On-sky performances of an optical phasing sensor based on a cylindrical lenslet array for segmented telescopes.

New optical phasing sensor technologies have been studied with a test bench experiment, called Active Phasing Experiment, on-sky at the European Southern Observatory Very Large Telescope. One of the sensors was of the Shack-Hartmann type using cylindrical lenslets across the segment borders for the measurement of the phasing errors. With bright stars, the precision of the measurement of piston steps at a single border was better than 9 nm wavefront RMS, and the precision of the closed-loop correction of the piston errors of the segments across the whole mirror was better than 10 nm wavefront RMS. With dimmer stars of magnitude up to 14.5, precisions of the order of 22 nm wavefront RMS were obtained.