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Open Access Publications from the University of California

Measuring Segment Piston with a Dispersed Fringe Sensor on the Giant Magellan Telescope


The Giant Magellan Telescope (GMT) consists of seven 8.365 m segments with a separation of 0.345 m. A uniquechallenge for GMT lies in phasing these segments and, in particular, how to measure segment piston optically.Making segment piston measurements is relatively straight-forward when using diffraction-limited light, such aswavefront-corrected light at near infrared wavelengths. Unfortunately, we don’t have that luxury, since all of thatlight is passed to the science instrument. Instead, we must use stars 6’-10’ from the optical axis when guidingwith laser guide stars. The segment piston measurement can be made in two different ways using subaperturesthat span adjacent segments with dispersed broadband light. The Dispersed Hartmann Sensor takes the averagesthe Fourier phase of the dispersed fringes over baselines spanning the segment gap, while the Dispersed FringeSensor works by measuring the angle of the dispersed fringes. We explore the properties of the two approaches,discuss their relative advantages, and evaluate their performance using end-to-end simulations in YAO.

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