Manufacture of 8.4 m off-axis segments: a 1/5 scale demonstration

We describe the requirements for manufacturing and maintaining alignment of the 8.4 m off-axis segments of the Giant Magellan Telescope’s primary mirror, and a demonstration of the manufacturing techniques on the 1.7 m off-axis primary mirror of the New Solar Telescope. This mirror is approximately a 1/5 scale model of a GMT segment. We show that the stressed lap polishing system developed for highly aspheric primary and secondary mirrors is capable of figuring the GMT segments and the NST mirror. We describe an optical test with a null corrector consisting of a tilted spherical mirror and a computer-generated hologram, and derive accuracy requirements for the test. The criterion for accuracy of low-order aberrations is that the active support system can correct any figure errors due to the laboratory measurement, with acceptably small forces and residual errors.