An illumination system of a microlithographic projection exposure apparatus comprises an optical integrator, which includes a first optical raster plate and a second optical raster plate. The first second optical raster plate comprising an array of first lenses having, along a reference direction, a first focal length f1, and the second optical raster plate comprises an array of second lenses having, along the reference direction, a second focal length f2. The vertices of the first lenses and vertices of the second lenses are spaced apart by a distance d that is greater than the second focal length f2 so that d>1.01·f2. This ensures that laser pointing or another transient variation of the illumination of the optical integrator does not adversely affect the spatial irradiance distribution in a plane which is illuminated by the optical integrator.