This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

=================================================================================

After the corrections of lower order aberrations, the corrector normally introduces or leaves high order aberrations. For instance, the Ronchigram with a 54 mR aperture indicated in Figure 3646 shows the left 4^th order coefficient of order 0.5 - 2.0 mm limiting the probe shape and size after 3^rd order aberration correction by Nion quadrupole-octupole corrector. The pattern is strongly 4-fold symmetry with weak 2-fold structure in the opposing corners. Note that this uniform 4-fold symmetry was actually imposed by the octupoles in the corrector, so called parasitic aberrations.

Figure 3646. Ronchigram obtained with 4^th order coefficients corrected to < 60 µm [1].