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News Article | January 15, 2016

When astronomers point their telescopes up at the sky to see distant supernovae or quasars, they’re collecting light that’s traveled millions or even billions of light-years through space. Even huge and powerful energy sources in the cosmos are unimaginably tiny and faint when we view them from such a distance. In order to learn about galaxies as they were forming soon after the Big Bang, and about nearby but much smaller and fainter objects, astronomers need more powerful telescopes. Perhaps the poster child for programs that require extraordinary sensitivity and the sharpest possible images is the search for planets around other stars, where the body we’re trying to detect is extremely close to its star and roughly a billion times fainter. Finding earth-like planets is one of the most exciting prospects for the next generation of telescopes, and could eventually lead to discovering extraterrestrial signatures of life. Detectors in research telescopes are already so sensitive that they capture almost every incoming photon, so there’s only one way to detect fainter objects and resolve structure on finer scales: build a bigger telescope. A large telescope doesn’t just capture more photons, it can also produce sharper images. That’s because the wave nature of light sets a limit to the telescope’s resolution, known as the diffraction limit; the sharpness of the image depends on the wavelength of the light and the telescope’s diameter. As optical scientists, our contribution to the next generation of telescopes is figuring out how to craft the gargantuan mirrors they rely on to collect light from far away. Here’s how we’re perfecting the technology that will enable tomorrow’s astrophysical discoveries. The question is how to build something substantially bigger than the current generation of telescopes, which have effective diameters of 8 to 12 meters (26 to 40 feet). One of the biggest challenges is making a bigger mirror to collect the light. First, it helps to know the basic optical layout of a telescope, illustrated here by the Giant Magellan Telescope (GMT) that is being built in Chile. A large primary mirror collects incoming light and reflects it to a focus. The light is reflected a second time by the smaller secondary mirror, to form an image on an instrument located at a safe, accessible place below the primary mirror, where the image is recorded. A mirror much larger than eight meters, made of a single piece of glass, would be too expensive and too hard to handle. Everyone involved in building giant telescopes agrees that the solution is to make the primary mirror out of multiple smaller mirrors. Multiple pieces of glass are shaped and aligned to form one gigantic mirror, called a segmented mirror. Gaps between the segments are acceptable as long as the segments' surfaces lie on a continuous nearly parabolic surface, called the parent surface. The three extremely large telescope (ELT) projects now in development have made very different decisions about the design of this segmented primary mirror. Two of the ELTs, the European ELT and the Thirty Meter Telescope, have adopted the approach pioneered by the 10-meter Keck Observatory telescopes in Hawaii — they’ll make a giant mirror out of hundreds of 1.5-meter segments. The third project, the Giant Magellan Telescope, takes a different tack. Its 25-meter primary mirror will have only seven segments. They’re the largest single mirrors that can be made, the 8.4-meter (28-foot) honeycomb mirrors we produce here at the Richard F. Caris Mirror Lab at the University of Arizona. The GMT’s 3-meter secondary mirror also has seven segments, each paired with one of the primary mirror segments. Artist’s representation of the seven giant mirrors installed in the Giant Magellan Telescope. Giant Magellan Telescope — GMTO Corporation Big mirror segments guarantee a smooth surface over their entire large areas. The more segments there are in the primary mirror, the more its accuracy depends on their precise alignment to keep them on the parent surface. Because of the pairing of primary and secondary mirror segments in the GMT, the fine control needed to form sharp images can be done by moving the small, agile segments of the secondary mirror rather than the 8.4-meter primary segments. A second advantage of the 8.4-meter honeycomb mirrors is their strong legacy, including use in what is currently the world’s largest telescope, the Large Binocular Telescope here in Arizona. One of the challenges of using a large mirror is that it tends to bend under its own weight and the force of wind. The mirror is exposed to wind like a sail on a yacht, but it can only bend by about 100 nanometers before its images become too blurry. The best way to overcome this problem is to make the mirror as stiff as is practical, while also limiting its weight. We accomplish this feat by casting the mirror into a lightweight honeycomb structure. Each mirror has a continuous glass facesheet on top and an almost continuous backsheet, each about one inch thick. Holding the two sheets together is a honeycomb structure consisting of half-inch-thick ribs in a hexagonal pattern. Our honeycomb mirrors are 70 centimeters thick, making them stiff enough to withstand the forces of gravity and wind. But they’re 80 percent hollow and weigh about 16 tons each, light enough that they don’t bend significantly under their own weight. Mold for casting an 8.4-meter honeycomb mirror for the GMT. The glass will melt around the hexagonal boxes to form the honeycomb. Ray Bertram, Steward Observatory We start by melting glass into a complex mold that’s the negative of the honeycomb mirror we want to end up with. While the glass is molten, the furnace spins at five revolutions per minute; the