Unsupervised-clustering-driven noise-residue filter for phase images.

We analyze the characteristics of noise-induced phase inconsistencies, or residues, in a wrapped phase map. Because residues are the potential source of phase-error propagation, it is essential to filter them before two-dimensional phase unwrapping. We propose an unsupervised-clustering-driven noise-residue filter, and apply it as a preprocessing procedure of phase unwrapping. The filter is based on the fact that most residues are present in the form of adjacency caused by noisy wrapped phases. These noisy phases differ from the other correct ones numerically within the local k(1) x k(2) window containing the adjacent residues, and it is possible to group the correct and noisy wrapped phases into different clusters. The window size is determined adaptively according to the local noise level. The proposed procedure avoids constructing branch cuts, and converts path-following unwrapping to path independence, which improves the operating speed of phase unwrapping significantly. The tests performed on simulated and real projected fringe patterns confirm the validity of our approach in residue reduction, fringe preservation, and rapidity.

Active hexagonally segmented mirror to investigate new optical phasing technologies for segmented telescopes.

The primary mirror of the future European Extremely Large Telescope will be equipped with 984 hexagonal segments. The alignment of the segments in piston, tip, and tilt within a few nanometers requires an optical phasing sensor. A test bench has been designed to study four different optical phasing sensor technologies. The core element of the test bench is an active segmented mirror composed of 61 flat hexagonal segments with a size of 17 mm side to side. Each of them can be controlled in piston, tip, and tilt by three piezoactuators with a precision better than 1 nm. The context of this development, the requirements, the design, and the integration of this system are explained. The first results on the final precision obtained in closed-loop control are also presented.

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

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.