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Figure 10.1: Dithering.
Extended Objects
For an extended object which occupies a significant portion of the NICMOS field of view, the dithering technique does not apply to building background images. In this case, offsets to an adjacent field (chopping), chosen to be at least one camera field away in an arbitrary direction, are necessary. By offsetting in different directions a stacked and filtered sky can be created which removes the effect of contaminating objects in the offset fields (see Figure 10.2). As in the case of compact objects, these offsets might be quite small, but for large galaxies for example, they may need to be over considerable distances. The user will have the ability to specify the offset values and directions and their number in the Phase II instructions.