CGH-assisted metrology testbed for the Thirty Meter Telescope primary mirror

The Thirty Meter Telescope primary mirror consists of 492 1.4 m diameter hexagonal segments. A CGH-assisted interferometric testbed has been developed to quickly and accurately measure surface figure error of the 82 different segment prescriptions. In this paper, technical aspects of the testbed will be described, including interferometer design, techniques to reduce sensitivity to vibration and turbulence, use of CGH phase fiducials for 6 degree-of-freedom alignment of the interferometer to the Test Plate, proximity sensors for segment-to-Test Plate alignment, synthetic extended source technique to mitigate coherent artifacts, characterization of instrument transfer function, and system calibration. A surface figure error map of a "Type 0" full-scale segment will also be presented.