The stability of the electron beam current and position are of primary importance in EMs, especially in TEMs and high-voltage STEMs. The problems, related to the instability of the beam current, originate mainly from:
        i) Instability of objective lens current;
        ii) Instability of condenser lens current;
        iii) Variation of the high voltage of the gun;
        iv) Variation of the bias voltage in the gun;
        v) Variation of the filament current or gun emission over time, causing a small energy spread;
        vi) Variation of the aperture positions;
        vii) Instability of the high vacuum in the electron gun.

The problems, related to the instability of the beam position, originate mainly from:
        i) Instability of objective lens current, causing focus variation with time;
        ii) Instability of condenser lens current;
        iii) Variation of the accelerating voltage, causing a small energy spread;
        iv) Variation of the position of the filament or gun emission.