The C03 is a sensor head type optimized for moderate working distances and using retro-reflectors as target. The probe beam is enlarged so that its divergence is reduced and more optical power can be collected after reflection at the target mirror. The C03 sensor head type is based on the standard sensor head type C01, including the fiber collimation optics and the beam splitter. The beam splitter splits the beam into a reference and probe beam. The reference beam is reflected by an internal reference mirror, coated to one side of the beam splitter cube. In the C03 sensor head, the probe beam exits the head after being expanded by a telescope (two lenses) to signifi- cantly reduce the beam divergence. It is reflected off the target surface and tracks its relative displacement. The front surface of the beam splitter marks the absolute zero position of every PICOSCALE measurement as here the probe and reference beam are of equal length.