Progress on the GMT Matt Johns Carnegie Observatories, 813 Santa Barbara Street, Pasadena CA USA 91101

ABSTRACT The Giant Magellan Telescope (GMT) is being developed by a consortium of major US and international educational and research institutions. The 25 meter next-generation telescope will be located at in Chile. The project has completed the conceptual design of the telescope and enclosure and is currently in the Design Development Phase leading up to construction. Various refinements have been made to the telescope structure since the Conceptual Design. These include the modification of the upper truss structure to reduce image blur due to wind shake and the design of a 9 meter rotator for large Gregorian instruments. An integral field spectrograph has been added to the candidate list of first-generation instruments. The primary mirror for GMT consists of seven 8.4 meter diameter segments. The first of the six, highly aspheric, off-axis segments has been cast and generated at the SOML with completion of the mirror expected in 2009. The metrology for polishing the segments is currently being installed in the new test tower at SOML. Verification tests that independently measure the mirror figure have been designed and are also being implemented. This paper summarizes the overall design and recent progress in the technical development of GMT and in characterizing the site. Keywords: Giant Magellan Telescope, GMT, ELT

1. INTRODUCTION

The Giant Magellan Telescope (GMT) consortium is developing a next-generation 25m extremely large telescope (ELT) for astronomical research at optical and infrared wavelengths. The science goals for GMT2 are aligned with those laid out by the GSMT committee in its report “Frontier Science Enabled by a Giant Telescope” and reflect the priorities of the most recent National Academy of Sciences Decadal Survey. The 25 meter GMT will provide the astronomical community with a powerful tool for addressing these goals. Members of the consortium are Carnegie Institution of Science, the Smithsonian Astrophysical Observatory, , Texas A & M University, the University of Arizona, The University of Texas at Austin, the Australian National University and Astronomy Australia Limited. Recently, the Korean Astronomy and Space Science Institute (KASI) has submitted a proposal to the Korean Ministry of Education, Science and Technology (MEST) to join the project on behalf of the Korean astronomical community. The broader US astronomical community is expected to have access to the GMT through arrangements with the national funding agencies. The conceptual design of the GMT telescope and facility was completed in 20063 and the GMT project is currently in the Design Development Phase. Engineering studies are underway to address key aspects of the concept leading towards a System Preliminary Design in 2010. A corporation has been formed, the GMTO Corporation, that will be ultimately responsible for constructing and operating the GMT facility. GMT will be located at Las Campanas Observatory in Chile. LCO is owned and operated by Carnegie Observatories and is known for its excellent observing conditions. A campaign to characterize several potential sites has been conducted over the past three years. A site, Cerro Las Campanas, has been identified for the GMT pending final approval by the partners. GMT is unique among the current ELT projects in using large, 8.4m, segments for its primary mirror. The 8.4 segments offer the advantage of large subapertures of well corrected wavefronts but require the development of techniques for producing the highly aspheric off-axis mirrors. In order to demonstrate the technology and establish the production pipeline for producing such mirrors, the GMT is fabricating the first off-axis segment, GMT1. Casting and generation of GMT1 is now complete and the final phase of grinding and polishing is underway as described below. 2. TELESCOPE

2.1. Optical configuration

The 25m GMT is designed around a primary mirror (M1) consisting of seven 8.4 m segments in a common alt-azimuth mount4 shown in Figure 1. The parent optical design of the telescope is aplanatic Gregorian with ellipsoidal primary and secondary mirror surfaces. The secondary mirror (M2) is also segmented with deformable face sheets for operation. The seven 1.1m M2 segments are conjugated to the primary mirror segments. This arrangement greatly simplifies the alignment of the telescope optics and allows deflections of the primary segments to be corrected by motions of the more agile M25. Corrections for windshake with fast tip tilt of the segments are also possible with this configuration.

