NIRISS AMI Dithers
Dithering is available for the AMI mode, with the 5-point subpixel dither pattern recommended.
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AMI dithering considerations
See also: NIRISS Aperture Masking Interferometry, NIRISS AMI Template APT Guide
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5-point subpixel dithering is strongly recommended for AMI observations in order to mitigate pixel-level effects and improve the effective sampling (Charles et al. 2026). To calculate exposure times, AMI community advice suggests evenly splitting the desired number of photoelectrons for a given binary point source contrast across the 5 sub-pixel dithered exposures (e. g., Desdoigts. et al. 2026).
Primary dithers
Table 1. Offsets for primary AMI dithers
Primary Dithers | ΔX (arcsec) | ΔY (arcsec) |
|---|---|---|
| 1 | 0.0000 | 0.0000 |
| 2 | -1.8309 | 1.1271 |
| 3 | -1.8307 | -1.0428 |
| 4 | 0.9155 | -1.1224 |
Subpixel dithers
Table 2. Offsets when not using subpixel dithers with AMI
| Subpixel Positions = NONE | ||
|---|---|---|
| Dither Step # | ΔX (arcsec) | ΔY (arcsec) |
| 1 | 0.0000 | 0.0000 |
Table 3. Offsets for a 5-step subpixel dither pattern with AMI
| Subpixel Positions = 5 | ||
|---|---|---|
| Dither Step # | ΔX (arcsec) | ΔY (arcsec) |
| 1 | 0.0000 | 0.0000 |
| 2 | 0.01633 | 0.01636 |
| 3 | -0.01633 | 0.01650 |
| 4 | -0.01633 | -0.01636 |
| 5 | 0.01633 | -0.01650 |
Table 4. Offsets for a 9-step subpixel dither pattern with AMI
| Subpixel Positions = 9 | ||
|---|---|---|
| Dither Step # | ΔX (arcsec) | ΔY (arcsec) |
| 1 | 0.0000 | 0.0000 |
| 2 | 0.0216 | 0.0000 |
| 3 | 0.0216 | 0.0216 |
| 4 | 0.0000 | 0.0217 |
| 5 | -0.0216 | 0.0218 |
| 6 | -0.0216 | 0.0001 |
| 7 | -0.0216 | -0.0216 |
| 8 | 0.0000 | -0.0217 |
| 9 | 0.0216 | -0.0218 |
Table 5. Offsets for a 25-step subpixel dither pattern with AMI
| Subpixel Positions = 25 | ||
|---|---|---|
| Dither Step # | ΔX (arcsec) | ΔY (arcsec) |
| 1 | 0.0000 | 0.0000 |
| 2 | 0.0131 | -0.0001 |
| 3 | 0.0262 | -0.0001 |
| 4 | 0.0262 | 0.0130 |
| 5 | 0.0262 | 0.0262 |
| 6 | 0.0131 | 0.0262 |
| 7 | 0.0000 | 0.0263 |
| 8 | -0.0131 | 0.0264 |
| 9 | -0.0262 | 0.0264 |
| 10 | -0.0262 | 0.0133 |
| 11 | -0.0262 | 0.0000 |
| 12 | -0.0262 | -0.0130 |
| 13 | -0.0262 | -0.0262 |
| 14 | -0.0131 | -0.0262 |
| 15 | 0.0000 | -0.0263 |
| 16 | 0.0131 | -0.0264 |
| 17 | 0.0262 | -0.0264 |
| 18 | 0.0262 | -0.0133 |
| 19 | 0.0131 | -0.0132 |
| 20 | 0.0000 | -0.0132 |
| 21 | -0.0131 | -0.0131 |
| 22 | -0.0131 | 0.0001 |
| 23 | -0.0131 | 0.0132 |
| 24 | 0.0000 | 0.0132 |
| 25 | 0.0131 | 0.0131 |
Dither patterns for direct imaging with AMI mode
In addition to the main AMI dither patterns mentioned above, which are for observations with the non-redundant mask (NRM), the AMI mode also offers 3 additional dither patterns for optional direct imaging to mitigate effects of bad pixels, intra-pixel sensitivity variations, persistence, and flat field errors. The spatial offsets of these dither patterns with respect to the first pointing (Primary Dithers = NONE) are tabulated below.
Table 6. Offsets for dithers in AMI direct imaging mode
| Pattern Name | Step # | ΔX (pixels) | ΔY (pixels) | ΔX (arcsec) | ΔY (arcsec) |
|---|---|---|---|---|---|
| Image Dithers = NONE | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| Image Dithers = 2 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 4.500 | 4.500 | 0.2942 | 0.2945 | |
| Image Dithers = 3 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 4.333 | 4.333 | 0.2831 | 0.2833 | |
| 3 | 8.667 | 8.667 | 0.5668 | 0.5673 | |
| Image Dithers = 4 | |||||
| 1 | 0.000 | 0.000 | 0.0000 | 0.0000 | |
| 2 | 6.000 | 2.500 | 0.3923 | 0.1624 | |
| 3 | 2.500 | 7.000 | 0.1634 | 0.4595 | |
| 4 | -3.500 | 4.500 | -0.2289 | 0.2971 |
References
Charles M, Desdoigts L, Pope B, et al. 2026, PASA, 43, 48C
Image reconstruction with the JWST interferometer
Desdoigts L, Pope B, Charles M, et al. PASA. Published online 2026:1-37. Doi:10.1017/pasa.2026.10194
Amigo: a Data-Driven Calibration of the JWST Interferometer.