The resolution of electron microscopes (EMs) is partially limited by: i) The electrical stability of the EM systems, e.g. the stabilities of the high voltage and the lens currents; ii) External disturbances e.g. mechanical vibration, contamination, charging, fluctuation of stray magnetic fields, and the nonuniform magnetic properties of the pole-piece material used.
Note that we can consider mechanical and electrical instabilities as a kind of incoherent aberration.
The instabilities of the accelerating voltage and/or lens currents in the EMs could be contributory factors to the chromatic aberration (Cs), but the two are now negligible because of the high electrical stability of modern power supplies. For TEM systems, stability of 10 ppm or better are usually needed.
Electrical stability is normally ensured by the double feed-back circuitry:
i) A slow loop to control drift.
ii) A fast loop for ripple.