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. [1] 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.
[1] 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).