In round beam electron optics, spherical aberration is ordinarily the largest of the beam aberrations. While aberration corrected TEMs and STEMs are available, the deployed fleet of electron microscopes largely lacks this capability, since the correctors (multi-pole lenses) require major changes to the microscope.
Two investigations, both undertaken in collaboration with the Ernst-Ruska Center for Microscopy and Spectroscopy with Electrons, attack spherical aberration correction in electron round-beam optics, taking minimum disturbance to the existing optics as a design constraint. One employs an off-axis holographic corrector, which introduces spherical aberration counter to the on-axis term. While achieving good cancellation is a apparently a bit delicate, the authors show Si  resolved to 1.36Å.
The second investigation makes good on the long-sought, on-axis foil corrector. This they have sculpted in a silicon nitride membrane by focussed ion-beam milling. Advantageously, the corrector shape depends only on the spherical aberration of the optics and not on the beam voltage. Inevitably, there is a small amount of noise introduced by inelastic scattering in the membrane, but this is quite small.
- V Grillo, et al., “Towards a holographic approach to spherical aberration correction in scanning transmission electron microscopy”, arXiv:1705.04903
- R Shiloh, et al., “Spherical aberration correction in a scanning transmission electron microscope using a sculpted foil”, arXiv:1705.05232