Figure 1. GMT on its pier. The six outer M1 mirrors and mirror cells are identical. A seventh off-axis mirror cell assembly and mirror segment will be provided to facilitate coating operations and minimize down-time. The mirrors will be re-coated in their cells in a coating plant in the nearby Facility Building. The Gregorian focus is 5.5 m below the vertex of the primary mirror. Science instruments mount on an Instrument Platform (IP) below the primary mirror. There are no Nasmyth platforms but a Coude room is provided for gravity invariant instruments. The GMT structure allows a field of view up to 24΄. A corrector and atmospheric dispersion compensator have been designed with a 20΄ FOV for multi-object spectroscopy in the visible. The corrector/ADC is removed for infrared/AO operation. Table 1 summarizes some of the optical parameters of GMT.

Table 1. GMT Optical Parameters Parameter Value M1 diameter 25.4 m M1 focal length 18.0 m Effective aperture (collecting area) 21.9 m Effective aperture (diffraction limit) 24.5m Final focal length 202.74m Final focal ratio f/8 Focal plane scale 1.017″/mm Field of view (with corrector) 20΄ 2.2. Structure

GMT uses an alt-az mount with all-sky coverage down to a minimum elevation angle of 28°. Hydrostatic bearings are provided for the azimuth and elevation axes. The Optical Support Structure (OSS) consists of the elevation journals (C- ring assembly), a connector frame that holds the primary mirror cells, the upper truss supporting secondary mirror assembly, and the Gregorian Instrument Platform. The OSS rides on a turntable that rotates on a track atop the pier and provides the azimuth motion. Stress relieved welded steel structures are used throughout except for the upper truss. The truss and secondary mirror assembly are designed to minimize obscuration of the seven subapertures of the telescope for lower emissivity in the IR and increased throughput at all wavelengths. This truss assembly is a composite of steel and carbon fiber reinforced plastic (CFRP) tubing which combines high stiffness to resist windshake and low weight. The height of GMT is 38.7 meters above its pier structure. The entire structure stands 50 meters above ground level including the pier. The elevation axis is 25.4 meters above ground level. The telescope structure, excluding the pier, weighs 1,125 metric tons and has a 4.5 Hz lowest vibrational mode. The lowest mode including the pier and estimated LCO soil stiffness is 4.3 Hz.

2.3. Windshake

Image blur caused by windshake of the telescope structure was analyzed during the GMT conceptual design for various combinations of wind speed, enclosure venting, drive constraints, and structure damping6,7. The dynamic response analysis (DRA) was performed using the time-series wind velocities and forces measured on the Gemini South telescope. Fast tip-tilt of the secondary mirror segments to compensate for wind shake was not assumed in the analysis but is an option with the adaptive secondary mirror. The study concluded that, while the structure nominally came close to meeting the natural seeing error budget without fast tip-tilt compensation, further optimization was desirable to provide an additional margin of safety in light of uncertainties in the model input parameters.

Figure 2. Optimized secondary mirror truss.

This optimization has been carried out producing in an additional ~25% reduction in image blur. Details of the analysis and results are reported by Gunnels, et. al.8. Figure 2 shows the optimized structure. Table 2 shows the calculated image blur for the 75th and 90th percentile wind speeds characteristic of the GMT site.

Table 2. Image blur due to windshake.

Condition 1 Condition 2 Natural seeing Image Motion Mean Wind Speed Open Vents Closed Vents Locked Rotor Locked Rotor Error Budget Specification 2% Damping 2% Damping 13 m/s θrms=0.161 arcsec θrms=0.063 arcsec 90th percentile 9.5 m/s θrms=0.086 arcsec θrms=0.034 arcsec θrms=0.043 arcsec 75th percentile θ80=0.218 arcsec θ80=0.086 arcsec θ80=0.109 arcsec

2.4. Instrument mounting

GMT science instruments will be mounted on a platform below the primary mirror, Figure 3a. Small and intermediate size instruments will be located above the platform and be addressed by one or more pick-off mirrors inserted in the Gregorian beam. AO instruments will also reside on top of the IP and be fed by the AO relay optics described below. A service lift built into the enclosure floor will raise instruments to the level of the IP for installation and servicing on the telescope. A 9 m diameter instrument rotator below the platform, shown in Figure 3b, will carry the large survey instruments and mid-IR instruments that require a view of the sky with minimal reflections. The Gregorian Instrument Rotator (GIR)8 will be supported on hydrostatic bearings and will be driven to take out field rotation produced by alt-azimuth tracking. Walkways and stairs around the perimeter of the GIR provide service access. The GIR can be exchanged with the lift platform in the base of the pier. Instruments will normally remain powered up and ready for deployment. Rapid switching will allow programs to use multiple instruments during a night and program changes to adapt to varying observing conditions.

