An apparatus for inspecting a surface of a sample, wherein the apparatus comprises a multi beam charged particle generator for generating an array of primary charged particle beams (3,23,33,43,53), and a charged particle optical system with an optical axis (8, 28, 38,48,58) comprising: a first lens system (13, 213, 37, 310) for focusing the primary charged particle beams (3, 23, 33, 43, 53) into a first array of separated spots in an intermediate plane, and a second lens system (14, 214, 313, 314) for directing the primary charged particle beams (3, 23, 33, 43, 53) from the intermediate plane towards the sample surface (15, 215, 315) and which comprises an electromagnetic or electrostatic objective lens which is common for at least the primary charged particle beams (3, 23, 33, 43, 53) for focusing all primary charged particle beams into a second array of individual spots on the sample surface (15, 215, 315), wherein the apparatus comprises a position sensitive backscatter charged particle detector (11, 211, 311, 411, 511) positioned in or near the intermediate plane, wherein said detector comprises one or more through openings for passing said primary charged particle beams (3, 23, 33, 43, 53) therethrough and wherein said second lens system (14, 214, 313, 314) is arranged to project backscattered charged particles from the second array of individual spots on the sample surface into an array of backscattered charged particle spots (44, 54) on the detector, characterized in that the second lens system (14, 214, 313, 314) comprises one or more magnetic lenses which are adapted to rotate the array of primary charged particle beams (3, 23, 33, 43, 53) around the optical axis (8, 28, 38, 48, 58) on their way from said intermediate plane towards the sample (15, 215, 315), to position the second array with respect to the first array at an angle larger than 0 degrees and preferably smaller than 180 degrees around the optical axis (8, 28, 38, 48, 58), and wherein the one or more magnetic lenses are adapted to rotate the array of backscattered charged particle spots (44, 54) with respect to the first array at an angle larger than 0 degrees and preferably smaller than 360 degrees around the optical axis (8, 28, 38, 48, 58). a first lens system (13, 213, 37, 310) for focusing the primary charged particle beams (3, 23, 33, 43, 53) into a first array of separated spots in an intermediate plane, and a second lens system (14, 214, 313, 314) for directing the primary charged particle beams (3, 23, 33, 43, 53) from the intermediate plane towards the sample surface (15, 215, 315) and which comprises an electromagnetic or electrostatic objective lens which is common for at least the primary charged particle beams (3, 23, 33, 43, 53) for focusing all primary charged particle beams into a second array of individual spots on the sample surface (15, 215, 315), wherein the apparatus comprises a position sensitive backscatter charged particle detector (11, 211, 311, 411, 511) positioned in or near the intermediate plane, wherein said detector comprises one or more through openings for passing said primary charged particle beams (3, 23, 33, 43, 53) therethrough and wherein said second lens system (14, 214, 313, 314) is arranged to project backscattered charged particles from the second array of individual spots on the sample surface into an array of backscattered charged particle spots (44, 54) on the detector, characterized in that the second lens system (14, 214, 313, 314) comprises one or more magnetic lenses which are adapted to rotate the array of primary charged particle beams (3, 23, 33, 43, 53) around the optical axis (8, 28, 38, 48, 58) on their way from said intermediate plane towards the sample (15, 215, 315), to position the second array with respect to the first array at an angle larger than 0 degrees and preferably smaller than 180 degrees around the optical axis (8, 28, 38, 48, 58), and wherein the one or more magnetic lenses are adapted to rotate the array of backscattered charged particle spots (44, 54) with respect to the first array at an angle larger than 0 degrees and preferably smaller than 360 degrees around the optical axis (8, 28, 38, 48, 58).