nonisochromaticity

When the incidence direction of an electron beam to an energy filter is tilted from the optical axis of the filter, the energy spectrum undergoes an energy shift ⊿E = ⊿E(α) (α is the incidence angle with respect to the optical axis) due to the second-order aberration of the filter. This energy shift is termed "nonisochromaticity." When a finite-size energy slit is inserted, the energy range selected by the filter changes with the angular position of the slit.