In STEM, there is only a condenser system, which is a lens used to form a fine probe. Sometimes this lens is also called objective lens. The correction of the spherical aberration (Cs) of the pre-field lens is much cheaper comparing to post-field corrections. The main advantage of such correction is to reduce the beam tails so that a fine beam can be positioned at a specified column of atoms and does not spread its intensity into neighboring columns significantly. This tail spilling is critical for high resolution Z-contrast (HAADF) imaging and EELS analysis.
As a result of second-order aberrations in the TEM-EELS system, the focusing is imperfect but these aberrations can be corrected by curving the polepiece edges, allowing an energy resolution better than 1 eV for an entrance angle of up to several milliradians (of the order of 0.3 º). Furthermore, by curving the polepieces of the magnetic prism and by using weak multipole lenses for fine tuning, these aberrations can be almost eliminated.
By using a two-dimensional CCD array as the detector, Gatan imaging filters (GIFs) used both quadrupole and sextupole lenses to correct spectrometer aberrations.