Optical phasing of a segmented mirror with sub-nanometer precision: experimental results of the APE Internal Metrology

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.