Dithering is available for the AMI mode, with the 5-point subpixel dither pattern recommended.

AMI dithering considerations

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

The dither patterns for the AMI mode consist of "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 strongly recommended for AMI observations. This subpixel dither pattern enables adequate pixel phase coverage using the drizzling technique developed for Hubble data, and has shown marked improvement in AMI’s binary point source contrast. In terms of exposure time calculations for subpixel dithered exposures, STScI's NIRISS team suggests  splitting up the desired number of photoelectrons for a given binary point source contrast evenly between the 5 subpixel dithered exposures.

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Primary dithers

Table 1. Offsets for primary AMI dithers

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Subpixel dithers

Table 2. Offsets when not using subpixel dithers with AMI

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

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

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

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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

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