An aberration correcting device for correcting aberrations of an electron beam in an electron microscope, wherein the electron microscope comprises an electron-optical column comprising electron optical elements for projecting said electron beam from an electron source onto a target, wherein the aberration correcting device comprises: a first and a second electron mirror (12, 14), each comprising an electron beam reflecting face (13, 15), an intermediate space, wherein the intermediate space comprises an input side for inputting the electron beam into the intermediate space, and an exit side for exiting the electron beam out of the intermediate space, wherein the first and second electron mirror (12, 14) are arranged at opposite sides of the intermediate space, and wherein the reflective face of the first electron mirror and the reflective face of the second mirror are arranged to face said intermediate space, wherein the first mirror is arranged at the exit side and is configured to receive the electron beam coming from the input side and to reflect the electron beam via the intermediate space towards the second mirror, wherein the second mirror is arranged at the input side and is configured to receive the electron beam coming from the first mirror at a reflection position on the second mirror and to reflect the electron beam via the intermediate space towards the exit side, wherein the aberration correcting device is configured such that an incoming electron beam passes said second mirror at a position spaced apart from the reflection position on the second mirror, wherein at least one of said first and second electron mirrors (12, 14) is configured to provide a correcting aberration to a reflected electron beam, wherein said at least one of said first and second mirrors (12, 14) comprises an electrostatic mirror, wherein said electrostatic mirror comprises at least three electrodes, wherein two of said at least two electrodes are configured for transmitting an electron beam and for, in use, providing an electrostatic lens, wherein the aberration correcting device is configured to at least partially compensate for the aberration of one or more said electron optical elements in the electron microscope, and wherein the aberration correcting device further comprises a magnetic deflector which is arranged in said intermediate space and is placed in a different position from the first and second electron mirror (12, 14), which magnetic deflector is configured for separating the incoming and reflected electron beam of said first and/or said second electron mirror (12, 14). a first and a second electron mirror (12, 14), each comprising an electron beam reflecting face (13, 15), an intermediate space, wherein the intermediate space comprises an input side for inputting the electron beam into the intermediate space, and an exit side for exiting the electron beam out of the intermediate space, wherein the first and second electron mirror (12, 14) are arranged at opposite sides of the intermediate space, and wherein the reflective face of the first electron mirror and the reflective face of the second mirror are arranged to face said intermediate space, wherein the first mirror is arranged at the exit side and is configured to receive the electron beam coming from the input side and to reflect the electron beam via the intermediate space towards the second mirror, wherein the second mirror is arranged at the input side and is configured to receive the electron beam coming from the first mirror at a reflection position on the second mirror and to reflect the electron beam via the intermediate space towards the exit side, wherein the aberration correcting device is configured such that an incoming electron beam passes said second mirror at a position spaced apart from the reflection position on the second mirror, wherein at least one of said first and second electron mirrors (12, 14) is configured to provide a correcting aberration to a reflected electron beam, wherein said at least one of said first and second mirrors (12, 14) comprises an electrostatic mirror, wherein said electrostatic mirror comprises at least three electrodes, wherein two of said at least two electrodes are configured for transmitting an electron beam and for, in use, providing an electrostatic lens, wherein the aberration correcting device is configured to at least partially compensate for the aberration of one or more said electron optical elements in the electron microscope, and wherein the aberration correcting device further comprises a magnetic deflector which is arranged in said intermediate space and is placed in a different position from the first and second electron mirror (12, 14), which magnetic deflector is configured for separating the incoming and reflected electron beam of said first and/or said second electron mirror (12, 14).