A. image space  stitching (internal  stitching) refers to the image stitching using multiple detectors, which can be done directly, or by using optical elements. In both ways, inner stitching arranges the detectors in a certain order to form a whole receiving target plane. Some have proposed stitching by reflective prismatic decomposition, dividing the image plane into four parts which respectively belong to four detectors; it requires no moving components and thus maintains a compact structure. However, it necessitates a relatively long back focal length (BFL) to allow for the placement of the reflecting prism. Moreover, the energy efficiency would be low, and the lateral dimension of the system would be large. Within the infrared band, the focal planes of cooled detectors are enclosed in the Dewar flask, therefore, the direct stitching of the focal planes is inappropriate, and considering the lengths of the detectors, stitching by reflective prismatic decomposition or time-share pointing would require a larger BFL thus a longer overall structure. B. Object stitching (Outer stitching) is the stitching of the observation area in the object space. It involves the placement of multiple cameras in certain relative angles and positions to obtain an image of a large field of view. Each camera lens is responsive to a detector, for instance, “horizontal 1”, “vertical 1”, crisscross pattern, and “T” type. Some theses have put forward the crisscross pattern stitching of four surveying cameras to achieve the measurement of a large field of view. This is a simple arrangement, yet it requires cameras with the same optical parameters, and the optical parallax is inevitable, which brings about inaccuracy of FOV stitching. The cooled infrared detector is quite expensive, thus increasing the processing cost.