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Users can select a predefined number of dither steps and sizes for the wide field slitless spectroscopy (WFSS) mode of JWST NIRISS. Dithering is required for WFSS to mitigate the undersampling of the JWST point spread function by NIRISS at wavelengths < 2 μm.

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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.

The 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: 2, 3 ,4 ,6, 8, 12, and 16 steps. The first step corresponds to the initial pointing.
  • Dither step amplitude: 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 redshift. 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. Offsets of the SMALL dither patterns for the WFSS mode

# of DithersStep #

ΔX pixels

ΔY pixels

ΔX arcsec

ΔY arcsec

210.0000.0000.00000.0000
 23.5003.5000.22890.2303
3     
 10.0000.0000.0000.000
 24.3334.3330.28330.2851
 38.6678.6670.56680.5703
4     
 10.0000.0000.00000.0000
 25.0002.5000.32700.1645
 32.5006.0000.16350.3948
 4-2.5004.500-0.16350.2961
6     
 10.0000.0000.00000.0000
 2-2.0003.500-0.13080.2303
 3-4.6678.500-0.30520.5593
 42.6676.0000.17440.3948
 54.3332.0000.28340.1316
 66.667-2.5000.4360-0.1645
8     
 10.0000.0000.00000.0000
 25.0002.5000.32700.1645
 33.5007.0000.22890.4606
 4-1.5004.500-0.09810.2961
 5-2.500-1.000-0.1635-0.0658
 62.5001.5000.16350.0987
 71.0006.0000.06540.3948
 8-4.0003.500-0.26160.2303
12     
 

1

0.0000.0000.0000.000
 25.0002.5000.32700.1645
 33.5007.0000.22890.4606
 4-1.5005.500-0.09810.3619
 5-2.500-1.000-0.1635-0.0658
 62.0001.5000.13080.0987
 71.0006.0000.06540.3948
 8-4.5003.500-0.29430.2303
 9-2.0002.000-0.13080.1316
 102.5004.5000.16350.2961
 111.5009.0000.09810.5922
 12-4.0006.500-0.26160.4277
16     
 10.0000.0000.0000.000
 25.0002.5000.32700.1645
 34.5007.0000.29430.4606
 4-1.5005.500-0.09810.3619
 5-2.500-1.500-0.1635-0.0987
 62.500-1.0000.1635-0.0658
 71.0006.0000.06540.3948
 8-5.0003.500-0.32700.2303
 9-3.5001.0000.1635-0.0658
 103.5004.5000.22890.2961
 112.0009.5000.13080.6251
 12-4.0005.000-0.26160.3290
 13-6.0001.500-0.39240.0987
 14-1.0003.000-0.06540.1974
 15-0.5008.000-0.3270.5264
 16-6.5007.500-0.42510.4935

 

Table 2. Offsets of the MEDIUM dither patterns for the WFSS mode

# of DithersStep #

ΔX pixels

ΔY pixels

ΔX arcsec

ΔY arcsec

2     
 10.0000.0000.00000.0000
 26.5006.5000.42510.4277
3     
 10.0000.0000.00000.0000
 26.3336.3330.41420.4167
 312.66712.6670.82840.8284
4     
 10.0000.0000.00000.0000
 28.0004.5000.52320.2961
 33.50011.0000.22890.7238
 4-3.5007.500-0.22890.4935
6     
 10.0000.0000.00000.0000
 2-4.0007.500-0.26160.4935
 3-7.66714.500-0.50140.9541
 43.66711.0000.23980.7238
 57.3334.0000.47960.2632
 610.667-3.5000.6976-0.2303
8     
 10.0000.0000.00000.0000
 28.0004.5000.52320.2961
 33.50011.0000.22890.7238
 4-3.5007.500-0.22890.4935
 5-4.000-2.000-0.2616-0.1316
 64.0002.5000.26160.1645
 7-0.5009.000-0.03270.5922
 8-7.5005.500-0.49050.3619
12     
 10.0000.0000.00000.0000
 28.0004.5000.52320.2961
 33.50011.0000.22890.7238
 4-3.0006.500-0.19620.4277
 5-4.500-1.500-0.2943-0.0987
 64.0003.0000.26160.1974
 7-1.0008.500-0.06540.5593
 8-7.5005.000-0.49050.3290
 9-2.5002.000-0.16350.1316
 106.5007.5000.42510.4935
 111.00012.0000.06540.7896
 12-5.5009.5000-0.35970.6251
16     
 10.0000.0000.0000.000
 28.0003.5000.52320.2303
 33.50011.0000.22890.7238
 4-3.5007.500-0.22890.4935
 5-4.500-2.500-0.2943-0.1645
 65.0002.0000.32700.1316
 7-1.0009.5000-0.06540.6251
 8-7.5005.000-0.49050.3290
 9-2.0002.500-0.13080.1645
 106.5007.0000.42510.4606
 111.50013.5000.09810.8883
 12-6.00010.000-0.39240.6580
 13-5.0001.500-0.32700.0987
 143.0006.0000.19620.3948
 15-1.50012.000-0.09810.7896
 16-8.5008.500-0.55590.5593

