Fabrication and testing of the first 8.4-m off-axis segment for the Giant Magellan Telescope

H. M. Martin; R. G. Allen; J. H. Burge; D. W. Kim; J. S. Kingsley; M. T. Tuell; S. C. West; C. Zhao; T. Zobrist

doi:10.1117/12.857494

19 July 2010 Fabrication and testing of the first 8.4-m off-axis segment for the Giant Magellan Telescope

H. M. Martin, R. G. Allen, J. H. Burge, D. W. Kim, J. S. Kingsley, M. T. Tuell, S. C. West, C. Zhao, T. Zobrist

Proceedings Volume 7739, Modern Technologies in Space- and Ground-based Telescopes and Instrumentation; 77390A (2010) https://doi.org/10.1117/12.857494
Event: SPIE Astronomical Telescopes + Instrumentation, 2010, San Diego, California, United States

Abstract

The primary mirror of the Giant Magellan Telescope consists of seven 8.4 m segments which are borosilicate honeycomb sandwich mirrors. Fabrication and testing of the off-axis segments is challenging and has led to a number of innovations in manufacturing technology. The polishing system includes an actively stressed lap that follows the shape of the aspheric surface, used for large-scale figuring and smoothing, and a passive "rigid conformal lap" for small-scale figuring and smoothing. Four independent measurement systems support all stages of fabrication and provide redundant measurements of all critical parameters including mirror figure, radius of curvature, off-axis distance and clocking. The first measurement uses a laser tracker to scan the surface, with external references to compensate for rigid body displacements and refractive index variations. The main optical test is a full-aperture interferometric measurement, but it requires an asymmetric null corrector with three elements, including a 3.75 m mirror and a computer-generated hologram, to compensate for the surface's 14 mm departure from the best-fit sphere. Two additional optical tests measure large-scale and small-scale structure, with some overlap. Together these measurements provide high confidence that the segments meet all requirements.

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H. M. Martin, R. G. Allen, J. H. Burge, D. W. Kim, J. S. Kingsley, M. T. Tuell, S. C. West, C. Zhao, and T. Zobrist "Fabrication and testing of the first 8.4-m off-axis segment for the Giant Magellan Telescope", Proc. SPIE 7739, Modern Technologies in Space- and Ground-based Telescopes and Instrumentation, 77390A (19 July 2010); https://doi.org/10.1117/12.857494

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