The electron wavefunction in the front focal plane is a function of scattering semi-angle α, given by,
χ -- The phase shift induced by objective lens.
For an ideal lens, χ(α) is equal to 0.
Figure 2773 shows the full view of of the practical double-sextupole Cs correctors for STEM and for TEM, respectively. The positions of the correctors with respect to the front and back focal planes for STEM and TEM, respectively, are also indicated.
Figure 2773. Double-sextupole spherical aberration correctors for: (a) STEM and (b) TEM.