Each of the IRAC dither patterns is available in three sizes, large (default), medium, and small. For most of the patterns, the scaling of the large, medium, and small patterns is approximately in the ratio 4:2:1. Exceptions are the small cycling pattern, which is about 1/5 of the size of the large cycling pattern and has a lower-weighted inner region to reduce the numbers of small separation dithers, and the 4-point subarray pattern where the scaling is 4:3:1.5. For all the patterns, the sub-pixel dithering is maintained, independent of scale. Note that the cycling dither table will wrap around once the final (311th) element is reached. For example, selecting a starting point of 310 and three dithers per map, the first grid position will use points 310, 311 and 1 from the table, the second 2, 3 and 4 and so on. This pattern has a 1/2 sub-pixel sampling pattern superposed on it, starting with point 1 and repeating continuously every four points (at point 311, the final cycle is simply truncated early, thus patterns which wrap around the table will miss a sub-pixel dither point).

The Gaussian dither pattern has a 1/2 sub- pixel pattern, with the 5th point at sub-pixel (1/4,1/4). The 9-point and 16-point patterns were designed to be the optimum size for 1/3 and 1/4 sub-pixel dithering, respectively. The "random 9" pattern is based on a uniform random distribution. The "spiral 16" pattern was designed to provide a pattern which is both compact and has a good figure of merit for self-calibration. The 12-point and 36-point Reuleaux dither patterns have a 1/2 and 1/4 sub-pixel sampling and sample spatial frequencies in a fairly uniform manner. The subarray 4-point dither pattern has a 1/2 sub-pixel sampling and the subarray 9-point dither pattern has 1/3 sub-pixel sampling.

The current IRAC dither patterns are described in general terms in the IRAC Instrument Handbook.

The dither tables are given below. The offsets are in pixels, in (v,w) coordinates (where +v is in the direction of the BCD frame +x-axis and +w is in the direction of BCD frame -y-axis):

Full array:

• 5-point gaussian, small scale
• 5-point gaussian, medium scale
• 5-point gaussian, large scale

• 9-point random, small scale
• 9-point random, medium
• 9-point random, large

• 12-point Reuleaux, small
• 12-point Reuleaux, medium
• 12-point Reuleaux, large

• 16-point spiral, small
• 16-point spiral, medium
• 16-point spiral, large

• 36-point Reuleaux, small
• 36-point Reuleaux, medium
• 36-point Reuleaux, large

• 311-cycling, small
• 311-cycling, medium
• 311-cycling, large

Sub-array:

• 4-point random, small
• 4-point random, medium
• 4-point random, large

• 9-point Reuleaux, small
• 9-point Reuleaux, medium
• 9-point Reuleaux, large