In certain imaging applications, such as infrared search and track (IRST), for example, it is desirable for the imaging sensor to scan large fields of regard at a high rate and with diffraction limited performance. Approaches to achieving these goals include using back-scanned sensors, or line-scan imagers with large fields of view. In order to increase the integration time for a scanned two-dimensional (2-D) imaging sensor, the technique of back-scanning is often used to provide step/stare coverage. FIGS. 1A-1B illustrate this concept.
FIG. 1A shows the nominal case for a sensor system in which an afocal telescope 110 is configured to receive incoming electromagnetic radiation 120 and direct the radiation via imaging optics 130 to an imaging sensor 140, which is frequently a focal plane array (FPA). Back-scanning with a mirror 150 behind the afocal telescope 110 provides an agile method to increase the integration time for the FPA 140 by at least partially compensating for movement of the sensor system. Specifically, referring to FIG. 1B, as the system scans in object space, the back-scan mirror 150 tilts to attempt to keep the image fixed on the FPA 140 during the integration time. Thus, the larger sensor system, or a portion thereof, may be scanned at the nominal rate while the smaller back-scan mirror 150 provides the fast motions to implement the step/stare function. This is further illustrated in FIGS. 2A-2B.
Referring to FIG. 2A, there is illustrated a schematic representation of the back-scanned sensor's field of view (i.e., field of view of the sensor 140) within the field of view 210 of the afocal telescope 110. The sensor's field of view is scanned through the afocal telescope's field of view 210, as represented by arrow 220. Box 230a represents the sensor's field of view at a first or earlier point in time, with a corresponding target point position 235a, and box 230b represents the sensor's field of view at a second or later point in time, with a corresponding target point position 235b. The target point 235 effectively sweeps through the telescope's field of view 210 during the back-scan operation. FIG. 2B shows the corresponding back-scanned sensor field of view 230 in object space. In FIG. 2B, the afocal telescope's field of view 210 is scanning to the right, as represented by arrow 240, while the back-scan mirror 150 moves to keep the FPA's field of view 230 fixed during the integration time (also referred to as exposure time). Back scanning holds the sensor field of view 230 fixed in object space as the afocal telescope field of view 210 scans.