NIRISS WFSS Dithers

The JWST NIRISS wide field slitless spectroscopy (WFSS) mode has a predefined number of dither steps and sizes. Dithering is required to mitigate NIRISS PSF undersampling at wavelengths < 2 μm.

See also: NIRISS Wide Field Slitless Spectroscopy, NIRISS WFSS Recommended Strategies, NIRISS WFSS Template APT Guide

The NIRISS wide field slitless spectroscopy (WFSS) observing sequence can be constructed using the GR150R grism, GR150C grism, or a combination of both grisms. Dithering is required for WFSS observations because the JWST point spread function (PSF) is heavily undersampled by NIRISS when observing at the wavelengths covered by WFSS. Dithers can significantly improve the effective sampling of the PSF.

Dither pattern steps for WFSS are chosen to have sufficient spacing (typically ≥3 pixels) along both rows and columns to accommodate exposures with GR150R and GR150C grisms. The dither patterns are chosen to achieve the best possible pixel-phase sampling for a given number of dither positions. 

Words in bold are GUI menus/
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bold italics are buttons in GUI
tools or package parameters.

The selection of dither patterns for WFSS is made by specifying 2 parameters:

  • Number of dither steps (Image Dithers): 2, 3 ,4 ,6, 8, 12, and 16 steps. The first step corresponds to the initial pointing.
  • Dither step amplitude (Pattern Size): SMALL (~0.3"), MEDIUM ( ~0.6"), or LARGE (~1.2"). 

As to the latter parameter, consider that smaller dither amplitudes yield the benefit of higher conservation of pixel phase sampling across the detector (due to varying geometric distortion), while larger dither amplitudes allow one to step over larger targets and hence offer a better mitigation of flat-fielding errors for astronomical scenes involving such targets. The user should choose the best compromise for a given science case. The MEDIUM patterns (the default choice in the APT template for the WFSS mode) was chosen to be appropriate for extragalactic studies at moderate to high redshifts. The SMALL patterns are thought to be useful for sparse fields of compact sources, while the LARGE patterns should be useful for observing astronomical scenes involving relatively large objects (e.g., galaxies at relatively low redshift). 

The spatial offsets with respect to the initial pointing (step #1 in all patterns listed below) are given in Tables 1-3 for the NIRISS WFSS dither patterns. Note: when the option GRISM = BOTH is selected in the APT template for NIRISS WFSS, the initial pointing depends on the grism (GR150R or GR150C) and the filter. This is described in the NIRISS WFSS Recommended Strategies article. 

As of APT v2020.2, the NIRISS WFSS APT template also supports optional 3-position dither patterns for the direct image(s) that are taken after a dithered set of grism images (see the NIRISS WFSS Recommended Strategies article for details on when to use these). The spatial offsets of these patterns are given in Table 4 below. 

The NIRISS WFSS mode can also be used in coordinated parallel mode. When this option is selected, additional customized dither patterns become available through the primary instrument's APT template. These customized dither patterns have been designed to work well for both the primary and the selected parallel instrument mode.  