Figure 3. Instrument mounting. (a) Instrument Platform. (b) Gregorian Instrument Rotator. (c) Coude room.

A Coude room on the azimuth platform provides a location for instruments that require a gravity invariant mounting. Optical relay or a fiber feeds to the Coude may be accommodated. The fiber-fed high precision radial velocity SHARPS spectrometer would be placed here, for example. 3. PRIMARY MIRROR SEGMENTS

3.1. Primary Mirror Segment 1 (GMT1)

The primary mirrors for GMT are being produced at the Mirror Lab (SOML) using techniques developed previously on their 3.5m, 6.5m, and 8.4m cast borosilicate mirrors9. The six off-axis segments of the primary mirror are particular challenging due to their highly aspheric surfaces: the height variation around the perimeter of each segment is 14 mm P-V. This is roughly 10 times the departure of the LBT 8.4 m mirrors, previously the most highly aspheric mirrors made by SOML. A decision was made early in the preliminary phase of the project to fabricate the first segment, GMT1, in order to buy down the technical and programmatic risk. This involved adapting the casting, generating and stressed-lap polishing techniques employed in making the previous generation of 8m mirrors. The metrology for measuring the front surface of the off-axis mirrors is the most challenging part of the fabrication because of the highly aspheric surfaces and because of tighter tolerances on the mirror figure and the requirement for repeatability over many years of production to ensure the segments match each other when they come together in the telescope. It takes approximately 3.5 years to produce a single GMT segment. The segments are processed in successive stages (casting, rear surface generation and attachment of supports, front surface generation, front surface grinding and polishing). The fabrication pipeline will have multiple segments at various stages of completion. Once the pipeline is filled, a new segment will be completed every 10-12 months. The metrology used in figuring the mirrors must be stable over the period of time it will take to produce the full complement of 8 segments.

Figure 4. GMT1 at the end of front surface generation with the 3.75 m test sphere for the GMT Principal Test in the foreground.

GMT1 was cast in July 2005. The spin casting technique at SOML produces a rough parabolic surface that is cylindrically symmetric about the spin axis of the furnace. The mold that produced the internal honeycomb structure of the blank followed the asymmetric height variation of the final surface. Excess glass was subsequently removed during diamond wheel generating to give the front surface of the blank a uniformly thick faceplate. The generation of the blank was completed in January 2008. Loose abrasive grinding is currently underway in preparation for polishing of the mirror to its final figure. The work is being done with the 2m stressed lap at SOML with completion expected in 2009. Figure 4 shows GMT1 at the completion of generation and the 3.75m test sphere in its cell. The fabrication process is described in detail by Martin, et. al.10. 3.2. Metrology

The metrology for measuring the surface figure is the key to producing the primary mirror segments11. The mirrors will be polished to a surface accuracy of around 25 nm RMS, comparable to what has been achieved at SOML on previous 8.4m on-axis segments. The actual surface specification is a structure function with the additional constraint that the radius of curvature of the mirrors (Rc = 36m) must be held to a tolerance of +0.5mm in order that they all match. The strategy for achieving these requirements is two-fold: 1. Use multiple redundant tests capable of measuring critical parameter(s) with sufficient inherent precision to independently demonstrate compliance with the specifications, 2. Compensate for residual errors in fabrication using the active mirror supports in the telescope. Compensators include (a) the ability to accurately position the segments with 6 degrees of freedom, (b) active figure control of the mirror surface using the distributed force actuations of the mirror support system, and (c) position control of the secondary mirror segments. A set of four tests, listed in Table 3, are being developed to measure the surface figure. The Principal Test12 is used for day-to-day testing to guide polishing of the front surface. Two additional tests, the Laser Tracker Plus Test (LTP)13,14 and the Pentaprism Test (PT) 13,15, are sensitive to low-order aberrations (astigmatism, coma, trefoil, spherical aberration, etc.) in the surface and provide two independent verifications of the Principal Test results. A fourth test, the Shear Test, is sensitive to high frequency errors. The basic metrology for measuring the GMT segments has been developed on the 1.7 meter off-axis primary mirror segment being produced for the New Solar Telescope (NST)16,17. The NST is a 1/5 scale analog of the GMT segments. The tests will be modified for the on-axis central segment but are, in general, similar.

