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The Giant Magellan Telescope (GMT) design consists of seven circular 8.4-m diameter mirror segments that are separated by large > 30 cm gaps, making them susceptible to fluctuations in optical path differences (piston) due to flexure, segment vibrations, wind buffeting, temperature effects, and atmospheric seeing. If we wish to utilize the full 25.4-m diffractionlimited aperture of the GMT for high-contrast natural guide star adaptive optics (NGSAO) science (e.g., direct imaging of habitable zone earth-like planets around late type stars), the seven mirror segments must be co-phased to well within a fraction of a wavelength. The current design of the GMT involves seven adaptive secondary mirrors, a dispersed fringe sensor, and a pyramid wavefront sensor (PyWFS) to measure and correct the total path length between segment pairs, but these methods need to be tested “end-to-end” in a lab environment if we hope to officially retire the GMT high risk item of phasing performance. We present the design and working prototype of a “GMT High-Contrast Adaptive Optics phasing Testbed” (p-HCAT) which leverages the existing MagAO-X ExAO instrument to demonstrate segment phase sensing and simultaneous AO-control for high-contrast GMT NGSAO science. We present the first test results of closed-loop piston control with one GMT segment using MagAO-X’s PyWFS and a novel Holographic Dispersed Fringe Sensor (HDFS) with and without simulated atmospheric turbulence. We show that the PyWFS alone was able to successfully control piston without turbulence within 12-33 nm RMS for 0 λ/D – 5 λ/D modulation, but was unsuccessful at controlling segmented piston with generated ∼ 0.6 arcsec and ∼ 1.2 arcsec seeing turbulence due to non-linear modal cross-talk and poor pixel sampling of the segment gaps on the PyWFS detector. We report the success of an alternate solution to control segmented piston using the novel HDFS while controlling all other modes with the PyWFS purely as a slope sensor (piston mode removed). This “second channel” WFS method worked well to control piston to within 50 nm RMS and ± 10 μm dynamic range under simulated 0.6 arcsec and 1.2 arcsec atmospheric seeing conditions. These results suggest that a PyWFS alone is not an ideal piston sensor for the GMT and likely other Giant Segmented Mirror Telescopes (GSMTs) as well. Therefore, an additional “second channel” piston sensor, such as the novel HDFS, is strongly suggested.
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