This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

=================================================================================

In many TEM operations, the field of view changes when the specimen is tilted as shown in Figure 2946 (a), so that we often need to re-center the area of interest using the specimen traverse controls. In the figure, the light-orange solid circle represents the tilt axis (vertical to the plane of the paper), while the small blue rectangular area represents the interesting area in the specimen. It can be seen that the area of interest translates a distance (x) when the samples is tilted at angle α. This area change causes especially inconveniency when specimen tilting is applied during observation of diffraction pattern, because the diffraction pattern comes from the continually changing area of the specimen. The black solid and dash lines represent the TEM specimen before and after tilt operation, respectively, and α indicates the tilting angle. In order to avoid the movement of field of view, this area must be positioned at the point defined by the intersection of the optic axis and tilt axis, which is called axis-centred tilting, as shown in Figure 2946 (b). In optimized conditions, this intersection should be located into eucentric plane.

Figure 2946. (a) Specimen traverse induced by specimen tilting when there is a mismatch between optic axis and tilt axis, and (b) No specimen traverse induced by specimen tilting when there is no mismatch between optic axis and tilt axis.

Note that in the case of Figure 2946 (a), some TEM systems can still achieve axis-centred tilting by computer controls, which adjusts all the five axes, including three translations (x, y, and z translations) and two tilts ( x and y tilts). Such stages are often called double-eucentric stages.  In this setup, the specimen height and translations are re-adjusted corresponding to the tilts (from -60° to +60°).

Note that for TEMs with top-entry stages, it is usually noneucentric, i.e. the specimen translates out of the selected area if it is tilted.