The Nion Aberration Corrector consists of seven elements – four quadrupoles (red) and three octupoles (blue) – placed between the two condenser lenses (bottom) and the objective lens (top) of the VG Microscopes HB501 STEM.  The quadrupoles shape the beam into pencil cross sections inside the octupoles. The octupoles then provide correction for spherical aberration in the objective lens. Summing the octupole strengths (on the left) shows that there is a net +1 strength in both the x-z and y-z planes. The corrector is installed into the volume previously occupied by scan coils, under the objective lens. Electrical and mechanical stabilities of 0.2 ppm are crucial to the successful operation of this device. The image on the right shows the corrector hardware mounted underneath the microscope objective lens.
 N. Dellby, O.L. Krivanek, P.D. Nellist, P.E. Batson, and A.R. Lupini, Progress in Aberration-corrected Scanning Transmission Electron Microscopy, J. Electron Microscopy 50, 177-185 (2001).