Dither Tables
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 lowerweighted inner region to reduce the numbers of small separation dithers, and the 4point subarray pattern where the scaling is 4:3:1.5. For all the patterns, the subpixel 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 subpixel 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 subpixel dither point).
The Gaussian dither pattern has a 1/2 sub pixel pattern, with the 5th point at subpixel (1/4,1/4). The 9point and 16point patterns were designed to be the optimum size for 1/3 and 1/4 subpixel 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 selfcalibration. The 12point and 36point Reuleaux dither patterns have a 1/2 and 1/4 subpixel sampling and sample spatial frequencies in a fairly uniform manner. The subarray 4point dither pattern has a 1/2 subpixel sampling and the subarray 9point dither pattern has 1/3 subpixel 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 +xaxis and +w is in the direction of BCD frame yaxis):
Full array:
Subarray:
