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.
For compact objects, such as point sources, background subtraction can
be achieved by moving the target across the camera field of view (see Figure D.1
). A dither pattern, which involves movements of a few arcsec 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.
For an extended object, which occupies a significant portion of the
NICMOS field of view (star fields, nebulae, galaxies), the dithering technique cannot be applied to build background images. In this case, offsets to an adjacent field (chopping) chosen to be at least one camera field away in an arbitrary (or user specified) direction, are necessary. By offsetting in different directions, a stacked and filtered sky image can be created which removes the effect of contaminating objects in the offset fields (see Figure D.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 has the ability to specify the offset value, directions, and the number of offsets in the Phase II pattern parameter specification.