The EELS spectrometer is a Wien Filter type mounted within a high voltage electrode.[1,2] The filter is designed for a center pass energy of 100-200 eV, giving it a very high energy dispersion -- of order mm/eV. This center pass energy is defined by a very accurate 15-1200V scanning power supply which is in turn connected to the microscope high voltage. This design is capable of about 60 meV energy resolution at a collection half angle of 10 mR at the specimen, and a center pass energy definition of better than 20 meV. In the figure below, I show a design intended to optimize the spectrometer for operation with the aberration corrected probe. The Wien Filter has a first order focus in the energy dispersive direction, but does not modify the electron paths in the perpendicular direction. Therefore we use a combination of a cylindrical lens and a weak quadrupole to produce focusing in the energy dispersive direction and to maintain a narrow beam in the spectrometer in the other direction. This design suggests that a 15 meV resolution should be possible using 15 mR collection half angle at the specimen.
 P.E. Batson, High Resolution Electron Energy Loss Spectrometer for the Scanning Transmission Electron Microscope, Rev. Sci. Inst. 57, 43-48 (1986).
 P.E. Batson, Parallel Detection for High Resolution Electron Energy Loss Studies in the Scanning Transmission Electron Microscope, Rev. Sci. Inst. 59, 1132-1138 (1988).
 P.E. Batson, High Resolution Spectrometer Coupling to the Sub-Angstrom IBM STEM, in Microscopy and Microanalysis, edited by D. Piston, J. Bruley, I.M. Anderson, P. Kotula, G. Solorzano, A. Lockey, and S. McKernan (Cambridge University Press, Cambridge, 2003), pp. 836 -837.