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Beam size as a function of the lens-to-sample distance D LS (N = 16 CRLs): horizontal FWHM (red squares), vertical FWHM (green circles). The error bars are shown below the symbol size. A parabolic approximation of the data is indicated by the lines. It can be used to determine the optimal focus position in the vertical (green dashed line) and horizontal (red solid line) directions.
