A software configurable optical test system (SCOTS) based on fringe reflection was implemented for measuring the
primary mirror segments of the Giant Magellan Telescope (GMT). The system uses modulated fringe patterns on an
LCD monitor as the source, and captures data with a CCD camera and calibrated imaging optics. The large dynamic
range of SCOTS provides good measurement of regions with large slopes that cannot be captured reliably with
interferometry. So the principal value of the SCOTS test for GMT is to provide accurate measurements that extend
clear to the edge of the glass, even while the figure is in a rough state of figure, where the slopes are still high.
Accurate calibration of the geometry and the mapping also enable the SCOTS test to achieve accuracy that is
comparable measurement accuracy to the interferometric null test for the small- and middle- spatial scale errors in
the GMT mirror.
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.
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.
We have developed a metrology system that is capable of measuring rough ground and polished surfaces alike, has
limited sensitivity to the nominal surface shape, and can accommodate surfaces up to 8.4 m in diameter. The system
couples a commercial laser tracker with an advanced calibration technique and a system of stability references to
mitigate numerous error sources. This system was built to guide loose abrasive grinding and initial polishing of the off-axis
primary mirror segments for the Giant Magellan Telescope (GMT), and is also being used to guide the fabrication of
the Large Synoptic Survey Telescope primary and tertiary mirrors. In addition to guiding fabrication, the system also
works as a verification test for the GMT principal optical interferometric test of the polished mirror segment to
corroborate the measurement in several low-order aberrations. A quantitative assessment of the system accuracy is
presented, along with measurement results for GMT, including a comparison to the optical interferometric test of the
polished surface.8
We have developed a metrology system that is capable of measuring rough ground and polished surfaces alike, is nearly
independent of the nominal surface shape, and can accommodate surfaces up to 8.4 m in diameter. The system couples a
commercial laser tracker with an advanced calibration technique and a system of external references. This system was
built to guide loose abrasive grinding and initial polishing of the off-axis primary mirror segments for the Giant
Magellan Telescope, and will be used to guide the fabrication of the Large Synoptic Survey Telescope primary and
tertiary mirrors as well. The results obtained using this system during the fabrication of the first segment of the Giant
Magellan Telescope are presented along with an assessment of the expected system accuracy.
The first of the 8.4 m off-axis segments for the primary mirror of the Giant Magellan Telescope is being manufactured at
the Steward Observatory Mirror Lab. In addition to the manufacture of the segment, this project includes the
development of a complete facility to make and measure all seven segments. We have installed a new 28 m test tower
and designed a set of measurements to guide the fabrication and qualify the finished segments. The first test, a laser-tracker
measurement of the ground surface, is operational. The principal optical test is a full-aperture interferometric test
with a null corrector that includes a 3.75 m spherical mirror, a smaller sphere, and a computer-generated hologram. We
have also designed a scanning pentaprism test to validate the measurement of low-order aberrations. The first segment
has been cast and generated, and is in the process of loose-abrasive grinding.
Surface measurements represent a significant part of the cost for manufacturing large aspheric optics. Both polished and
rough ground surfaces must be measured with high precision and spatial resolution. We have developed a system that
couples a commercial laser tracker with an advanced calibration technique and a system of external references. This
system was built to measure the off-axis primary mirror segments for the Giant Magellan Telescope where it will guide
loose abrasive grinding and initial polishing. The system is further expected to corroborate the optical interferometric
tests of the completed mirrors, in several low-order aberrations. The design, analysis, calibration, and measured
performance of this system will be presented.
Laser trackers have been developed that project laser beams and use optical systems to provide three dimensional coordinate measurements. The laser trackers incorporate a servo system to steer a laser beam so that it tracks a retro-reflector, such as a corner cube. The line of sight gimbal angles and the radial distance to the retroreflector are used to determine the coordinates of the retroreflector relative to the tracker. In this paper, we explore the use of the laser tracker to define the metrology for aligning optical systems, including the use of mirrors and windows. We discuss how to optimize the geometry to take advantage of the tracker's most accurate measurements. We show how to use the tracker for measuring angles as well as points.
The primary mirror for the 25-m Giant Magellan Telescope is made of seven circular segments, each of 8.4-m
diameter. The lack of axisymmetry and the steep aspheric departure present significant technical challenges for
the metrology. These segments will be measured interferometrically using a complex system of mirrors and
holograms to give a null test with high spatial resolution. While analysis predicts this system will meet
requirements, an additional set of measurements will be used to corroborate the principal interferometric
measurement. The set of tests, including these alternate surface measurements, assures that all aspects of the
mirror surface are measured completely and redundantly. The corroboration tests discussed in this paper are:
Direct surface profile using metrology system based on a laser tracker, measuring low order shape errors
Shear testing with full aperture interferometer, separating small scale errors in the null test from those in
the mirror
Slope testing with scanning pentaprism, measuring low order shape errors and sampling small scale errors
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