Dual-wavelength low-coherence instantaneous phase-shifting interferometer to measure the shape of a segmented mirror with subnanometer precision

Abstract

We present a noncontact 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 the 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 achieved precision: 0.48 nm rms in piston and 74 nrad rms in tip/tilt.

Measuring the cophasing state of a segmented mirror with a wavelength sweep and a Zernike phase contrast sensor

“…The internal metrology positioning error is translated into a Gaussian distribution of errors added to the position of the individual segments in the input complex wavefront. This error is an accumulation of the internal metrology measurement error [31] on the positioning system of the test bench [13]. A previous study [13] mentions that the RMS on the error is 0.75 nm over 5 hours.…”

“…After setup, the internal metrology [31] drives the segmented mirror to a piston configuration, while images at different wavelengths are recorded.…”

“…In order to apply the family and random configurations, a reference measurement system called the internal metrology [31] is used. The internal metrology is a dual-wavelength Michelson interferometer, using a four points phase shift polarisation encoded scheme to retrieve the phase of each wavelength.…”