1. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)1 Buddy Martin a, Jim Burge a,b, Brian Cuerden a, Warren Davison a, Jeff Kingsley a, Cary Kittrell a, Randy Lutz a, Steve Miller a, Chunyu Zhao b and Tom Zobrist b a Steward Observatory, University of Arizona b College of Optical Sciences, University of Arizona Progress in manufacturing the first 8.4 m off-axis segment for the Giant Magellan Telescope

2. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)2 The GMT GMT is designed to guarantee smooth wavefront over large subapertures, with minimal reliance on active alignment and phasing. 24 m f/0.7 primary mirror consists of seven 8.4 m segments, the largest that can be made. Adaptive Gregorian secondary is segmented to match primary, with 1.1 m segments. Fine alignment, adaptive correction, and phasing are done with small, agile secondary segments. GMT design is simplest way to achieve a coherent wavefront over 24 m aperture.

3. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)3 GMT primary mirror segments 8.4 m honeycomb sandwich segments, similar to LBT primary mirrors. Use mature technology for fabrication, support and thermal control. Off-axis segments have 14 mm aspheric departure. Makes fabrication interesting. Makes testing a challenge. See talk by Burge, paper 41 (Wednesday 14:20).

4. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)4 Overview of manufacturing plan Fabrication is similar to that of an LBT primary mirror. Spin-casting and generating the aspheric surface are complete. Currently in loose-abrasive grinding with stressed lap. Testing systems support all stages of fabrication, provide redundant measurements of critical parameters. Enhanced laser-tracker measuring system for early stages, also corroborates optical test for large-scale figure. Principal optical test provides full-aperture, high-resolution interferometric measurement, with large non-axisymmetric null corrector. Scanning pentaprism test provides independent measurement of low-order aberrations. All measurements are made in new 28 m vibration-isolated test tower. Manufacturing system for first segment will enable efficient production of all remaining segments.

5. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)5 Casting Mirror blank was cast in spinning furnace, July 2005. Mold consists of 1681 ceramic fiber boxes in silicon carbide tub. Tops of boxes follow shape of aspheric surface; no two are identical.

6. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)6 Preparation of rear surface Rear surface was machined and polished flat. Load spreaders were bonded to the surface with RTV. Load spreaders are the interface between the 165 support actuators and the mirror. Distribute axial loads from an actuator to 1-4 points on mirror. Distribute lateral loads to 3 holes in back plate, so loads are applied in compression.

7. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)7 Generating Generating the front surface introduces the 14 mm aspheric departure. Segment is centered on turntable. Position of tool (red spindle) is adjusted as a function of segment rotation angle. To minimize errors due to backlash in machine, use monotonic vertical motion and weave horizontally. Surface is 9 x less sensitive to horizontal errors in tool position. Tool follows contours of constant height, superposed on downward spiral path. z r θ

8. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)8 Loose-abrasive grinding Loose-abrasive grinding and polishing are done with stressed laps, with help from small-tool orbital polisher. Stressed lap has computer-controlled actuators that bend aluminum plate. Lap shape matches local curvature as it moves across aspheric surface. Allows use of large, stiff lap with strong passive smoothing. First day of loose-abrasive grinding. 1.2 m stressed lap is faced with pitch and ceramic tiles.

9. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)9 Polishing mm Aspheric departure of 8.4 m segment. Circle represents 1.2 m stressed lap. GMT off-axis segment has 14 mm aspheric departure, but mostly astigmatism. Shape changes for 1.2 m stressed lap are mild, similar to those for LBT primary mirrors. Control software is more involved, as lap’s shape depends on its position in parent coordinates. Measure lap’s position and rotation angle in segment coordinates, and segment’s rotation angle. Transform to lap’s position and rotation angle in parent coordinates. Look up commands for bending actuators. Segment on turntable Axis of parent Stressed lap

10. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)10 1.7 m off-axis mirror for New Solar Telescope For practice, we polished the 1.7 m off-axis primary mirror for the New Solar Telescope at Big Bear Solar Observatory. Mirror surface is nearly a 1/5 scale model of the GMT segment, 2.7 mm asphericity. Optical test used a computer-generated hologram as null corrector. severe distortion alignment of mirror introduces combination of coma and astigmatism Mirror was finished to 16 nm rms surface error over 1.6 m clear aperture. Special mapping polynomials are used to correct for image distortion.

11. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)11 Accuracy of the measurement is critical to GMT. The ability to correctly manufacture the optical surface on the primary segments depends on the ability to accurately measure the shape. Principal optical test is a full-aperture interferometric test. Produces high-resolution map (>400 pixels, λ/100 height resolution). Guides figuring and qualifies finished mirror. Large asymmetric null corrector compensates for 14 mm asphericity. See Burge et al, paper 41 (Wednesday 14:20). Scanning pentaprism test independently measures low-order aberrations. See Su et al, paper 146 (Tuesday poster session). Shear test with principal test isolates potential errors on small and medium scales. Enhanced laser tracker measurement guides early stages, provides further confirmation of large-scale figure. See Zobrist et al, paper 147 (Tuesday poster session). Measurement of GMT primary mirror segments

12. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)12 Test of 3.75 m fold sphere Test optics for GMT segment GMT off-axis segment 28 m vibration-isolated tower was installed 2006-07. Supports all GMT tests, plus LSST, future 6.5 m and 8.4 m mirrors. New test tower at Mirror Lab

13. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)13 GMT principal optical test 3.75 m fold sphere interferometer for in situ test of fold sphere GMT segment 0.76 m fold spherevibration-insensitive interferometer computer-generated hologram, part of null corrector reference hologram, used to validate wavefront and align system point-source microscope, used to align fold sphere System can be aligned within tolerances using hologram alignment patterns, coordinate-measuring machine, point-source microscope and laser trackers. See Burge et al, paper 41. ~100 μm alignment tolerances ~10 μm alignment tolerances

14. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)14 3.75 m fold sphere Figure of fold sphere will be measured in situ and subtracted. Accuracy of correction depends on slope errors, and magnitude of small-scale structure that cannot be subtracted. Finished fold sphere meets requirements: < 2 nm/cm rms slope error small-scale errors < 15% of GMT segment specification Overall accuracy < 20 nm rms over clear aperture. Fold sphere on test tower, GMT segment in backgroundFold sphere after aluminizing

15. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)15 Scanning pentaprism test Scanning pentaprism measures slope errors by producing collimated beams parallel to parent axis. Displacement of focused spot is measured with camera in focal plane. Pentaprism rail lies in plane perpendicular to parent axis. Hub rotates rail to scan different diameters. Scanning pentaprism test as implemented for GMT off-axis segments. Pentaprism rail is suspended from tower.

16. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)16 Pentaprism test of 1.7 m off-axis NST mirror We measured the NST mirror with a prototype scanning pentaprism test as well as a full-aperture interferometric test. This was done in late 2007 before the mirror was finished. The pentaprism test measures only the aberrations listed below. For comparison, we fit the same polynomials to the interferometric data. The results agree within the uncertainties of both tests. pentaprism measurementinterferometric test nm surface aberration interferometerpentaprism nm rms surface astigmatism 0° 8 9 ± 23 astigmatism 45° 0 -2 ± 23 coma 0°-87-98 ± 12 coma 90° -4 16 ± 12 trefoil 0°-50-32 ± 35 trefoil 30° 9 23 ± 30 spherical-32-35 ± 8

17. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)17 sphere-mounted retro- reflector for laser tracker Retroreflector for interferometer and position sensing detector (PSD) assemblies in 4 places at edge of mirror laser tracker & distance-measuring interferometers (DMI) PSD10% BS DMI retroreflector DMI laser and remote receivers laser tracker DMIs Laser Tracker Plus

18. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)18 Laser Tracker Plus measurement of 3.75 m mirror We measured the 3.75 m fold sphere made for the GMT principal test. R = 25.5 m, tracker distance = 22 m 93 sample points, measured 4 DMIs with each sample Subtracted best-fit sphere (R = 25.497 m) after DMI correction: 0.75 μm rms before DMI correction: 1.4 μm rms

19. Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, SPIE 7018 (2008)19 Summary General development 1.7 m NST prototype is finished. Pentaprism test agrees with interferometric test. First off-axis segment Optical surface has been generated. Loose-abrasive grinding is underway. Measurement systems for all GMT segments 28 m tower is complete. Laser Tracker Plus system has been demonstrated to sub-micron accuracy. Principal optical test is designed, including alignment methods. 3.75 m fold sphere for principal test is ready. Pentaprism test is designed, prototype has been demonstrated. Manufacturing system for first segment will enable efficient production of all GMT segments.