NIRISS Imaging Dithers

The JWST NIRISS imaging mode has a predefined number of dither steps and sizes. Dithering is required for NIRISS imaging for various reasons (see below). When used as a coordinated parallel mode, NIRISS imaging will follow the dither pattern of the prime observing mode.

See also: NIRISS Imaging, NIRISS Imaging APT TemplateJWST Parallel Observations, APT Coordinated Parallel Observations

NIRISS imaging is severely affected by undersampling at λ ≤ 4 μm, requiring dithers to improve sampling of the PSF. Dithering can also mitigate the impact of ghosts on the final combined image, flat-fielding errors, and detector artifacts like bad pixels and occulting spots. The LARGE dither pattern works best to minimize the impacts of ghosts on the final combined spectral product.

The selection of dither patterns 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. A minimum of 4 dither steps is recommended.
  • Dither step amplitude (Pattern Size): SMALL (~0.3"), MEDIUM ( ~0.6"), or LARGE (~1.2").  Note that the average pixel scale for NIRISS is 0.066"/pixel.

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 and imaging modes) 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 dither step sizes for each pattern are the same for the WFSS and and imaging modes.

When NIRISS imaging is used as a coordinated parallel observing mode, the prime instrument's template will allow the user to select customized dither patterns that work well for both the prime and the parallel instrument modes. 

Table 1. Imaging 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. Imaging 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. Imaging 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




Notable updates
  •  
    Updated to support NIRISS imaging as a prime mode in Cycle 2
Originally published