Science and Technology

Science Case and Technical Requirements

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The GMT Science Case flows from the Decadal Survey’s report “Astronomy and Astrophysics in the New Millennium.”

The GMT Science Book focuses on those areas of frontier science best explored with a large aperture ground-based telescope. This document can be downloaded here. (pdf)

The GMT Science Technical Requirements for the telescope and associated instruments and facilities flow from the science priorities listed in the GMT Science Book. These requirements are used to optimize the telescope design and development process, and to define the goals and requirements for the GMT first generation instruments.

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Primary Mirror

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MirrorLabVideoStill Inside Mirror Lab

The principal measure of the power of an optical/IR telescope is the diameter of its primary aperture.

Larger apertures translate into both greater light collecting area and, potentially, higher angular resolution. GMT’s primary mirror comprises seven segments that work together like a single mirror with the resolving power of a telescope 24 .5 meters (over 80 feet) in diameter. Each of GMT’s seven mirror segments is 8.4 meters in diameter. The limitation of the size of a single mirror segment is related to the technology available to manufacture and transport such a mirror. GMT’s 8.4-meter mirror segments are being developed at the University of Arizona’s Richard F. Caris Mirror Laboratory (RFCML).

The mirrors are made of low expansion glass molded into a light-weight honeycomb structure. The mirror segments are ground and polished to a precise optical prescription. The final polished surface departs from the desired shape by no more than 1/20 of a wavelength of green light, or approximately 25 nanometers. After polishing, the surface is coated with a thin layer of aluminum to achieve maximum reflectivity.

One of the mirror segments is mounted at the center axis of the telescope. The other six mirrors are mounted surrounding the center mirror segment. Each mirror segment is mounted into its own “cell,” a complex active support system that keeps the mirror in the proper position relative to the other segments at all times.

The most challenging aspect of making the GMT’s mirror segments arises from the asymmetric shape of the six outer segments. These mirrors have a steeply curved shape similar to that of a potato chip. The peak departure from a symmetric mirror is 14 mm at the edge, amounting to 28,000 waves of green light. A new suite of test instruments and procedures were developed to test these mirrors.

Fabrication

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fab_01 GMT1 mold is prepared by installation of hexagonal silica fiber cores.
[Photo: RFCML]
fab_02 GMT1 mold is loaded with 18 tons of glass in preparation for casting.
[Photo: RFCML]
fab_03 Glass is melted while entire furnace assembly is rotated.
[Photo: Pat McCarthy]
fab_04 The bottom side mirror blank is cleaned after successful casting.
[Photo: RFCML]
fab_05 “Load spreaders” are added on bottom side to distribute weight.
[Photo: RFCML]
fab_06 The top surface of the mirror is slowly polished into optical precision.
[Photo: RFCML]
fab_07 A computer controlled “stressed lap” tool is used to polish GMT’s aspheric surfaces. [Photo: RFCML]

The GMT mirrors are made at The Mirror Lab in Tucson, AZ.

Mirror fabrication occurs in 3 stages:

  1. Cast the mirror blank by melting glass in a rotating mold;
  2. Perform rough grinding of the front and back surfaces;
  3. Polish the front surface to optical tolerances.

In addition to these steps, the polished glass must be installed into its mirror cell. After it is transported to the mountain-top telescope site, the glass is given its reflective aluminum coating to make it a mirror. Finally the mirror cell assembly is mounted on the telescope structure for alignment and testing.

The mirror “blank” is formed by melting glass in a mold formed by high-temperature resistant refractory material.

The mold consists of a tub made of silicon carbide cement, lined with ceramic fiber. This tub will form the bottom half of a furnace to melt the glass. The mold is filled with about 1,700 alumina-silica fiber hexagonal boxes that form a honeycomb structure. The top of the boxes must follow the aspheric shape of the final mirror surface; no two are identical!

