The Eucentric Position
The eucentric position is the horizontal center of the objective lens. The sample must be set to this position. To do this the entire sample holder is raised or lowered.
TEMs are set up so that magnification, camera length, and correct focus are set to this reference position. When the sample height (in the Z-direction) is set at the eucentric position, one can tilt the sample around its axis without the image of the sample moving across the projection screen.
The eucentric height can be set by adjusting the objective lens current to a specific known setting for a specific voltage (e.g. 80kV, 100kV or 200kV), and then raising or lowering the Z-height until the image is in focus. The "wobbler" button can be used as a focus aid where one brings the vibrating split image together into one static image.
Alternatively, if the objective lens current is not known for the correct sample height, the sample Z-height is raised or lowered until, when tilted, the axis of the sample remains centered.
This is how the beam path looks when set up for selected area diffraction (SAD) and the sample height is incorrectly set (left) and correctly set (right). Observe what happens to the magnification and focus.