centrifugal force pushes the glass' surface into the concave parabolic shape that can focus light from a distant star. Watch the video below to see the construction of the honeycomb mold and the spin-casting process. The spin-cast mirror surface doesn’t yet have the optical quality needed to make sharp images. But spinning gives it the right overall curvature and saves our having to grind out 14 tons of glass from a flat surface — almost as much glass as is left in the finished mirror. Next we need to polish the surface to an accuracy of a small fraction of the light’s wavelength, so it will form the sharpest images possible. The mirror surface has to match the ideal, nearly parabolic surface to about 25 nanometers — about 3 ten-thousandths of the width of a human hair. That’s really, really smooth; if the mirror were scaled up to the size of North America, the tallest mountain would be one inch high and the deepest canyon would be one inch low. To guide our polishing, the first step is to create a superfine contour map of the mirror’s surface, with steps of less than 10 nanometers. As our “ruler,” we use red laser light; its divisions are the light’s wavelength — about 630 nanometers — and it can be read to about one hundredth of a division. The measuring instrument illuminates the mirror surface, collects the reflected light, and compares the path lengths of the rays reflected by different locations on the mirror. A ray that reflects off a high spot will have a shorter path than a ray that hits a low spot. The instrument uses this information to construct the contour map of the mirror’s surface. The basic principle of polishing is to rub the surface with a disk-shaped tool, removing glass selectively from the spots that are too high. A fine abrasive such as rouge (iron oxide) slowly removes glass, atom by atom, through mechanical and chemical processes. Figuring is removing glass explicitly from high spots identified in the contour map, for example by having the tool rub there longer. This is effective on scales larger than about 10 centimeters. Smoothing is what happens when you rub a stiff tool over a rough surface: the tool naturally sits on the high spots and removes more material there, even without any guidance from a contour map. This is effective on scales smaller than 10 centimeters. Both methods are more difficult when the mirror surface is aspheric, meaning its curvature changes from point to point, which is very much the case for the GMT segments. An 8.4-meter mirror for the Large Synoptic Survey Telescope being polished at the Richard F. Caris Mirror Lab. Steward Observatory, CC BY-ND We’ve developed several new polishing tools to address the challenges of polishing large mirrors for telescopes. One essential feature of any polishing tool is that it match the shape of the mirror surface to an accuracy of around 1 micron. The larger tool in the background is a complex electro-mechanical system that changes the shape of a stiff aluminum disk as it moves over the surface, so it always matches the local curvature of the mirror. The smaller tool in the foreground is much simpler. Similar to Galileo’s reinvention of a carnival toy as an astronomical telescope, our new idea came from Silly Putty — a non-Newtonian fluid that flows like a liquid over a long period of time but acts like a solid on short timescales. We harness those intrinsic properties to achieve both figuring and smoothing. Our tool, containing Silly Putty enclosed by a thin rubber diaphragm, slowly moves over the surface of the mirror while simultaneously rapidly orbiting around itself. The Silly Putty is stiff over the quick period of the orbit, which smooths out small-scale irregularities in the mirror surface. Over the longer time it takes to move across the mirror, the Silly Putty flows easily, so the tool always matches the surface’s shape. As a result, it removes glass at a predictable rate and in a predictable pattern that doesn’t vary as it moves across the mirror. The Giant Magellan Telescope as it will look after construction on Cerro Las Campanas in Chile. Giant Magellan Telescope — GMTO Corporation, CC BY Here at the Mirror Lab, we finished making the first Giant Magellan Telescope segment in 2012. After a pause for work on two other mirrors, the lab is in the process of grinding Segments 2 and 3. Segment 4 has just finished cooling to room temperature after spin-casting in September 2015. We are well on the way to manufacturing the full 25-meter primary mirror. Getting these near-perfect mirrors from our lab in Arizona to a mountaintop in Chile presents another set of challenges. They travel by tractor-trailer on land, and by freight ship from California to Chile. The keys to safe transport are distributing the weight of the mirror over hundreds of support points and having several layers of suspension between the mirror and the road or ship deck. The GMT project schedule calls for a preliminary first light, with four segments installed in the telescope, in 2022. We expect all seven segments to be scanning the cosmos starting in 2024. Many of us who work on the GMT see it as the way to open new windows into the universe, as the Hubble Space Telescope (HST) has done over the last 25 years. That orbiting telescope was a generous gift to the next generation from the people who worked on the project for decades before it launched. HST’s deep space images amazed, motivated and inspired many of us on Earth. The GMT project team dreams of passing on a similar gift for future generations. Buddy Martin, Project Scientist at the Steward Observatory and Associate Research Professor of Optical Sciences, University of Arizona and Dae Wook Kim, Assistant Professor of Optical Sciences, University of Arizona. This article was originally published on The Conversation. Read the original article.