The honeycomb sandwich structure is formed by melting the glass into a mold which is the negative of the desired honeycomb shape. The glass fills the voids in this mold and becomes a honeycomb structure. The main components of the mold are a cylindrical tub which sets the outside shape and alumina-silica hexagonal columns or cores which form the voids in the honeycomb. Each of the hollow core boxes is held down against the floatation force with a special silicon carbide bolt. The cylindrical tub is held together by bands of Inconel 601 which wrap around the outside of the mold in a manner similar to hoops of steel around the staves of a barrel. After the honeycomb structure has been cast and cooled, the mold material is removed by breaking it apart with a high pressure water spray.

After the mold is prepared, the glass is placed inside the mold. Blocks of low expansion glass made by the Ohara Corporation of Japan are inspected and weighed. A total of 18 tons of glass are loaded into the mold, one piece at a time. Finally, the furnace lid is placed on top of the mold.

The mirror is made using a unique “spin cast” process whereby the furnace is rotated as the glass melts. This gives the mirror surface a rounded, or parabolic shape. The mirror will still require additional shaping by grinding to achieve optical tolerances. However, this process saves several tons of glass and significantly shortens the annealing and grinding time because the glass is already in a parabolic shape. Heating coils in the walls and lid of the furnace raise the temperature to 1160°C (or 2120°F) as it spins at 5 rpm. The temperature is maintained for four hours to allow the glass to melt and fill the mold. The glass is then cooled rapidly to 900°C, and then cooled more slowly for three months to avoid strains in the final mirror.

When the glass has cooled, the successfully cast mirror is lifted out of its mold. When tilted vertically, the mold’s attached “floor tiles” are visible on the rear surface of the glass. The tiles are removed and a high-pressure water spray is used to clean out the fiber boxes. This leaves behind a piece of glass with a honeycomb-like structure; the mirror is mostly empty space. This significantly reduces the mirror’s weight and enables its temperature to stabilize much more rapidly than a solid glass mirror.

The mirror is inverted, and the rear surface and edges are lapped and polished. 165 “loadspreaders” are bonded to distribute the weight of the mirror and provide a permanent attachment points to mount the mirror to its active support system.

The mirror is turned face up, and the front surface is ground to its approximate final shape with a series of diamond grinding wheels. The mirror’s surface is then polished to precise specifications.

The polishing system employs a “stressed-lap” polishing tool, which was developed for highly aspheric surfaces. The lap consists of a polishing disk which bends actively to match varying curvature of surface. This provides passive smoothing traditionally associated with spherical surfaces.

As the mirror is polished, it is tested using multiple, redundant measurements to assure it is precisely figured. This phase is called Optical Metrology and it is the most demanding part of all.

More information: “How do you build a mirror for one of the world’s biggest telescopes”, The Conversation, January 15, 2016.

Optical Metrology

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met_top An 82-foot high optical tower is
used to test each mirror to
extremely high precision.
met_top Technician inside optical tower
at Mirror Lab.

Due to the mirror arrangement in the GMT, each mirror has to be highly aspheric–that is, the mirror’s face has a steeply curved slope. One side of the mirror is thicker than the other, differing by as much as 14 mm.

The difficulty of shaping each mirror segment is compounded by the fact that these large segments must have the exact same curvature in order to perform together. To achieve the stringent demand for accuracy, each mirror is periodically taken off of the polishing machine and carefully measured. The results of these measurements, in turn, guide the polishing program as it progresses.

The principal test to compensate for the unprecedented asymmetry of the mirror’s surface is conducted using a laser interferometer. Instead of measuring the mirror directly, a beam is first bounced off of two mirrors at oblique angles, and then passed through a computer generated hologram. With the aspheric departure removed, interferometric surface measurements may be taken.

In theory, this is complicated, and in practice it is even more difficult. One of the bounce-off mirrors, called fold spheres, must be quite large: 3.8 meters in diameter. And, it must be positioned a considerable distance away from the primary mirror segment. The Test Tower protruding above the mirror lab actually had to be rebuilt to support this large fold sphere and accommodate the width of the interferometer beam.

