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Chopping and Dithering Strategies

The most efficient strategy for removing the background from a science exposure strongly depends on the nature of the target and of the science to be accomplished. In general two types of targets can be defined: compact and extended.

Compact Objects

For compact objects, such as point sources, background subtraction can be achieved by moving the target across the camera field of view (see Figure 10.1). A dither pattern, which involves movements of a few arcsecs from one exposure to the next, can then be used. This is an efficient way to build background images, since the target is present in each exposure, and a background image can be created from the stacking and filtering of all exposures.

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.

Figure 10.2: Chopping

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