In the upper row, measured wavefronts behind the mirror M2 are shown at the position of the wavefront sensor. The inital wavefront shows a dominant presence of astigmatic contributions (a), wavefront after pitch optimization of M2 (b) and the final wavefront after alignment of roll and x of M2 (c). The peak-to-valley value for each wavefront is given in λ and could be reduced to about half of the initial value. The lower row shows the intensity profiles at the focus position of M2. They were retrieved by Fresnel back-propagation of the intensity profile measured at the position of the wavefront sensor. The focus size in the x and y direction is given as the FWHM for the cross-marked positions. The focus size could be significantly reduced to a circular shape of about 25.5 µm each in the x and y directions.