Electron spectrometers may determine electron energy based on time of flight, retarding potential (effectively a high-pass filter), resonant collision or curvature in a deflecting field (magnetic or electric).
Owing to these effects, sector based designs are commonly used in electron spectrometers.
The effective potential in the solution of motion in a magnetic or electric system with rotational symmetry leads to radial focusing onto a mean radius.
By breaking the rotational symmetry a bit and varying the electrostatic potential along the mean path of the spherical aberration is minimized.
Photoelectron spectra from single crystals exhibit a dependency on the emission angle, and the entrance slit is needed at the entrance of the hemispherical electron analyzer in angle-resolved photoemission spectroscopy and related techniques.