Green G.M.,Harvard - Smithsonian Center for Astrophysics | Schlafly E.F.,Max Planck Institute for Astronomy | Finkbeiner D.P.,Harvard - Smithsonian Center for Astrophysics | Rix H.-W.,Max Planck Institute for Astronomy | And 13 more authors.
Astrophysical Journal | Year: 2015

We present a three-dimensional map of interstellar dust reddening, covering three-quarters of the sky out to a distance of several kiloparsecs, based on Pan-STARRS 1 (PS1) and 2MASS photometry. The map reveals a wealth of detailed structure, from filaments to large cloud complexes. The map has a hybrid angular resolution, with most of the map at an angular resolution of 3′.4-13′.7, and a maximum distance resolution of ∼25%. The threedimensional distribution of dust is determined in a fully probabilistic framework, yielding the uncertainty in the reddening distribution along each line of sight, as well as stellar distances, reddenings, and classifications for 800 million stars detected by PS1. We demonstrate the consistency of our reddening estimates with those of twodimensional emission-based maps of dust reddening. In particular, we find agreement with the Planck t353GHz -based reddening map to within 0.05 mag in E (B-V) to a depth of 0.5 mag, and explore systematics at reddenings less than E (B-V) ≈ 0.08 mag. We validate our per-star reddening estimates by comparison with reddening estimates for stars with both Sloan Digital Sky Survey photometry and Sloan Extension for Galactic Understanding and Exploration spectral classifications, finding per-star agreement to within 0.1 mag out to a stellar E (B-V) of 1 mag. We compare our map to two existing three-dimensional dust maps, by Marshall et al. and Lallement et al., demonstrating our finer angular resolution, and better distance resolution compared to the former within ∼3 kpc. The map can be queried or downloaded at We expect the three-dimensional reddening map presented here to find a wide range of uses, among them correcting for reddening and extinction for objects embedded in the plane of the Galaxy, studies of Galactic structure, calibration of future emission-based dust maps, and determining distances to objects of known reddening. © 2015. The American Astronomical Society. All rights reserved.

Bouchez A.H.,GMTO Corporation | McLeod B.A.,Smithsonian Astrophysical Observatory | Acton D.S.,Ball Corporation | Kanneganti S.,Smithsonian Astrophysical Observatory | And 3 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2012

The 25 m Giant Magellan Telescope consists of seven circular 8.4 m primary mirror segments with matching segmentation of the Gregorian secondary mirror. Achieving the diffraction limit in the adaptive optics observing modes will require equalizing the optical path between pairs of segments to a small fraction of the observing wavelength. This is complicated by the fact that primary mirror segments are separated by up to 40 cm, and composed of borosilicate glass. The phasing system therefore includes both edge sensors to sense high-frequency disturbances, and wavefront sensors to measure their long-term drift and sense atmosphere-induced segment piston errors. The major subsystems include a laser metrology system monitoring the primary mirror segments, capacitive edge sensors between secondary mirror segments, a phasing camera with a wide capture range, and an additional sensitive optical piston sensor incorporated into each AO instrument. We describe in this paper the overall phasing strategy, controls scheme, and the expected performance of the system with respect to the overall adaptive optics error budget. Further details may be found in specific papers on each of the subsystems. © 2012 SPIE.

