Beam-astigmatism in laser interferometry

We have developed a null laser interferometer capable of picometer resolution in fringe division for the measurement of the silicon (220) lattice spacing by X-ray interferometry. With a view to achieving 10/sup -9/ measurement uncertainty, we have given special attention to the effects of diffraction. An analysis of the measurements which has brought to light wavefront astigmatism, the effect of which had so far been overlooked, is presented. This article shows how astigmatism affects measurements, describes how we determine beam parameters, and compares theoretical and experimental results.