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A phase retrieval technique to measure and correct residual segment piston errors of large aperture optical telescopes

The success of the Keck telescopes’ segmented mirror technology provided a basis for the development of other large and extremely large telescopes. We investigate ways to optimize the performance of the segmented mirror telescope further to (1) take on the challenges of high contrast imaging to characterize habitable zone exoplanets, (2) enable visible adaptive optics (AO), and (3) fully benefit from recent extreme AO developments. The current status of Keck telescope phasing using the phasing camera system (PCS) is briefly presented. A phase retrieval technique is presented that uses AO science instrument images to improve the phasing of the telescope primary mirror. The technique was tested on the Keck telescopes, and the first experimental results are presented along with the limitations of this approach. The static, semi-static, and dynamic nature of the residual segment piston errors are discussed, along with possible elevation-dependent residual segment piston errors. We propose that the technique be periodically used at Keck observatory to monitor and improve telescope phasing. We discuss the significance of the technique for AO observations with the existing and future large aperture optical telescopes. The ultimate goal is to push large aperture ground-based telescopes to their performance limits and make them competitive with space telescopes in terms of PSF stability to enable breakthrough science.