The representative raster field of view was presented in Chap. 9. Sometimes this raster does not cover the entire area that is to be covered. The strip mapper was also discussed; this mapper can be generated by either a whiskbroom or a pushbroom scanner. The instantaneous field of view is the field that is covered by the optical system at any instant. It can have a single detector, a linear array, or a two-dimensional array as the defining field stop. The resolution element, reselm, or picture element, pixel, is the field subtended by a single detector element. The field of regard is the entire field that is covered by the system; it can be larger than the instantaneous field of view. It is also possible for the field of regard to be the same as the instantaneous field, or all three field can be identical. In trackers, the three fields are often identical. In staring systems with two-dimensional arrays, the instantaneous field and the regard field are identical, but there are many reselms in it. When a single detector is used to cover a raster, the reselm is the same as the instantaneous field, but the field of regard is the full frame. The instantaneous field and field of regard are usually specified in degrees, while the reselms are given as milliradians or microradians. These are illustrated in Fig. 12-1.
In this section various ways to scan the reselms or the instantaneous field over the field of regard are discussed. There is, as always, a tradeoff. Staring systems require very good, wide-field optics, and it has already been shown how much more difficult this is than very good, narrow-field optics. However, the scanners all require moving parts that are less reliable and require more space and more power. In some cases the starers also require choppers.
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