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The 11 patterns are listed in Table 10.1, together with applicable parameters, such as the allowed values for the number of steps in the pattern (Num_Pos), for the dither size and for the chop size. In addition, the figure number where the pattern is graphically shown is given in column 5 of Table 10.1. Offset sizes and number of steps in a pattern affect the amount of overhead time required to perform an observation (see Chapter 9). The effects of dithering or chopping on an astronomical image are shown in a set of examples in the next section.
Figure 10.3: Dither Patterns
Chop Patterns
The chop patterns are recommended for measuring the background adjacent to extended targets. For each chop pattern, half of the exposures are taken on the target (position 1). The names of the chop patterns are: ONE-CHOP, TWO-CHOP, FOUR-CHOP, and EIGHT-CHOP. The ONE-CHOP pattern produces one image of the target and one image of the background. the TWO-CHOP pattern produces one image (with two exposures) of the target and two background images, with the background fields positioned on opposite sides of the target. The two other patterns increase the number of target-background pairs to four and eight, respectively. A large number of background images may be required if they contain a large number of contaminating sources or if the background is highly structured. The four chop patterns are shown in the figure below:Figure 10.4: Chop Patterns
Combined Patterns
The combined patterns permit dithering interleaved with chops to measure the background. They are recommended for simultaneous mapping and background subtraction during observations of extended targets, beyond 1.6 microns. Three combined patterns are implemented: SPIRAL-DITH-CHOP, XSTRIP-DITH-CHOP, and YSTRIP-DITH-CHOP. Their characteristics are analogous to the dither patterns SPIRAL-DITH, XSTRIP-DITH, and YSTRIP-DITH, respectively, with the addition that each dither step is coupled with a background image obtained by chopping. The three combined patterns are shown in Figure 10.5.
Figure 10.5: Combined Patterns