This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
Figure 3602a shows a Ronchigram pattern taken at Gaussian focus from an amorphous thin film in TEM. The green circle marks the unaberrated part of Ronchigram that should be selected by objective aperture for STEM imaging.
Figure 3602a. Ronchigram pattern taken at Gaussian focus.
Figure 3602b shows typical Ronchigrams taken at the edge of an amorphous carbon film. At defoci (defined by z-height), there is a distance between the electron cross-over and the point on the specimen along the optic axis. At large underfocus, electron rays at all angles cross the optic axis after the specimen and it shows a shadow image of the specimen edge. At small underfocus, low-angle rays cross the optic axis after the specimen, while high-angle rays cross before the specimen due to spherical aberration. Therefore, the shadow image changes in magnification as a function of the angle. The low-angle asymmetry indicates the presence of astigmatism. At Gaussian focus, the lowest-angle rays cross the axis at the specimen, while higher-angle rays cross before the specimen due to the spherical aberration. The coma free axis is defined at this focus and all alignment and positioning of detectors and apertures can be performed with respect to the low-angle “disk”. Defocus and spherical aberration can effectively cancel each other at those lowest angles. Axial astigmatism can be accurately corrected by using the stigmator coils, resulting in circularly symmetric Ronchigram features. At overfocus, rays at all angles cross the axis before the specimen.
Figure 3602b. Ronchigrams of a thin amorphous carbon (C) film at: (a) Large underfocus, (b) Small underfocus, (c) Gaussian focus, and (d) Overfocus.
The Ronchigrams of a thin <110> silicon (Si) film in Figure 3602c shows the diffraction effects and fringes arising from the specimen periodicities. The visibility of the characteristic fringes depends on the precision of specimen tilt and the degree of probe coherence in a specific crystalline orientation. Figure 3602c (a) shows the Ronchigram at small underfocus. The lattice fringes are visible near the Ronchigram center and become extremely distorted at high angles because of the spherical aberration. Figure 3602c (b) shows the Ronchigram near Scherzer focus. The central fringes are large and wide. Figure 3602c (c) shows the Ronchigram at slight overfocus. The fringe spacing decreases with increasing angle from the Ronchigram center.
Figure 3602c. The Ronchigrams of a thin <110> Si film: (a) At small underfocus, (b) Near Scherzer focus, and (c) At slight overfocus.
 E.M. James, N.D. Browning, Practical aspects of atomic resolution imaging and analysis in STEM, Ultramicroscopy 78 (1999) 125-139.