First cophasing of a segmented mirror with a tunable filter and the pyramid wavefront sensor

Abstract

Optical cophasing has a key role in ensuring that segmented mirror telescopes reach their best performance. To measure and correct segments misalignment it is necessary to have a wavefront sensor (WFS) in the telescope optical path. All the cophasing WFS suffer the phase ambiguity problem that limits the piston error measurements to a unit of wavelength. To overcome this problem we have developed a new cophasing technique based on the wavelength sweep. This paper will present the results of laboratory and on-sky tests of this technique, comparing them with the expected performance obtained in a previous work through numerical simulations. The laboratory test was carried out on the Active Phasing Experiment bench at ESO premises in Garching. We measured wavefront piston errors up to 15μm with an accuracy better than 0.25μm on a pupil conjugate segmented mirror using the Pyramid Phasing Sensor (PYPS) and a commercial tunable filter. We tested the possibility of propagating the differential piston measurements over the segmented mirror to cophase it, obtaining a residual surface error less than 0.2μm rms. The first on-sky test of the WST was carried out at William Hershel Telescope (WHT) using the NAOMI segmented mirror. We checked the effects of atmospheric turbulence on the measurements of large piston errors up to 15um wavefront and it was obtained an accuracy of 0.5μm, which is in agreement with simulation.