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

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

Jul 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010spie.7739e...6m&link_type=abstract

Modern Technologies in Space- and Ground-based Telescopes and Instrumentation. Edited by Atad-Ettedgui, Eli; Lemke, Dietrich.

Computer Science

1

Scientific paper

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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