Patent ID: 8274046

Claim:
A monochromator, comprising: a first dispersion unit and a second dispersion unit aligned with a straight optical axis configured to deflect a charged particle beam with a normal energy and an energy spread, the charged particle beam propagating along the optical axis and including charged particles with the normal energy passing straight through said each dispersion unit and charged particles with energy deviations from the normal energy being deflected by said each dispersion unit in a same direction as a dispersion direction, wherein a deflection angle of each charged particle generated by each dispersion unit depends on a dispersion power thereof and energy deviation of each charged particle, and wherein the dispersion powers and the dispersion directions of the first and second dispersion units are respectively equal; a plate with an energy-limit aperture at a central plane between the first and second dispersion units, wherein the energy-limit aperture is aligned with the straight optical axis; and a beam adjustment element fastened to focus the charged particle beam before the charged particle beam enters into the first dispersion unit so as to form a real crossover of the charged particle beam at the energy-limit aperture, wherein a virtual crossover is formed between the first dispersion unit and the energy-limit aperture after the charged particle beam exits from the second dispersion unit.