Three additional methods are used to verify the mirror’s shape and direct the polishing activity:

  • The scanning pentaprism test is an independent measurement of low-order aberrations that guards against the possibility of a mistake in the implementation of the principal test.
  • The laser tracker measurement supports surface generation and loose-abrasive grinding processes by providing an independent measurement of radius of curvature and astigmatism.
  • The shear test, performed by displacing the segment by about ±0.5 m around the optical axis, will confirm the accuracy of the principal test on small scales.

For information regarding the science and implementation of mirror metrology, refer to the following publications:

Science Instruments

The GMTO Board of Directors has adopted an instrument development plan that follows the recommendations of the GMT Instrument Development Advisory Panel. Instrument development will be staged to match the technical development of the telescope and its adaptive optics system. Currently we are moving forward with four instruments and one facility fiber positioning system, summarized below. The summaries link to more information and related publications.

2015 ELT Detector Workshop

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Visible Echelle Spectrograph – G-CLEF
A high resolution, highly stable, fiber-fed visible light Echelle spectrograph well suited to precision radial velocity observations, investigations in stellar astrophysics and studies of the intergalactic medium. G-CLEF will operate from 350nm to 950nm with spectral resolutions ranging from 25,000 to 120,000.

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Visible Multi-Object Spectrograph – GMACS
A high throughput, general purpose multi-object spectrograph optimized for observations of very faint objects. GMACS will be used for studies of galaxy evolution, evolution of the IGM and circumstellar matter, and studies of resolved stellar populations, among other applications.

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Near-IR IFU and Adaptive Optics Imager – GMTIFS
A general purpose, AO-fed near-infrared (0.9 to 2.5 microns) integral field spectrograph and adaptive optics imager. The IFU mode will support multiple spaxel scales with spectral resolutions of 5,000 or 10,000.

gmtnirs_th

IR Echelle Spectrograph – GMTNIRS
An AO-fed high-resolution, 1-5 micron narrow-field spectrograph aimed at studies of pre-main sequence objects, extrasolar planets, debris disks, and other mid-IR targets. The baseline configuration provides spectral resolutions ranging from 50,000 to 100,000.

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Facility Fiber Optics Positioner – MANIFEST
A facility fiber positioning system that covers GMT’s full corrected 20 arcmin field of view. MANIFEST can feed G-CLEF and GMACS simultaneously with fiber bundles that may be configured to increase spectroscopic multiplexing, spectral resolution, and other scientific capabilities.

comcam

Commissioning Camera – ComCam
The commissioning camera will be used to validate the Ground Layer Adaptive Optics (GLAO) performance of the GMT facility Adaptive Optics System. It is also needed for the initial alignment of the telescope and for verifying the natural seeing optical performance in the Direct Gregorian Narrow Field (DGNF) mode.

SPIE Publications: Copyright (2010, 2012) Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.

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Las Campanas

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The GMT will be built on a peak in the Andes Mountains near several existing telescope facilities at Las Campanas, Chile. The Las Campanas Observatory (LCO) location was selected for its high altitude, dry climate, dark skies, and unsurpassed seeing quality, as well as its access to the southern sky. Las Campanas Peak (“Cerro Las Campanas”), one of many peaks within LCO, has an altitude of over 2,550 meters (approximately 8,500 feet).

The GMT project is in the fortunate position of having clear access to an already developed site: road access, water, electrical power and communications are already in place. The site has a long history of excellent performance. Light pollution is negligible and likely to remain so for decades to come. The weather pattern has been stable for more than 30 years. There are also many interesting objects that are primarily visible from the southern hemisphere such as the large and small Magellanic clouds, which are our closest neighboring galaxies, and our own galactic center.

More Information on the Las Campanas site: Click to view/download a pdf file.

