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.  

Introduction

The NIRSpec integral field unit (IFU) spectroscopy mode has several dither and nod options available. Dithers are offsets of the target position that will be used to mitigate 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 significantly spatially extended on the scale of the offset. 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.  The NIRSpec IFU dither and nod options are also illustrated in Figure 1.

Table 1.  NIRSpec IFU dither and nod options

Offset option

APT keyword

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 (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 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 ~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.

4-point nod

4-POINT-NOD

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 point-like and compact sources (less than 0.2"–0.3" extents).





Cycling patterns


 

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). Users must also choose, for the CYCLING option, the STARTING POINT and NUMBER OF POINTS parameters, or, for the SPARSE-CYCLING option, the POINTS parameter.


CYCLINGSIZEAvailable in three 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 three 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/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 with spatial PSF optimizing sub-pixel offsets.

Figure 1. IFU nod and dither patterns

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 can be selected as 2 to 9 point dither patterns using the first 2 to 9 points in the cycling pattern.

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 TYPEPosition indexX (dispersion)Y (spatial)

NONE

1

0.000

0.000

 

 

 

 

2-POINT-NOD

 

1

-0.825

-0.775

2

0.775

0.825

 

 

 

 



4-POINT-DITHER

 

1

-0.825

-0.775

2

0.775

-0.825

3

-0.825

0.825

4

0.775

0.775

 

 

 

 



4-POINT-NOD

 

1

-0.825

-0.775

2

0.775

-0.825

3

-0.825

0.825

4

0.775

0.775

 

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

Scroll across to see all columns

OFFSET TYPEPosition indexSmallMediumLarge
  X (dispersion)Y (spatial)X (dispersion)Y (spatial)X (dispersion)Y (spatial)
CYCLING or SPARSE-CYCLING1-0.125-0.075-0.225-0.175-0.425-0.375
20.0750.1250.1750.2250.3750.425
3-0.0750.075-0.1750.175-0.3750.375
40.125-0.1250.225-0.2250.425-0.425
5000000
6-0.025-0.1-0.025-0.2-0.025-0.4
70.0250.10.0250.20.0250.4
8-0.10.025-0.20.025-0.40.025
90.1-0.0250.2-0.0250.4-0.025
10-0.009-0.043-0.018-0.086-0.036-0.172
110.08850.0490.1770.0980.3540.196
120.029-0.10050.058-0.2010.116-0.402
13-0.12350.076-0.2470.152-0.4940.304
140.0415-0.0820.083-0.1640.166-0.328
150.02850.08950.0570.1790.1140.358
16-0.0550.0435-0.110.087-0.220.174
17-0.11350.0045-0.2270.009-0.4540.018
18-0.01150.088-0.0230.176-0.0460.352
19-0.049-0.009-0.098-0.018-0.196-0.036
20-0.0930.069-0.1860.138-0.3720.276
210.067-0.02750.134-0.0550.268-0.11
220.0905-0.030.181-0.060.362-0.12
230.014-0.0250.028-0.050.056-0.1
24-0.09250.036-0.1850.072-0.370.144
250.10750.0350.2150.070.430.14
260.002-0.04050.004-0.0810.008-0.162
27-0.065-0.003-0.13-0.006-0.26-0.012
280.07750.0590.1550.1180.310.236
29-0.03350.014-0.0670.028-0.1340.056
30-0.05-0.0265-0.1-0.053-0.2-0.106
310.0190.1230.0380.2460.0760.492
320.1170.03650.2340.0730.4680.146
330.11750.07250.2350.1450.470.29
340.121-0.02050.242-0.0410.484-0.082
35-0.1220.0775-0.2440.155-0.4880.31
360.02950.01850.0590.0370.1180.074
37-0.1235-0.0735-0.247-0.147-0.494-0.294
380.069-0.01850.138-0.0370.276-0.074
39-0.0925-0.074-0.185-0.148-0.37-0.296
40-0.1060.0395-0.2120.079-0.4240.158
41-0.047-0.0845-0.094-0.169-0.188-0.338
42-0.0110.068-0.0220.136-0.0440.272
430.04950.0350.0990.070.1980.14
44-0.0385-0.0335-0.077-0.067-0.154-0.134
45-0.06550.087-0.1310.174-0.2620.348
460.1185-0.02050.237-0.0410.474-0.082
47-0.07350.0755-0.1470.151-0.2940.302
48-0.0220.0935-0.0440.187-0.0880.374
49-0.0765-0.025-0.153-0.05-0.306-0.1
50-0.0275-0.025-0.055-0.05-0.11-0.1
51-0.0070.0345-0.0140.069-0.0280.138
52-0.0505-0.057-0.101-0.114-0.202-0.228
530.076-0.11050.152-0.2210.304-0.442
540.0665-0.09950.133-0.1990.266-0.398
55-0.104-0.1205-0.208-0.241-0.416-0.482
560.04350.0820.0870.1640.1740.328
57-0.0940.077-0.1880.154-0.3760.308
58-0.0440.035-0.0880.07-0.1760.14
590.10150.02850.2030.0570.4060.114
600.0335-0.1110.067-0.2220.134-0.444

 

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.

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. 

 


 

Related links

Near Infrared Spectrograph, NIRSpec
NIRSpec Overview
NIRSpec IFU Mode

Last updated

Published April 19, 2017

 

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