Principle for creating a vertical laser reference: a pellicle beam splitter above a flat liquid mirror is tilted in a such a way that the green laser goes back through a pinhole filmed by a webcam i.e. the fraction of the green laser light transmitted is perfectly vertical and can be used as a reference to align the metrology gondola above the C′ point (Fig. 7) of the metrology table. The dashed lines show the light rays if the beam splitter is misaligned. The reached accuracy is within one to two arcsec.