 

Table 3. Offsets of the LARGE dither patterns for the WFSS mode

# of Dithers

Step #

ΔX pixels

ΔY pixels

ΔX arcsec

ΔY arcsec

2     
 10.0000.0000.00000.0000
 212.50012.5000.81750.8225
3     
 10.0000.0000.00000.0000
 212.33312.3330.80660.8115
 324.66724.6671.61321.6231
4     
 10.0000.0000.00000.0000
 216.0008.5001.04640.5593
 37.50022.0000.49051.4476
 4-7.50015.500-0.49051.0199
6     
 10.0000.0000.00000.0000
 2-8.00014.500-0.52320.9541
 3-14.66728.500-0.95921.8753
 46.66722.0000.43601.4476
 514.3338.0000.93740.5264
 620.667-7.5001.3516-0.4935
8     
 10.0000.0000.00000.0000
 216.0008.5001.04640.5593
 37.50022.0000.49051.4476
 4-7.50015.500-0.49051.0199
 5-8.000-4.000-0.5232-0.2632
 68.0004.5000.52320.2961
 7-0.50018.000-0.03271.1844
 8-15.50011.500-1.01370.7537
12     
 10.0000.0000.0000.000
 216.0008.5001.04640.5593
 37.50022.0000.49051.4476
 4-7.50015.500-0.49051.0199
 5-6.500-4.500-0.4251-0.2961
 610.0004.0000.65400.2632
 71.00018.5000.06541.2173
 8-13.50011.000-0.88290.7238
 9-4.0006.500-0.26160.4277
 1012.50014.0000.81750.9212
 113.00028.0000.19621.8424
 12-9.50021.500-062131.4147
16     
 10.0000.0000.0000.000
 216.0008.5001.04640.5593
 37.50022.0000.49051.4476
 4-7.50015.500-0.49051.0199
 5-6.500-3.500-0.4251-0.2303
 610.0004.0000.65400.2632
 71.50018.5000.09811.2173
 8-13.00011.000-0.85020.7238
 9-4.0006.500-0.26160.4277
 1012.50014.0000.81750.9212
 113.50027.5000.22891.8095
 12-9.00021.000-0.58861.3818
 13-11.0002.500-0.71940.1645
 145.00010.0000.32700.6580
 15-3.50024.000-0.22891.5792
 16-16.50017.500-1.07911.1515
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Related links

JWST User Documentation Home

JWST Proposing Considerations

JWST Dithering Overview

NIRISS Observing Modes

NIRISS Wide Field Slitless Spectroscopy

NIRISS Instrument

NIRISS GR150 Grisms
NIRISS Filters 

NIRISS Detector

JWST Detector MULTIACCUM Integration
NIRISS Detector Readout Patterns

Recommended Strategies

NIRISS WFSS Recommended Strategies

Exposure Time Calculator

JWST Exposure Time Calculator Overview

Astronomer's Proposal Tool

JWST Astronomers Proposal Tool Overview
NIRISS Wide Field Slitless Spectroscopy Template APT Guide

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References

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

JWST technical documents

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Last updated

Published July 11, 2017


 

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UpdatedJuly 11, 2017
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TeamNIRISS 
AuthorRavindranath, Goudfrooij, La Massa 
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