In-column magnetic energy filters have been designed so as to have the central plane of symmetry of magnetic field and anti-symmetry of the imaging electron trajectory to the dispersion direction in order to cancel 2nd order aberrations. A new S filter, on the other hand, has the anti-symmetry to the mid point of the magnetic field and has the symmetry to the imaging trajectory. In the previous filters, the magnets locate only one side of the optical axis, but the symmetry condition of S filter gives a good balancing filter shape to the optical axis of the incident beam. In all of the previous filters, the number of focuses in the x-direction is 3 for the dispersion rays, but its number increases to 4 in the S filter, giving the dispersion of 6 micrometers eV at 300 kV for the practical design and 10 (mu) m/eV for the ideal case.
"S filter: a compact high-dispersion in-column energy filter", Proc. SPIE 4510, Charged Particle Detection, Diagnostics, and Imaging, (21 December 2001); doi: 10.1117/12.451267; https://doi.org/10.1117/12.451267