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Chopping and Dithering Patterns

 There are a set of 11 pre-designed patterns available for NICMOS observations. They include: 4 dither patterns, 4 chop patterns, and 3 dither-chop patterns. For each of these, the observer will be able to specify, during the Phase II Proposal submission, the total number of positions desired (2 to 40), the dither size (0 to 40 arcsecs), the chop size (0 to 1800 arcsecs), and the orientation of the pattern on the sky. The option POS-TARG will be still available for offsetting the telescope and creating custom-design patterns, but there are a number of advantages in using the pre-designed patterns:

Dither Patterns

 The dither patterns are recommended for the background subtraction from observations of point sources (beyond 1.6 microns), for the construction of mosaics (maps) of regions of the sky, and for the reduction of flat field uncertainties. The four dither patterns are called SPIRAL-DITH, SQUARE-WAVE-DITH, XSTRIP-DITH, and YSTRIP-DITH. Most of the names are self-explanatory: the SPIRAL-DITH pattern produces a spiral around the first pointing; the SQUARE-WAVE-DITH pattern covers extended regions by moving along a square-wave shape; the XSTRIP-DITH and the YSTRIP-DITH patterns map a strip of the sky along the x and y directions of the detector (as defined in Figure 4.23). The difference between the XSTRIP-DITH and the YSTRIP-DITH patterns is that the first moves by default along the grism dispersion axis, while the second moves orthogonal to the grism dispersion axis. The four patterns are illustrated below:

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


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