Magnetic energy filter

A magnetic energy filter having plural magnetic fields and designed to deflect a trajectory of an electron beam extending from an entrance window to an exit slit, said trajectory being in a plane, said magnetic energy filter comprising:

• said magnetic fields being at least four in number and including first, second, third, and fourth magnetic fields, the trajectory of the electron beam passing through said at least four magnetic fields in the order first, second, third, and fourth;
• said plural magnetic fields defining an electron trajectory that has a rotational symmetry axis located midway between said second and third magnetic fields, said axis being perpendicular to said plane;
• and
• said second and third magnetic fields located on opposite sides of a line and being opposite in polarity, said line being in said plane and intersecting said rotational symmetry axis at a point.