Large capture range cophasing with the Liquid Crystal Tunable Filter

The construction of ELT primary mirrors requires the integration of a hundreds of segments.(1,2) A frequent. substitution of some segment is foreseen allowing the primary alumimization. Mechanical integration has to reduce residual piston errors up to the capture range of optical cophasing sensors. Enlarging this range, will relax the mechanical integration requirements, speeding up meaningfully the integration operations. In this work we tested the performance of the Wavelength Sweeping Technique(3) (WST): a cophasing technique that allow a large piston capture range, with any need of initial calibration. To apply the WST is necessary couple a Liquid Crystal Tunable Filter (LCTF) to a phasing WaveFront Sensor (WFS). In laboratory we measured segment edge steps on a MEMS-DM applying the WST to the PYramid Phasing Sensor(4) (PYPS). We measured 72 wavefront phase steps from 0.5 to 3 mu m with an accuracy of less than 0.2 mu m. With numerical simulations we inquired the possibility to propagate WST edge step measures to cophase a segmented mirror. To do this we realized a cophasing algorithm and we compared its performace in different observing environments. The algorithm converges in 3 divided by 5 iterations and all the residual edge steps on the mirror are in the capture range of high resolution cophasing techniques.