Monochromator and radiation source with monochromator

A monochromator for charged particle optics or for electron microscopy, the charged particles emanating from a radiation source disposed upstream of the monochromator, the radiation source having an image into which the charged particles virtually enter at a first angle with respect to an optical axis in a first plane and at a second angle with respect to the optical axis in a second plane, the monochromator comprising:

• a selection aperture having a selection aperture plane;
• at least one first deflection element disposed upstream of said selection aperture and having first electrodes generating a first electrostatic deflecting field, said first deflecting field producing a dispersion of the charged particles in the selection aperture plane for selecting charge particles of a desired energy interval;
• and
• at least one second deflection element disposed downstream of said selection aperture and having second electrodes generating a second electrostatic deflection field which eliminates the dispersion of said at least one first deflecting field, said first and said second deflection elements having a design other than spherically shaped, wherein potentials applied to said first and said second electrodes cause charged particles which virtually enter the image of the radiation source at respective first angles in the first plane to be differently focused than charged particles which virtually enter the image of the radiation source at respective second angles in the second plane, with charged particles of one energy being point focused exclusively in said selection aperture plane, wherein zero-crossings of deflections of the charged particles in the first and the second planes only coincide at a same axial position at said selection aperture plane.