The invention relates to a method for determining an imaging function of a mask inspection microscope, wherein the mask inspection microscope comprises an imaging optical element, a tube, a recording device, an object stage, an illumination unit for measurement with transmitted light and an illumination unit for measurement in reflection, comprising the following method steps: a) measuring the intensities in the pupil plane of the imaging optical element in a reflective measurement, b) measuring the intensities in the pupil plane of the imaging optical element in a transmitted-light measurement, d) Determining the imaging function of the intensities of the imaging optical element, d) determining the imaging function of the intensities of the illumination optical element comprised in the illumination unit for the transmitted-light measurement.
Furthermore, the invention relates to a mask inspection microscope for determining the deviation of an actual structure from a desired structure on an object, comprising an imaging optical element, a tube, a recording device, an additional optical module, an illumination unit for measurement with transmitted light, an illumination unit for measurement in reflection, and an object stage, wherein the object stage is configured to move to a position with a deviation of less than 100 nm, in particular of less than 20 nm, a calculation unit, in which the calculation unit is configured to calibrate the mask inspection microscope.