Table 1. WFSS mode offsets for the SMALL dither patterns

# of Dithers
(Image Dithers)
Step #

ΔX pixels

ΔY pixels

ΔX arcsec

ΔY arcsec

210.0000.0000.00000.0000

23.5003.5000.22890.2296
3





10.0000.0000.0000.000

24.3334.3330.28330.2842

38.6678.6670.56680.5685
4





10.0000.0000.00000.0000

25.0002.5000.32700.1632

32.5006.0000.16350.3946

4-2.5004.500-0.16350.2972
6





10.0000.0000.00000.0000

2-2.0003.500-0.13080.2312

3-4.6678.500-0.30520.5614

42.6676.0000.17440.3945

54.3332.0000.28330.1305

66.667-2.5000.4360-0.1667
8





10.0000.0000.00000.0000

25.0002.5000.32700.1632

33.5007.0000.22890.4602

4-1.5004.500-0.09810.2969

5-2.500-1.000-0.1635-0.0651

62.5001.5000.16350.0981

71.0006.0000.06540.3950

8-4.0003.500-0.26160.2318
12





1

0.0000.0000.0000.000

25.0002.5000.32700.1632

33.5007.0000.22890.4602

4-1.5005.500-0.09810.3628

5-2.500-1.000-0.1635-0.0651

62.0001.5000.13080.0982

71.0006.0000.06540.3950

8-4.5003.500-0.29430.2320

9-2.0002.000-0.13080.1324

102.5004.5000.16350.2958

111.5009.0000.09810.5925

12-4.0006.500-0.26160.4295
16





10.0000.0000.0000.000

25.0002.5000.32700.1632

34.5007.0000.29430.4599

4-1.5005.500-0.09810.3628

5-2.500-1.500-0.1635-0.0981

62.500-1.0000.1635-0.0666

71.0006.0000.06540.3950

8-5.0003.500-0.32700.2321

9-3.5001.000-0.22890.0669

103.5004.5000.22890.2955

112.0009.5000.13080.6253

12-4.0005.000-0.26160.3306

13-6.0001.500-0.39240.1006

14-1.0003.000-0.06540.1980

15-0.5008.000-0.03270.5273

16-6.5007.500-0.42500.4961


Table 2. WFSS mode offsets for the MEDIUM dither patterns

# of Dithers
(Image Dithers) 
Step #

ΔX pixels

ΔY pixels

ΔX arcsec

ΔY arcsec

2





10.0000.0000.00000.0000

26.5006.5000.42510.4263
3





10.0000.0000.00000.0000

26.3336.3330.41410.4154

312.66712.6670.82830.8308
4





10.0000.0000.00000.0000

28.0004.5000.52310.2941

33.50011.0000.22890.7237

4-3.5007.500-0.22890.4952
6





10.0000.0000.00000.0000

2-4.0007.500-0.26160.4954

3-7.66714.500-0.50140.9577

43.66711.0000.23980.7237

57.3334.0000.47950.2614

610.667-3.5000.6975-0.2338
8





10.0000.0000.00000.0000

28.0004.5000.52310.2941

33.50011.0000.22890.7237

4-3.5007.500-0.22890.4952

5-4.000-2.000-0.2616-0.1306

64.0002.5000.26160.1635

7-0.5009.000-0.03270.5931

8-7.5005.500-0.49040.3646
12





10.0000.0000.00000.0000

28.0004.5000.52310.2941

33.50011.0000.22890.7237

4-3.0006.500-0.19620.4292

5-4.500-1.500-0.2943-0.0975

64.0003.0000.26160.1965

7-1.0008.500-0.06540.5604

8-7.5005.000-0.49040.3317

9-2.5002.000-0.16350.1325

106.5007.5000.42510.4922

111.00012.0000.06540.7904

12-5.5009.5000-0.35970.6276
16





10.0000.0000.0000.000

28.0003.5000.52310.2282

33.50011.0000.22890.7237

4-3.5007.500-0.22890.4952

5-4.500-2.500-0.2943-0.1634

65.0002.0000.32700.1303

7-1.0009.5000-0.06540.6262

8-7.5005.000-0.49040.3317

9-2.0002.500-0.13080.1653

106.5007.0000.42510.4593

111.50013.5000.09810.8890

12-6.00010.000-0.39240.6607

13-5.0001.500-0.32700.1003

143.0006.0000.19620.3944

15-1.50012.000-0.09810.7911

16-8.5008.500-0.55580.5626


Table 3. WFSS mode offsets for the LARGE dither patterns

# of Dithers
(Image Dithers) 

Step #

ΔX pixels

ΔY pixels

ΔX arcsec

ΔY arcsec

2





10.0000.0000.00000.0000

212.50012.5000.81740.8199
3





10.0000.0000.00000.0000

212.33312.3330.80650.8089

324.66724.6671.61301.6179
4





10.0000.0000.00000.0000

216.0008.5001.04630.5553

37.50022.0000.49041.4473

4-7.50015.500-0.49041.0235
6





10.0000.0000.00000.0000

2-8.00014.500-0.52310.9578

3-14.66728.500-0.95911.8822

46.66722.0000.43601.4476

514.3338.0000.93730.5228

620.667-7.5001.3515-0.5004
8





10.0000.0000.00000.0000

216.0008.5001.04630.5553

37.50022.0000.49041.4473

4-7.50015.500-0.49041.0235

5-8.000-4.000-0.5231-0.2612

68.0004.5000.52310.2941

7-0.50018.000-0.03271.1861

8-15.50011.500-1.01360.7623
12





10.0000.0000.0000.000

216.0008.5001.04630.5553

37.50022.0000.49041.4473

4-7.50015.500-0.49041.0235

5-6.500-4.500-0.4250-0.2946

610.0004.0000.65390.2606

71.00018.5000.06541.2186

8-13.50011.000-0.88280.7288

9-4.0006.500-0.26160.4295

1012.50014.0000.81740.9187

113.00028.0000.19621.8440

12-9.50021.500-0.62121.4194
16





10.0000.0000.0000.000

216.0008.5001.04630.5553

37.50022.0000.49041.4473

4-7.50015.500-0.49041.0235

5-6.500-3.500-0.4250-0.2287

610.0004.0000.65390.2606

71.50018.5000.09811.2185

8-13.00011.000-0.85010.7286

9-4.0006.500-0.26160.4295

1012.50014.0000.81740.9187

113.50027.5000.22891.8109

12-9.00021.000-0.58851.3863

13-11.0002.500-0.71930.1680

145.00010.0000.32700.6574

15-3.50024.000-0.22891.5824

16-16.50017.500-1.07901.1580


Table 4. WFSS mode offsets for the 3-step "Direct Imaging" patterns*

Dither Size

Step #ΔX pixelsΔY pixelsΔX arcsecΔY arcsec
SMALL10.0000.0000.00000.0000

27.500-2.0000.4904-0.1340

35.0002.5000.32700.1632






MEDIUM10.0000.0000.00000.0000

211.500-3.0000.7520-0.2011

37.0003.5000.45770.2285






LARGE10.0000.0000.00000.0000

223.500-7.0001.5367-0.4683

315.0006.5000.98090.4238


* The MEDIUM-size pattern is used for the optional direct imaging dithers when the NIRISS WFSS mode is used as prime mode in a coordinated parallel observation and a customized dither pattern is selected.



References

Goudfrooij, P. 2015 JWST-STScI-004466
NIRISS Dither Patterns for the WFSS and Imaging Observing modes




Latest updates
  •  
    Added description of the optional three-position "Direct Imaging" dithers introduced in APT 2020.2 and Table 4.

  •   
    Added brief description of the new filter-dependent primary dither positions
Originally published