Controlled focusing and stigmating in the conventional and scanning transmission electron microscope

For linear transfer conditions and objects with isotropic statistics, both the sharpness of the point spread function and its departure from rotational symmetry show up in the shape of the autocorrelation peak of the electron micrograph. It is therefore suggested that this function be used for interactive or automatic focusing and stigmating of instruments with online digital image readout. Model computations suggest that this method works satisfactorily at reduced levels of exposure.