Table 3. GMT primary mirror front surface metrology Test Description Measures All aberrations below the spatial Principal Test Full aperture interferometric null test cutoff frequency. Verification test #1: Interferometer stabilized laser Laser Tracker Plus Low order aberrations. tracker to profile the surface along multiple scan lines. Verification test #2: Scanning pentaprism plus Pentaprism Test reference beam to measure slope errors along multiple Low order aberrations. scan lines. Verification test #3: Rotate the mirror about its optical Shear Test axis under the interferometer and difference the High spatial frequency errors. interferograms to measure local slope errors.

The tests are summarized below and described in detail in references 13, 14 and 15.

3.2.1. Principal Test In-process testing of the GMT segments uses a phase-shifting interferometer capable of imaging the full surface of GMT1 in a single frame. The test configuration is shown schematically in Figure 5. The segments are tested face-up in the tower. A 3.75 m diameter spherical mirror at the top folds the beam and allows the test to fit within the maximum height of the polishing lab. This fold mirror also removes most of the astigmatism introduced by viewing the off-axis segment face-on. The interferometer assembly (SAM) contains a second fold sphere to provide additional correction and a computer generated hologram (CGH) to correct residual wavefront errors. The removable Alignment CGH and Point Source Microscope (PSM) provide the means to align the internal components of SAM and align SAM with respect to the rest of the test optics.

Figure 5. Principal Test configuration.

The relative positions of the subassemblies and the mirror segment under test need to be determined within a tolerance of approximately +100 microns. To achieve this, a laser tracker will be mounted in the tower to sequentially measure the relative positions of reflective targets mounted on each of the subassemblies. The internal components in SAM must be positioned relative to each other to around 10μm. This will be achieved with careful metrology during assembly using an alignment hologram and point source microscope to check the position of M2. A separate interferometer mounted at ground level will monitor the shape of the front surface of the large fold sphere. Its position will also be monitored by the laser tracker. The measured misalignments in the test optics and deflections in the fold sphere will be backed out of the GMT1 measurement to get the final results. The pre-existing test tower at SOML was too narrow to accommodate the 36 m radius of curvature of GMT. The old tower was demolished and a new tower has been erected in its place. The current status is that the new tower is finished, the interferometer is being assembled, and the two fold spheres are being figured and close to being completed.

3.2.2. Laser Tracker Plus A laser tracker is used to profile the front surface of the segment in the Laser Tracker Plus test, shown in Figure 6. The laser tracker, made by FARO, measures the distance and x-y position of a sphere mounted retroreflector (SMR) that is scanned across the surface with a pulley system. The laser tracker is mounted as high as practical in the test tower to minimize the angle between the tracker beam and vector normal to the surface. Four Hewlett Packard distance measuring interferometers looking at fixed targets on the edge of the segment monitor the segment’s position relative to the laser tracker head. The required accuracy of each measurement is better than 2 microns rms. The measurement accuracy achieved on the NST mirror with a laser tracker was 0.5 microns rms. The LTP will be used during grinding to get the mirror figure close enough to its desired shape at the time of transition between grinding and polishing to allow fringes to be measurable with the Principle Test. The LTP will also provide a periodic check of the Principle Test results during figuring and at final acceptance. The GMT LTP has been assembled and mounted in the test tower. It currently is being tested on the 3.75m fold sphere.

Figure 6. Laser Tracker Plus test.

