This paper presents a non-contact optical metrology measuring the pistons and tip/tilt angles of the 61 hexagonal
segments of a compact-sized segmented mirror. The instrument has been developed within the scope of a design
study for a European Extremely Large Telescope (E-ELT). It is used as reference sensor for cophasing of the
mirror segments in closed loop control. The mirror shape is also measured by different types of stellar light-based
phasing cameras whose performances will be evaluated with regard to a future E-ELT. Following a description
of the system architecture, the second part of the paper presents experimental results demonstrating the level of
precision: 0.48nm RMS in piston and 0.074 μrad RMS in tip and tilt.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.