial)
CYCLING or SPARSE-CYCLING1-0.2328-0.0774-0.3365-0.1830-0.6478-0.4998
20.12920.18550.23240.29120.54140.6080
3-0.0259-0.13330.0774-0.24150.3867-0.5657
4-0.07760.2415-0.18110.3496-0.49230.6740
50.00000.00000.00000.00000.00000.0000
6-0.15520553-0.25870.0566-0.56970.0603
70.00000.16040.00000.2673-0.00030.5880
80.05170.10630.15500.10500.46450.1013
9-0.1552-0.1585-0.1553-0.2654-0.1556-0.5860
10-0.0629-0.0010-0.0703-0.0376-0.0927-0.1477
110.05810.11560.13130.15660.35040.2797
12-0.0157-0.07530.0082-0.16150.0800-0.4202
13-0.20510.1534-0.30740.2196-0.61490.4183
14-0.0002-0.05170.0341-0.12230.1369-0.3339
15-0.01630.16850.00720.24470.07780.4735
16-0.12000.1107-0.16550.1484-0.30230.2617
17-0.19260.0615-0.28670.0665-0.56910.0815
18-0.06600.1671-0.07550.2425-0.10420.4686
19-0.11250.0433-0.15310.0360-0.27490.0144
20-0.16720.1440-0.24420.2039-0.47560.3837
210.031430.01780.0868-0.00640.2529-0.0789
220.06060.01420.1354-0.01230.3595-0.0920
23-0.03430.0218-0.02270.00030.0120-0.0643
24-0.16660.1016-0.24320.1333-0.47330.2285
250.08170.09740.17050.12620.43670.2128
26-0.04920.0021-0.0476-0.0326-0.0426-0.1365
27-0.13240.0512-0.18620.0493-0.34780.0435
280.04450.12860.10850.17830.30050.3273
29-0.09330.0725-0.12100.0848-0.20420.1218
30-0.11380.0208-0.1552-0.0013-0.2794-0.0678
31-0.02810.2116-0.01250.31660.03440.6316
320.09350.09910.19010.12920.47970.2193
330.09410.14530.19110.20620.48190.3886
340.09840.02600.19840.00720.4979-0.0489
35-0.20320.1553-0.30430.2228-0.60800.4253
36-0.01510.07740.00930.09290.08250.1395
37-0.2051-0.0384-0.3074-0.1000-0.6148-0.2848
380.03390.02930.09100.01280.3036-0.0367
39-0.1666-0.0395-0.2432-0.1018-0.4733-0.2889
40-0.18330.1063-0.27110.1412-0.53490.2457
41-0.1101-0.0536-0.1490-0.1254-0.2659-0.3408
42-0.06540.1415-0.07450.1997-0.10190.3745
430.00970.09820.05070.12770.17340.2160
44-0.09950.0117-0.1314-0.0166-0.2270-0.1014
45-0.13300.1677-0.18730.2417-0.35030.4669
460.09530.02600.19320.00730.4865-0.0488
47-0.14300.1520-0.20380.2173-0.38670.4132
48-0.07900.1743-0.09730.2545-0.15210.4951
49-0.14670.0231-0.21000.0025-0.4003-0.0593
50-0.08580.0224-0.10860.0013-0.1769-0.0620
51-0.06040.0984-0.06620.1280-0.08360.2167
52-0.1144-0.0183-0.1562-0.0665-0.2818-0.2113
530.0426-0.08880.1054-0.18410.2934-0.4698
540.0308-0.07460.0858-0.16030.2504-0.4175
55-0.1809-0.0990-0.2671-0.2010-0.5260-0.5069
560.00230.15860.03820.22830.14600.4374
57-0.16840.1543-0.24630.2210-0.48020.4214
58-0.10630.0996-0.14270.1300-0.25210.2211
590.07420.08910.15810.11250.40950.1826
60-0.0101-0.08880.0175-0.18410.1004-0.4698

X and Y positions above are expressed as offsets in arcseconds relative to the IFU aperture center. 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.




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