In optics, aberration is a flaw in the imaging properties of optical systems such as lenses or mirrors that causes light to be blurred, distorted, or scattered over a region of space rather than focused in a point. The aberration of an optical system is defined as any deformation of the image with respect to the object that is not due to defects in the design of the system, but is due to the general laws of reflection and refraction. They can be divided into axial and extra-axial aberrations, depending on whether the object is on or off the optical axis of the system. Axial aberrations are axial chromatic aberration and spherical aberration; extra-axial aberrations are astigmatism, coma, field curvature, distortion and extra-axial chromatic aberration.

In images provided by an electronic optical system produced by electric and magnetic fields, chromatic aberration, spherical aberration, and aberration due to ionization of the residual gas within the electronic optical system may also occur. Chromatic aberration is due to the fact that the beam of charged particles passing through the system is never perfectly monochromatic (monoenergetic) because the particles are always emitted from the source (cathode) with a statistical energy distribution; therefore, the electronic optical system causes scattering of the trajectories, resulting in blurred images. The aperture aberration is due to the fact that the trajectories are not perfectly paraxial, i.e. they form too large an angle with the optical axis. The third type of electronic aberration is caused by the collisional ionization of the residual gas, which concentrates the beam in a damaging way due to its arrangement around the beam.

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