On-sky multiwavelength phasing of segmented telescopes with the Zernike phase contrast sensor

Future extremely large telescopes will adopt segmented primary mirrors with several hundreds of segments. Cophasing of the segments together is essential to reach high wavefront quality. The phasing sensor must be able to maintain very high phasing accuracy during the observations, while being able to phase segments dephased by several micrometers. The Zernike phase contrast sensor has been demonstrated on-sky at the Very Large Telescope. We present the multiwavelength scheme that has been implemented to extend the capture range from +/-lambda/2 on the wavefront to many micrometers, demonstrating that it is successful at phasing mirrors with piston errors up to +/- 4.0 mu m on the wavefront. We discuss the results at different levels and conclude with a phasing strategy for a future extremely large telescope. (C) 2011 Optical Society of America