Design of a truth sensor for the GMT laser tomography adaptive optics system

The GMT laser tomography adaptive optics (LTAO) system design has a truth sensor guiding on a natural guide star. The truth sensor is used to measure telescope segment piston errors and measure slowly varying non- common path aberrations. The challenge lies in measuring segment piston using faint natural guide stars and the wavefront delivered by the LTAO system. This requires a sensor that can make a direct phase measurement. It is demonstrated that an infrared, AO-corrected, unmodulated pyramid or roof wavefront sensor can make the required measurements at 10 Hz for stars brighter than magnitude 17 at H- or K-band.

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