3.2.3. Pentaprism The pentaprism test consists of a fixed collimated light source and a pentaprism on a traveling stage mounted on a rail that is in a plane perpendicular to the axis of the parent optical surface for the off-axis segment (Figure 7). The pentaprism re-directs the light beam down to the surface of the segment where it is reflected up and comes to a focus near prime focus. A camera records the position of the image. Slope errors on the mirror surface will cause the position of the spot to change as the prism scans across the mirror. A second, fixed prism (not shown) on the rail provides a reference spot and a differential measurement of the spot displacement is made. This eliminates errors due to relative motions of the test components and the segment. The rail is clocked with respect to the segment to scan across different diameters. The GMT pentaprism camera and rail will be mounted in the SOML test tower. Design of the equipment is well advanced and ready for construction.

Figure 7. Pentaprism test. Figure 8 shows the pentaprism test on the NST mirror. Four discrete clocking angles are provided. The repeatability of the test in separate sets of measurements was 26 nm rms.

Figure 8. Pentaprism test on the NST mirror.

3.2.4. Accuracy of Tests The expected accuracy of the four tests has been analyzed and is summarized in Table 4. The residual errors shown in the table are correctable with the active optics system in the telescope to the accuracy that they can be measured with the active optics wavefront sensors. Table 4. Test accuracy for primary mirror segment fabrication.

4. ADAPTIVE OPTICS

An adaptive optics capability will be provided to take advantage of the effective 25 m diffraction limit of the telescope. The components of the AO system shown in Figure 9 consist of the deformable secondary mirror (AOSM), wavefront sensors, optical relay to the AO instruments and system18. A phasing camera (not shown) and edge sensors on the primary and secondary mirror will maintain wavefront continuity between the subapertures. Six laser beacons will create artificial reference stars in the sodium layer of the atmosphere. There will be lasers located on the azimuth platform. The laser beams will be relayed across the elevation axis and along the upper truss to a projector above the secondary mirror. The design and operation of the AO system has been described in the GMT Conceptual Design document3. Current efforts focus on modeling AO performance, design of the optical relay and wavefront sensors, and designing the phasing system.

Figure 9. GMT AO schematic Table 5 lists the AO modes that will be initially supported. More advanced AO modes, multi-conjugate AO (MCAO) and multi-object AO (MOAO), are future developments.

Table 5. GMT AO modes Focal Field Mode Coverage Science λ Imaging station (max) Diff. limited Laser tomography Folded J,H,K 1΄ (J,H,K) All sky 0.019″ at K (LTAO) Direct L,M 4΄ (L,M) SR = 72% Ground Layer Δfwhm = 0.1″ All sky Direct J,H,K Up to 8΄ (GLAO) (K =0.4″ fwhm) Bright Extreme J, H, K 80% SR (H) sources Folded Point source (ExAO) L,M 1:108 contrast (V<8)

5. INSTRUMENTS

Proposed first generation GMT instruments are listed in Table 6. Preliminary design concepts with varying levels of maturity have been developed for each instrument. During the current Design Development Phase of the project, conceptual designs and cost studies will be completed and, at the end of that process, the first complement of instruments for construction will be selected. It is currently expected that 3 major instruments will be built in the first round. Table 6. GMT instrument concepts.

GMACS and NIRMOS are multi-object spectrometers that work in the visible and near-IR respectively. These large instruments mount below the Instrument Platform in the GIR as shown in Figure 10. NIRMOS is lowered approximately 1m down when the GMACS is in use. The fold mirrors, field lens, and slit masks for GMACS are parked to the side when NIRMOS is raised. Service platforms and doors through the GIR provide access to the instruments. Additional access for servicing large subassemblies is provided by hatches in the IP above and with the service lift from below. The high resolution optical and IR spectrometers, QSPEC19 and GMTNIRS, will be mounted above the IP. SHARPS will be located in the Coude room on the azimuth platform. The AO instruments, MIISE, HRCAM, and GMTIFS, are also located on top of the IP and will be fed by the AO relay.

