IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
spacing have no holes, even if there is only one dither per map point. All the patterns were constrained to
have no pair of dithers closer than three pixels in any run of four consecutive points. The cycling dither
table wraps around once the final (311th) element was reached. This pattern had a ½ sub-pixel sampling
pattern superposed on it, starting with point 1 and repeating continuously every four points (at point 311,
the final cycle was simply truncated early, thus patterns which wrap around the table missed a sub-pixe l
dither point). The five-point Gaussian pattern was a general use pattern suitable for shallow observations
where the exact sub-pixel sampling is unimportant. It had a ½ subpixel pattern, with the 5th point at subpixel
(¼,¼). Figure 3.1 shows the dither patterns at the default (large) scale. Figure 3.2 shows the cycling
dither patterns and the distribution of both the dithers and of the separation between dithers for each scale.
Table 3.1: Characteristics of the dither patterns.
Dither Pattern Scale Max dither
(pixels from (0,0))
Median dither
separation (pixels)
Sub-pixe l
dither pattern
Cycling Small 11 10.5 ½ pixel
Medium 119 53 ½ pixel
Large 161 97 ½ pixel
5-point random Small 26 23 ½ pixel
Medium 52 46 ½ pixel
Large 105 92 ½ pixe l
9-point random Small 16 14 1/3 pixel
Medium 34 28 1/3 pixel
Large 69 59 1/3 pixel
12-point Reuleaux Small 13 15 ½ pixel
Medium 27 30 ½ pixel
Large 55 59 ½ pixel
16-point spiral Small 16 12 ¼ pixel
Medium 32 23 ¼ pixel
Large 64 45 ¼ pixel
36-point Reuleaux Small 17 19 ¼ pixel
Medium 34 39 ¼ pixel
Large 67 78 ¼ pixel
Each of the IRAC dither patterns was 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
Operating Modes 28 Dithering Patterns