During mask inspection it is necessary to identify defects which also occur during wafer exposure. Therefore, the aerial images generated in the resist and on the detector have to be as far as possible identical. In order to achieve an equivalent image generation, during mask inspection the illumination and, on the object side, the numerical aperture are adapted to the scanner used. The invention relates to a mask inspection microscope for variably setting the illumination. It serves for generating an image of the structure (150) of a reticle (145) arranged in an object plane in a field plane of the mask inspection microscope. It comprises a light source (5) that emits projection light, at least one illumination beam path (3, 87, 88), and a diaphragm for generating a resultant intensity distribution of the projection light in a pupil plane (135) of the illumination beam path (3, 87, 88) that is optically conjugate with respect to the object plane. According to the invention, the diaphragm is embodied in such a way that the resultant intensity distribution of the projection light has at least one further intensity value between a minimum and a maximum intensity value.