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.

Main article: NIRISS Wide Field Slitless Spectroscopy
See also: 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 strongly 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. 

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

  • Number of dither steps (Image Dithers 1): 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 central pointing (step #1 in all patterns) are given in the tables below for the NIRISS WFSS dither patterns.


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

38.6678.6670.56680.5686
4





10.0000.0000.00000.0000

25.0002.5000.32700.1633

32.5006.0000.16350.3947

4-2.5004.500-0.16350.2973
6





10.0000.0000.00000.0000

2-2.0003.500-0.13080.2312

3-4.6678.500-0.30520.5615

42.6676.0000.17440.3946

54.3332.0000.28340.1305

66.667-2.5000.4360-0.1667
8





10.0000.0000.00000.0000

25.0002.5000.32700.1633

33.5007.0000.22890.4603

4-1.5004.500-0.09810.2970

5-2.500-1.000-0.1635-0.0652

62.5001.5000.16350.0981

71.0006.0000.06540.3951

8-4.0003.500-0.26160.2318
12





1

0.0000.0000.0000.000

25.0002.5000.32700.1633

33.5007.0000.22890.4603

4-1.5005.500-0.09810.3629

5-2.500-1.000-0.1635-0.0652

62.0001.5000.13080.0983

71.0006.0000.06540.3951

8-4.5003.500-0.29430.2320

9-2.0002.000-0.13080.1324

102.5004.5000.16350.2958

111.5009.0000.09810.5927

12-4.0006.500-0.26160.4295
16





10.0000.0000.0000.000

25.0002.5000.32700.1633

34.5007.0000.29430.4600

4-1.5005.500-0.09810.3629

5-2.500-1.500-0.1635-0.0981

62.500-1.0000.1635-0.0666

71.0006.0000.06540.3951

8-5.0003.500-0.32700.2321

9-3.5001.0000.16350.0669

103.5004.5000.22890.2955

112.0009.5000.13080.6255

12-4.0005.000-0.26160.3307

13-6.0001.500-0.39240.1006

14-1.0003.000-0.06540.1980

15-0.5008.000-0.3270.5273

16-6.5007.500-0.42510.4962


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

26.5006.5000.42510.4264
3





10.0000.0000.00000.0000

26.3336.3330.41410.4155

312.66712.6670.82830.8310
4





10.0000.0000.00000.0000

28.0004.5000.52310.2942

33.50011.0000.22890.7239

4-3.5007.500-0.22890.4953
6





10.0000.0000.00000.0000

2-4.0007.500-0.26160.4954

3-7.66714.500-0.50140.9578

43.66711.0000.23980.7238

57.3334.0000.47950.2614

610.667-3.5000.6975-0.2338
8





10.0000.0000.00000.0000

28.0004.5000.52310.2942

33.50011.0000.22890.7239

4-3.5007.500-0.22890.4953

5-4.000-2.000-0.2616-0.1306

64.0002.5000.26160.1636

7-0.5009.000-0.03270.5932

8-7.5005.500-0.49040.3647
12





10.0000.0000.00000.0000

28.0004.5000.52310.2942

33.50011.0000.22890.7239

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

106.5007.5000.42510.4923

111.00012.0000.06540.7905

12-5.5009.5000-0.35970.6277
16





10.0000.0000.0000.000

28.0003.5000.52310.2283

33.50011.0000.22890.7239

4-3.5007.500-0.22890.4953

5-4.500-2.500-0.2943-0.1634

65.0002.0000.32700.1303

7-1.0009.5000-0.06540.6263

8-7.5005.000-0.49040.3317

9-2.0002.500-0.13080.1653

106.5007.0000.42510.4594

111.50013.5000.09810.8892

12-6.00010.000-0.39240.6608

13-5.0001.500-0.32700.1003

143.0006.0000.19620.3945

15-1.50012.000-0.09810.7912

16-8.5008.500-0.55580.5627


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

212.50012.5000.81740.8200
3





10.0000.0000.00000.0000

212.33312.3330.80650.8091

324.66724.6671.61301.6282
4





10.0000.0000.00000.0000

216.0008.5001.04630.5554

37.50022.0000.49041.4476

4-7.50015.500-0.49041.0237
6





10.0000.0000.00000.0000

2-8.00014.500-0.52310.9579

3-14.66728.500-0.95911.8825

46.66722.0000.43601.4478

514.3338.0000.93730.5229

620.667-7.5001.3515-0.5005
8





10.0000.0000.00000.0000

216.0008.5001.04630.5554

37.50022.0000.49041.4476

4-7.50015.500-0.49041.0237

5-8.000-4.000-0.5231-0.2612

68.0004.5000.52310.2942

7-0.50018.000-0.03271.1863

8-15.50011.500-1.01360.7625
12





10.0000.0000.0000.000

216.0008.5001.04630.5554

37.50022.0000.49041.4476

4-7.50015.500-0.49041.0237

5-6.500-4.500-0.4250-0.2946

610.0004.0000.65390.2606

71.00018.5000.06541.2188

8-13.50011.000-0.88280.7289

9-4.0006.500-0.26160.4295

1012.50014.0000.81740.9189

113.00028.0000.19621.8443

12-9.50021.500-0.62121.4197
16





10.0000.0000.0000.000

216.0008.5001.04630.5554

37.50022.0000.49041.4476

4-7.50015.500-0.49041.0237

5-6.500-3.500-0.4250-0.2287

610.0004.0000.65390.2606

71.50018.5000.09811.2187

8-13.00011.000-0.85010.7288

9-4.0006.500-0.26160.4295

1012.50014.0000.81740.9189

113.50027.5000.22891.8112

12-9.00021.000-0.58851.3866

13-11.0002.500-0.71930.1680

145.00010.0000.32700.6575

15-3.50024.000-0.22891.5826

16-16.50017.500-1.07901.1582


1  Bold italics font style is used to indicate parameters, parameter values, and/or special requirements that are set in the APT GUI.



References

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




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