Figure 10. GIR assembly with GMACS and NIRMOS. 6. SITE

GMT will be located at Las Campanas Observatory, Chile. LCO has an established record of excellent seeing and stable seeing from decades of operations on the mountain. Over the past three years a campaign has been conducted to characterize three potential GMT sites 20,21. Quantities being monitored include (1) “seeing” measured with differential image motion monitors (DIMMs) and a multi-aperture scintillation sensor (MASS), (2) meteorological parameters including temperature, humidity, wind speed and direction, (3) precipital water vapor (PWV), and (4) cloud cover and transparency. Measurements are also being collected on Manqui Peak at the site of the Magellan 6.5 telescopes to serve as a benchmark. Two full years of simultaneous data has been recorded at the four sites. The quartile and 90 percentile seeing statistics are shown in Table 7. The measured seeing for the peak sites is identical within the statistical error. The Ridge site has slightly worse seeing. These results are consistent with the seeing campaign conducted in 1988-1989 for the Magellan Project and are borne out by the accumulated experience with the LCO telescopes.

Table 7. Seeing percentiles for LCO sites taken from November 2005 through November 2007.

FWHM 25% FWHM 50% FWHM 75% FWHM 90% Location arcsecs arcsecs arcsecs arcsecs

Manquis Ridge 0.55 0.68 0.87 1.10 Cerro Manqui 0.51 0.63 0.80 0.99 (Magellan) Cerro Alcaino 0.51 0.63 0.80 1.00 Cerro Las 0.51 0.64 0.81 1.00 Campanas

On the basis of the seeing measurements and a consideration of other factors including wind statistics and suitable topography to accommodate a large telescope facility, Cerro Las Campanas, Figure 11, has been identified as the site for GMT pending a general review by the project and ratification by the GMT Board. Currently a graded road runs to the summit and water and commercial power are available for development.

Figure 11. Cerro Las Campanas. 7. ENCLOSURE

Figure 12 shows the GMT enclosure and support facility on Cerro Las Campanas22. The structure stands 60m high by 55m in diameter. Vents in the walls of the enclosure provide a large open area for wind flushing of the telescope chamber. The vents can be throttled back during times of high wind. The main shutters also may be closed down around the telescope beam to reduce airflow and wind buffeting. An overhead crane is provided to remove the primary mirror assemblies from the telescope and translate them to ground level for re-coating in the Facility Building. The enclosure and telescope rotate independently of each other. The Facility Building provides space for assembling mirror cells and the coating plant, instrument assembly areas, offices for the support staff and visitors, mechanical systems (pumps, chillers, switch gear, etc.), and shops.

Figure 12. GMT Enclosure & Facility Building

8. PROJECT MILESTONES

Table 8. Project Milestones

Start of project December 2003 Conceptual Design Review February 2006 Begin Design Development Phase April 2007 GMTO Corporation formed August 2007 Construction Phase 2011-2018 First science use 2017

9. CONCLUSION

The GMT Project has completed the conceptual design of the telescope and facility and is currently in the Design Development Phase. An international consortium of partners has been assembled and set up the GMTO Corporation to complete the design, construct and operate the telescope. The first 8.4m off-axis segment of the primary mirror has been cast and is in the process of being finished in order to demonstrate the technology and establish the pipeline for producing these mirrors. A suite of natural seeing and adaptive optics instruments has been proposed and initial concept studies have been completed. Further development leading to a down select to the first generation instruments is planned. A site for GMT at Las Campanas Observatory has been identified on the basis of an on-going program of site characterization. The nominal GMT schedule has first science use starting in 2017.

10. ACKNOWLEGEMENTS

This material is based in part upon work supported by AURA through the National Science Foundation under Scientific Program Order No. 10 as issued for support of the Giant Segmented Mirror Telescope for the United States Astronomical Community, in accordance with Proposal No. AST-0443999 submitted by AURA.

