NIRISS AMI Dithers

Dithering is available, though not required, for the AMI mode.

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AMI dithering considerations

See also: NIRISS Aperture Masking Interferometry, NIRISS AMI Template APT Guide

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The dither patterns for the AMI mode are implemented as "primary" dithers that perform ~30 pixel offsets (with up to 4 positions within the 80 × 80 pixels science subarray) in conjunction with "secondary" dithers that are sub-pixel offsets (0.20–0.33 pixels) designed to obtain adequate pixel phase coverage. The offsets for the various dither step sizes and patterns are listed in Tables 1–5. Note that the dither offsets on this page are in the "Ideal" coordinate system


Five-point SUBPIXEL dithering is recommended for AMI observations. This dither pattern is designed to enable subsampling using the drizzling technique developed for Hubble data taken with pixel shifts that are wider than the Nyquist sampling limit. This subpixel sampling dither pattern should benefit shorter wavelength AMI filters.


Primary dithers


Table 1. Offsets for primary AMI dithers

Primary

Dithers

ΔX (arcsec)ΔY (arcsec)
10.00000.0000
2-1.83091.1271
3-1.8307-1.0428
40.9155-1.1224



Subpixel dithers


Table 2. Offsets when not using subpixel dithers with AMI

  Subpixel Positions = NONE
Dither Step #ΔX (arcsec)ΔY (arcsec)
10.00000.0000


Table 3. Offsets for a 5-step subpixel dither pattern with AMI

  Subpixel Positions = 5
Dither Step #ΔX (arcsec)ΔY (arcsec)
10.00000.0000
20.016330.01636
3-0.016330.01650
4-0.01633-0.01636
50.01633-0.01650


Table 4. Offsets for a 9-step subpixel dither pattern with AMI

  Subpixel Positions = 9
Dither Step #ΔX (arcsec)ΔY (arcsec)
10.00000.0000
20.02160.0000
30.02160.0216
40.00000.0217
5-0.02160.0218
6-0.02160.0001
7-0.0216-0.0216
80.0000-0.0217
90.0216-0.0218


Table 5. Offsets for a 25-step subpixel dither pattern with AMI

Subpixel Positions = 25
Dither Step #ΔX (arcsec)ΔY (arcsec)
10.00000.0000
20.0131-0.0001
30.0262-0.0001
40.02620.0130
50.02620.0262
60.01310.0262
70.00000.0263
8-0.01310.0264
9-0.02620.0264
10-0.02620.0133
11-0.02620.0000
12-0.0262-0.0130
13-0.0262-0.0262
14-0.0131-0.0262
150.0000-0.0263
160.0131-0.0264
170.0262-0.0264
180.0262-0.0133
190.0131-0.0132
200.0000-0.0132
21-0.0131-0.0131
22-0.01310.0001
23-0.01310.0132
240.00000.0132
250.01310.0131



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 NameStep #ΔX (pixels)ΔY (pixels)

ΔX (arcsec)

ΔY (arcsec)
Image Dithers = NONE





10.0000.0000.00000.0000
Image Dithers = 2





10.0000.0000.00000.0000

24.5004.5000.29420.2945
Image Dithers = 3





10.0000.0000.00000.0000

24.3334.3330.28310.2833

38.6678.6670.56680.5673
Image Dithers = 4





10.0000.0000.00000.0000

26.0002.5000.39230.1624

32.5007.0000.16340.4595

4-3.5004.500-0.22890.2971




Latest updates
  •  
    Updated AMI 5-step subpixel dither values for APT 2023.5 (release on Aug 24) and recommendation to use the 5-step sub-pixel dither pattern.

  •  
    Added (new) Table 5 and renamed previous Table 5 to Table 6. Also removed mention of native coordinate system.

  •  
    Table 5 updated to show all dither patterns available for direct imaging with AMI mode. Also updated the heading and text above table 5.
Originally published