An energy filter receiving an electronic beam oriented along an optical axis. The filter includes a deflecting system which deviates in a dispersion plane not including the optical axis the received beam and a dispersing system which guides the beam sent by the deflecting system on an optical path inscribed in the dispersion plane and returning to the deflecting system. The deflecting system brings back in alignment with the optical axis the beam coming from the dispersing system. The deflecting system consists of a single element ensuring the inverse deviations of the beam whether outgoing or incoming.