The invention relates to an aberration correcting device for correcting aberrations of focusing lenses in an electron microscope. The device comprises a first and a second electron mirror, each comprising an electron beam reflecting face. Between said mirrors an intermediate space is arranged. The intermediate space comprises an input side and an exit side. The first and second electron mirrors are arranged at opposite sides of the intermediate space, wherein the reflective face of the first and second mirror are arranged facing said intermediate space. The first mirror is arranged at the exit side and the second mirror is arranged at the input side of the intermediate space. In use, the first mirror receives the electron beam coming from the input side and reflects said beam via the intermediate space towards the second mirror. The second mirror receives the electron beam coming from the first mirror, and reflects the electron beam via the intermediate space towards the exit side. The incoming electron beam passes said second mirror at a position spaced apart from the reflection position on the second mirror. At least one of the electron mirrors is arranged to provide a correcting aberration to a reflected electron beam.