Alignment of the electron microscope, step 3. Alignment of the electron microscope, result of step 3 correction. The alignment procedure must result in the axes of all the image forming lenses being in line and on the mechanical axis of the instrument . Once all the imaging lenses are in line, an object point lying on the axis of the objective lens will be imaged at the center of the viewing screen whatever the magnification setting, although the image and object may be rotated with respect to each other. Fluctuation in the high voltage results in sharp definition of the image only near an axis or voltage center .
Image quality is affected least if the voltage center is in the center of the field of view.
Ideally the voltage and current centers should coincide , but in practice they do not do so exactly because axial asymmetries are hard to eliminate during manufacture of the lenses. Thus, the microscope has to be aligned with respect to either high-tension or lens current fluctuations. Since, in most microscopes, the voltage fluctuations are greater than the current fluctuations, it is desirable to align the voltage center. However, most microscopes do not allow for small, continuous changes in the accelerating voltage whereas the lens currents can be varied in a continuous fashion. Lens alignment in older model microscopes such as the Philips EM300, consists of physical translation of the lenses so that either the voltage or current center coincides with the physical microscope axis.