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