We will now rethink what we mean by a TEM, in a way that is more suitable for HRTEM, where the purpose is to maximize the useful detail in the image. (Note the word
here.) You should think of the microscope as an optical device that transfers information from the specimen to the image. The optics consists of a series of lenses and apertures aligned along the optic (symmetry) axis. What we would like to do is to transfer
the information from the specimen to the image, a process known as mapping. There are two problems to overcome and we can never be completely successful in transferring
the information. First, as you know from Chapter 6, the lens system is not perfect so the image is distorted and you lose some data because the lens has a finite size (Abbe’s theory). The second problem is we have to interpret the image using an atomistic model for the material.