Effect of segmented telescope phasing errors on adaptive optics performance

The largest existing optical telescopes and the next generation of extremely large telescopes are all segmented telescopes. The primary mirror segments must be phased in order to attain the diffraction limit of the telescope. In this paper, we investigate the effect of phasing errors on image quality delivered by the adaptive optics systems at the W. M. Keck Observatory using both simulations and on-sky data. We show that a ShackHartmann wavefront sensor is able to measure moderate phasing errors, which can be partially corrected by the deformable mirror. Phase retrieval using multiple short exposure images at and near the focal plane leads to phase maps that are consistent with telescope segment phasing errors of around 150 nm. In the future, we would like to use the phase maps to correct for the segment phasing errors.