Puzia T.H.,University of Santiago de Chile | Miller B.W.,Gemini Observatory | Trancho G.,Gemini Observatory | Trancho G.,GMTO Corporation | And 3 more authors.
Astronomical Journal | Year: 2013

We present the calibration of the spectroscopic Lick/IDS standard line-index system for measurements obtained with the Gemini Multi-Object Spectrographs known as GMOS-North and GMOS-South. We provide linear correction functions for each of the 25 standard Lick line indices for the B600 grism and two instrumental setups, one with 0.″5 slit width and 1 × 1 CCD pixel binning (corresponding to ∼2.5 Å spectral resolution) and the other with 0.″75 slit width and 2 × 2 binning (∼4 Å). We find small and well-defined correction terms for the set of Balmer indices Hβ, HγA , and HδA along with the metallicity sensitive indices Fe5015, Fe5270, Fe5335, Fe5406, Mg2, and Mgb that are widely used for stellar population diagnostics of distant stellar systems. We find other indices that sample molecular absorption bands, such as TiO1 and TiO2, with very wide wavelength coverage or indices that sample very weak molecular and atomic absorption features, such as Mg1, as well as indices with particularly narrow passband definitions, such as Fe4384, Ca4455, Fe4531, Ca4227, and Fe5782, which are less robustly calibrated. These indices should be used with caution. © 2013. The American Astronomical Society. All rights reserved.

Danks R.,Davies and Irwin Inc. | Smeaton W.,Davies and Irwin Inc. | Bigelow B.,GMTO Corporation | Burgett W.,GMTO Corporation
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2016

In the era of extremely large telescopes (ELTs), with telescope apertures growing in size and tighter image quality requirements, maintaining a controlled observation environment is critical. Image quality is directly influenced by thermal gradients, the level of turbulence in the incoming air flow and the wind forces acting on the telescope. Thus any ELT enclosure must be able to modulate the speed and direction of the incoming air and limit the inflow of disturbed ground-layer air. However, gaining an a priori understanding of the wind environment's impacts on a proposed telescope is complicated by the fact that telescopes are usually located in remote, mountainous areas, which often do not have high quality historic records of the wind conditions, and can be subjected to highly complex flow patterns that may not be well represented by the traditional analytic approaches used in typical building design. As part of the design process for the Giant Magellan Telescope at Cerro Las Campanas, Chile; the authors conducted a parametric design study using computational fluid dynamics which assessed how the telescope's position on the mesa, its ventilation configuration and the design of the enclosure and windscreens could be optimized to minimize the infiltration of ground-layer air. These simulations yielded an understanding of how the enclosure and the natural wind flows at the site could best work together to provide a consistent, well controlled observation environment. Future work will seek to quantify the aerothermal environment in terms of image quality. © 2016 SPIE.

Maitena J.,GMTO Corporation | Johnsa M.,GMTO Corporation | Tranchoa G.,GMTO Corporation | Sawyera D.,Cognition Corporation | Madyb P.,Cognition Corporation
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2012

Like many telescope projects today, the 24.5-meter Giant Magellan Telescope (GMT) is truly a complex system. The primary and secondary mirrors of the GMT are segmented and actuated to support two operating modes: natural seeing and adaptive optics. GMT is a general-purpose telescope supporting multiple science instruments operated in those modes. GMT is a large, diverse collaboration and development includes geographically distributed teams. The need to implement good systems engineering processes for managing the development of systems like GMT becomes imperative. The management of the requirements flowdown from the science requirements to the component level requirements is an inherently difficult task in itself. The interfaces must also be negotiated so that the interactions between subsystems and assemblies are well defined and controlled. This paper will provide an overview of the systems engineering processes and tools implemented for the GMT project during the preliminary design phase. This will include requirements management, documentation and configuration control, interface development and technical risk management. Because of the complexity of the GMT system and the distributed team, using web-accessible tools for collaboration is vital. To accomplish this GMTO has selected three tools: Cognition Cockpit, Xerox Docushare, and Solidworks Enterprise Product Data Management (EPDM). Key to this is the use of Cockpit for managing and documenting the product tree, architecture, error budget, requirements, interfaces, and risks. Additionally, drawing management is accomplished using an EPDM vault. Docushare, a documentation and configuration management tool is used to manage workflow of documents and drawings for the GMT project. These tools electronically facilitate collaboration in real time, enabling the GMT team to track, trace and report on key project metrics and design parameters. © 2012 SPIE.

