an assistant microscope add - on device for a surgical microscope comprises an objective inclined relative to an object plane with an object field , wherein the objective defines an objective plane and an objective axis perpendicular thereto forming an angle α larger than 0 ° to the surface normal of the object plane . a tube and an eyepiece having an eyepiece lens defining an eyepiece plane and an eyepiece axis perpendicular thereto . due to the oblique viewing angle , an intermediate image plane to be imaged by the eyepiece is inclined relative to the objective plane . the resulting aberration is eliminated by the eyepiece axis and the objective axis forming an angle β of larger than 0 °, wherein the angle β is so chosen that the eyepiece axis is substantially perpendicular to the intermediate image plane . consequently , the object field is imaged free of distortion and sharply across the entire image area .