An exposure pattern is computed which is used for exposing a desired pattern on a target by means of a blanking aperture array in a particle-optical lithography apparatus which has a finite number of defects, said desired pattern being composed of a multitude of image elements within an image area on the target: A list of defective blanking apertures is provided, comprising information about the type of defect of the defective blanking apertures; from the desired pattern a nominal exposure pattern is calculated as a raster graphics over the image elements disregarding the defective blanking apertures; the 'compromised' image elements (1105) are determined which are exposed by aperture images of defective blanking apertures; for each compromised element (1105), a set of neighboring image elements is selected as 'correction elements' (1104); for each compromised element, corrected dose values are calculated for the correction elements, said corrected dose values minimizing an error functional of the deviation of the dose distribution including the defects from the nominal dose distribution, under the constraint that each of the corrected dose values falls within the allowed doses; and a corrected exposure pattern (1103) is generated by substituting the corrected dose values for the nominal dose values at the correction elements.

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