Alignment and use of the optical test for the 8.4-m off-axis primary mirrors of the Giant Magellan Telescope
2010
The Giant Magellan Telescope has a 25 meter f/0.7 near-parabolic primary mirror constructed from seven 8.4 meter
diameter segments. Several aspects of the interferometric optical test used to guide polishing of the six off-axis
segments go beyond the demonstrated state of the art in optical testing. The null corrector is created from two obliquelyilluminated
spherical mirrors combined with a computer-generated hologram (the measurement hologram). The larger
mirror is 3.75 m in diameter and is supported at the top of a test tower, 23.5 m above the GMT segment. Its size rules out
a direct validation of the wavefront produced by the null corrector. We can, however, use a reference hologram placed at
an intermediate focus between the two spherical mirrors to measure the wavefront produced by the measurement
hologram and the first mirror. This reference hologram is aligned to match the wavefront and thereby becomes the
alignment reference for the rest of the system. The position and orientation of the reference hologram, the 3.75 m mirror
and the GMT segment are measured with a dedicated laser tracker, leading to an alignment accuracy of about 100
microns over the 24 m dimensions of the test. In addition to the interferometer that measures the GMT segment, a
separate interferometer at the center of curvature of the 3.75 m sphere monitors its figure simultaneously with the GMT
measurement, allowing active correction and compensation for residual errors. We describe the details of the design,
alignment, and use of this unique off-axis optical test.
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