Phasing the Giant Magellan Telescope: Lab Experiments and First On-sky Demonstration
Maggie Y. Kautz, Sebastiaan Y. Haffert, Laird M. Close, Jared R., Males, Olivier Guyon, Alexander D. Hedglen, Victor Gasho, Richard Demers,, Antonin Bouchez, Fernando Quir\'os-Pacheco, C\'edric Plantet, Avalon L., McLeod, Jay K. Kueny, Jialin Li, Joshua Liberman, Joseph D. Long

TL;DR
This paper introduces a holographic dispersed fringe sensor (HDFS) for segment piston correction in giant segmented telescopes, demonstrating lab and first on-sky phasing with high precision despite challenging conditions.
Contribution
The paper presents the development and initial on-sky demonstration of the HDFS, a novel holographic sensor capable of high dynamic range and linear differential piston measurement for segmented telescopes.
Findings
Achieved segment phasing within λ/11.3 residual piston WFE.
Demonstrated ~50 nm RMS residual piston WFE on-sky.
Successfully used HDFS for on-sky piston sensing in poor seeing conditions.
Abstract
The large apertures of the upcoming generation of Giant Segmented Mirror Telescopes will enable unprecedented angular resolutions that scale as /D and higher sensitivities that scale as for point sources corrected by adaptive optics. However, all will have pupil segmentation caused by mechanical struts holding up the secondary mirror [European Extremely Large Telescope and Thirty Meter Telescope] or intrinsically, by design, as in the Giant Magellan Telescope. These gaps will be separated by more than a typical atmospheric coherence length (Fried Parameter). The pupil fragmentation at scales larger than the typical atmospheric coherence length, combined with wavefront sensors with weak or ambiguous sensitivity to differential piston, can introduce differential piston areas of the wavefront known as "petal modes". Commonly used wavefront sensors, such as a…
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