Johns M.,GMTO Corporation | McCarthy P.,GMTO Corporation | Raybould K.,GMTO Corporation | Bouchez A.,GMTO Corporation | And 5 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2012

The Giant Magellan Telescope (GMT) is a 25-meter optical/infrared extremely large telescope that is being built by an international consortium of universities and research institutions. It will be located at the Las Campanas Observatory, Chile. The GMT primary mirror consists of seven 8.4-m borosilicate honeycomb mirror segments made at the Steward Observatory Mirror Lab (SOML). Six identical off-axis segments and one on-axis segment are arranged on a single nearly-paraboloidal parent surface having an overall focal ratio of f/0.7. The fabrication, testing and verification procedures required to produce the closely-matched off-axis mirror segments were developed during the production of the first mirror. Production of the second and third off-axis segments is underway. GMT incorporates a seven-segment Gregorian adaptive secondary to implement three modes of adaptive-optics operation: natural-guide star AO, laser-tomography AO, and ground-layer AO. A wide-field corrector/ADC is available for use in seeing-limited mode over a 20-arcmin diameter field of view. Up to seven instruments can be mounted simultaneously on the telescope in a large Gregorian Instrument Rotator. Conceptual design studies were completed for six AO and seeing-limited instruments, plus a multi-object fiber feed, and a roadmap for phased deployment of the GMT instrument suite is being developed. The partner institutions have made firm commitments for approximately 45% of the funds required to build the telescope. Project Office efforts are currently focused on advancing the telescope and enclosure design in preparation for subsystem- and system-level preliminary design reviews which are scheduled to be completed in the first half of 2013. © 2012 SPIE.

Farahani A.,GMTO Corporation | Kolesnikov A.,CPP | Cochran L.,CPP | Hull C.,GMTO Corporation | Johns M.,GMTO Corporation
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2012

The GMT (Giant Magellan Telescope) is a large ground-based telescope for astronomical research at optical and infrared wavelengths. The telescope is enclosed inside an Enclosure that rotates to follow the tracking of the telescope. The Enclosure is equipped with adjustable shutters and vents to provide maximum ventilation for thermal control while protecting the telescope from high wind loads, stray light, and severe weather conditions. The project will be built at Las Campanas Observatory in Chile on Cerro Las Campanas. The first part of this paper presents the wind tunnel test data as well as CFD (Computational Fluid Dynamics) study results for the GMT Enclosure. The wind tunnel tests include simulations for: a) Topography, b) Open Enclosure (all the shutters and vents open), and c) Closed Enclosure (all the vents and shutters closed). The CFD modeling was carried out for a wide range of conditions such as low and high wind speeds at various wind directions, and for the fully open and partially open Enclosure. The second part of this paper concerns the thermal effects of the Enclosure steel members. The wind speed and member sizes have been studied in relation to the required time to reach a defined temperature inside the Enclosure. This is one of the key performance characteristics of the Enclosure that can affect "Dome Seeing" significantly. The experimental data and theoretical predications have been used to identify the areas inside the Enclosure that need to be ventilated. The Enclosure thermal control strategy has been determined and an optimized system has been designed based on the final results. © 2012 SPIE.

Hull C.,GMTO Corporation
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2014

Hydrostatic bearings are a key component for many large telescopes due to their high load bearing capacity, stiffness and low friction. A simple technique is presented to model these bearings to understand the effects of geometry, oil viscosity, flow control, temperature, etc. on the bearings behavior. © 2014 SPIE.

Shectman S.,GMTO Corporation | Johns M.,GMTO Corporation
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

The 25-meter Giant Magellan Telescope (GMT) is one of the next generation of extremely large ground-based optical/infrared telescopes. GMT is currently under development by a consortium of major US and international university and research institutions. The telescope will be located at the Las Campanas Observatory in Chile. The GMT Project is mid-way through its Design Development Phase. This paper summarizes the organizational structure and status of the GMT Project and recent progress in the technical development of the various GMT subsystems. © 2010 SPIE.

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