11. REFERENCES

1. Johns, M., “The Giant Magellan Telescope (GMT)”, Proc. SPIE 6267, 2006. 2. GMT Science Working Group, “GMT Science Case”, www.gmto.org, 2004a 3. GMT Project, “Giant Magellan Telescope Conceptual Design Review”, www.gmto.org, 2006. 4. Gunnels, S., Davison, W., Cuerden, B., “The Giant Magellan Telescope (GMT) Structure”, in Astronomical Structures and Mechanisms Technology, J. Antebi, D. Lemke editors, Proceedings SPIE vol. 5495, 2004. 5. Johns, M., “GMT optical alignment, I”, GMT Document number 1339, 2005. 6. Kan, F. W., “GMT Dynamic Wind Response Study Report”, GMT document number 1356, 2005. 7. Kan, F. W., Eggers, D. W., “Wind vibration analysis of Giant Magellan Telescope”, Proc. SPIE 6271, 2006. 8. Gunnels, S., “The Giant Magellan Telescope (GMT): structure design update”, Proc. SPIE 7012, 2008; these proceedings. 9. Martin, H. M., Angel, J. R. P., Burge, J. H., Cuerden, B., Davison, W. B., Kingsley, J. S., Kot, L. B., Miller, S. D., Strittmatter, P. A., and Zhao, C., “Design and manufacture of the 8.4m primary mirror segments and supports for the GMT”, Proc. SPIE 6273, 2006. 10. Martin, H. M., Burge, J. H., Cuerden, B., Davison, W. B., Kingsley, J. S., Kittrell, C., Lutz, R. D., Miller, S. M., Zhao, C., Zobrist, T. L., “Progress in manufacturing the first 8.4m off-axis segment for the Giant Magellan Telescope”, Proc. SPIE 7018, 2008; these proceedings. 11. Burge, J. H., Davison, W. B., Zhao, C., Martin, H. M., “Development of surface metrology for the Giant Magellan Telescope primary mirror”, Proc. SPIE 7018, 2008; these proceedings. 12. Burge, J. H., L. Kot, L. B., Martin, H. M., Zhao, C., and Zehnder, R., “Design and analysis for interferometric testing of the GMT primary mirror”, Proc. SPIE 6273, 2006. 13. Burge, J. H., Zobrist, T. L., Kot, L. B., Martin, H. M., Zhao, C., “Alternate surface measurements for GMT primary mirror segments”, in Optomechanical Technologies for Astronomy, ed. E. Atad-Ettedgui, J. Antebi and D. Lemke, Proc. SPIE 6273, 2006. 14. Zobrist, T. L., Burge, J. H., Davison, W. B., Martin, H. M., “Measurement of large optical surfaces with a laser tracker”, Proc. SPIE 7018, 2008; these proceedings. 15. Su, P., Burge, J. H., Cuerden, B., Martin, H. M., “Scanning pentaprism measurement of off-axis aspherics”, Proc. SPIE 7018, 2008; these proceedings. 16. Martin, H. M., Burge, J. H., Cuerden, B., Miller, S. M., Smith, B., Zhao, C., “Manufacture of 8.4 m off-axis segments: a 1:5 scale demonstration”, Optical Fabrication, Metrology, and Material Advancements for Telescopes, ed. Eli Atad-Ettedgui and Phillippe Dierickx, SPIE 5494, 2004. 17. Martin, H. M., Burge, J. H., Miller, S. M., Smith, B. K., Zehnder, R., and Zhao, C., “Manufacture of a 1.7 m prototype of the GMT primary mirror segments” in Optomechanical Technologies for Astronomy, ed. E. Atad- Ettedgui, J. Antebi and D. Lemke, Proc. SPIE 6273, 2006. 18. Lloyd-Hart, M., Angel, J. R. P., “Design of the adaptive optics systems for GMT”, Proc. SPIE 6272, 2006. 19. Barnes, S. I., McQueen, P. J., “Q-Spec: a concept for the Giant Magellan Telescope high resolution optical spectrograph”, Proc. SPIE 7014, 2008; these proceedings. 20. Thomas-Osip, J. E., Johns, M. W., Phillips, M. M., Prieto, G., “Giant Magellan Telescope site evaluation and characterization at Las Campanas Observatory”, Proc. SPIE 7012, 2008; these proceedings. 21. Thomas-Osip, J. E., Bustos, E. B., Goodwin, M., Jenkins, C., Prieto, G., Tokovinin, A. A., “A campaign to compare three turbulence profiling techniques at Las Campanas Observatory”, Proc. SPIE 7014, 2008; these proceedings. 22. Teran, J., Neff, D. H., “Conceptual Design of the GMT Enclosure”, Proc. SPIE 6267, 2006. 23. Johns, M., “The Giant Magellan Telescope”, Proc. SPIE 6986, 2007.