Single images can encompass only the area seen at lowest magnification. Additionally, images with the SE and VP detectors tend to be unevenly bright at low magnification, brighter close to the detector and dimmer farther away. Also, the CL system can't make low-magnification images because of its light-collecting mirror. To image larger areas, there is a special program on the SEM computer called Smartstitch. Basically, you define a rectangular area by setting the upper-left and lower-right corners of the area you want a mosaic of, set the magnification, and press go. The mosaic is made of many small, partly overlapping images, typically at relatively high magnification, which are then "stiched" together to make one large, high-resolution mosaic image.
This is one of several images used to make the composite shown below. It was made with the VP detector at a magnification of 45x.
This is the full composite image. This is a grain mount with several materials that undergo cathodoluminescence. The variable brightness of the grains is actually from light emission. The stitching software attempts to adjust the brightness and position of the boundaries of individual images so they don't show on the composite.