 The energy dispersion is carried out for a narrow range of energies around the so-called pass energy in the direction perpendicular to the slit, so that the angular dispersion of the cylindrical lens is only preserved along the slit (typically ±15° over a 25 mm long, 0.1 mm wide slit).The hemispheres of the energy analyzer are kept at constant voltages so that the central trajectory is followed by those electrons that have the kinetic energy equal to the set pass energy; those with higher or lower energy end up closer to the outer or the inner hemisphere at the other end of the analyzer.This is where an electron detector is mounted, usually in the form of a 40 mm microchannel plate paired with a fluorescent screen.