Abstract
Bright field tilted beam illumination is commonly used to demonstrate the resolution of a transmission electron microscope by forming lattice fringes.The unscattered and diffracted beams are symmetrically positioned on either side of the optic axis so that the spherical, defocus and chromatic abberations will cancel. The cancellation of chromatic aberration is particularly important. However, the balancing of aberration occurs not only for the one diffraction direction across the axis from the unscattered beam but for an entire hollow cone of scattering vectors centered on the microscope axis and passing through the unscattered beam. This hollow cone projects as a circle in the back focal plane (BFP) of the microscope with zero net aberrations for one sideband of the scattering. Because the image amplitude is squared to form the image intensity, the single ring of cancelled aberrations in the BFP corresponds to a symmetrical pair of rings in the spatial power spectra of the micrograph. These ring pairs show up strongly in the experimental diffractograms, Figures 1, 3 and 4.
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