SPIE
Publications

 

Proc 9906 Ground-based and Airborne Telescopes VI
Proc 9906-35 GMT site, enclosure, and facilities design and development overview and update  Authors: Teran et al.
Proc 9906-37 Overview and status of the Giant Magellan Telescope Project (Invited Paper)  Authors: McCarthy et al.
Proc 9906-112 A 3D metrology system for the Giant Magellan Telescope  Authors: Rakich et al.
Proc 9908  Ground-based and Airborne Instrumentation for Astronomy VI
Proc 9908-68 Instrumentation progress at the Giant Magellan Telescope project (Invited Paper) Authors: Jacoby et al.
Proc 9908-72  GMTIFS: The Giant Magellan Telescope integral fields spectrograph and imager  Authors: Sharp et al.
Proc 9908-75  GMTNIRS: progress toward the Giant Magellan Telescope near-infrared spectrograph   Authors: Jaffee et al.
Proc 9908-76  The GMT-Consortium Large Earth Finder (G-CLEF): an optical Echelle spectrograph for the Giant Magellan Telescope (GMT)  Authors: Szentgyorgyi et al.
Proc 9908-375 The Giant Magellan Telescope multi-object astronomical and cosmological spectrograph (GMACS)  Authors: Prochaska et al.
Proc 9908-376 Optical design concept for the Giant Magellan Telescope Multi-object Astronomical and Cosmological Spectrograph (GMACS)  Authors: Schmidt et al.
Proc 9909  Adaptive Optics Systems V
Proc 9909-67 The GMT active and adaptive optics control strategies  Authors: Conan et al.
Proc 9911  Modeling, Systems Engineering, and Project Management for Astronomy VI
Proc 9911-41 Initial computational fluid dynamics modeling of the Giant Magellan Telescope site and enclosure  Authors: Danks et al.
Proc 9911-42 Computational fluid dynamics modeling and analysis for the Giant Magellan Telescope (GMT)  Authors: Ladd et al.
Proc 9912 Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II
Proc 9912-24 New and improved technology for manufacture of GMT primary mirror segments  Authors: Kim et al.
Proc 9912-30 Status of mirror segment production for the Giant Magellan Telescope  Authors: Martin et al.
Proc 9912-43 GMTIFS: the adaptive optics beam steering mirror for the GMT integral-field spectrograph Authors: Davies et al.
Proc 9913 Software and Cyberinfrastructure for Astronomy IV
Proc 9913-62 Software requirements flowdown and preliminary software design for the G-CLEF spectrograph  Authors: Evans, et al.
Proc 7735 Ground-based and Airborne Instrumentation for Astronomy III
Proc 7735-025 GMTNIRS (Giant Magellan Telescope near-infrared spectrograph): design concept    Authors: Lee, S. et al.
Proc 8444 Ground-based and AirborneTelescopes IV
Proc 8444-022 Progress on the Structural and Mechanical Design of the Giant Magellan Telescope    Authors: Sheehan, M. et al.
Proc 8444-029 GMT Enclosure Wind and Thermal Study    Authors: Farahani et al.
Proc 8444-030 Vibration mitigation for wind-induced jitter for the Giant Magellan Telescope    Authors: Glaese, R. M. et al.
Proc 8444-052 Giant Magellan Telescope: Overview    Authors: Johns, M. et al.
Proc 8444-075 Phasing Metrology System for the GMT    Authors: Acton, S.D. et al.
Proc 8444-114 Modeling seismic behavior of static supports of Giant Magellan Telescope    Authors: Kan, F. W. et al.
Proc 8444-117 Development of a fast steering secondary mirror prototype for the Giant Magellan Telescope    Authors: Cho, M. K. et al.
Proc 8446 Ground-based and Airborne Instrumentation for Astronomy IV
Proc 8446-050 The instrument development and selection process for the Giant Magellan Telescope    Authors: Jacoby, G. et al.
Proc 8446-301 GMTNIRS mechanical design    Authors: Beets, T. A. et al.
Proc 8446-289 MANIFEST instrument concept and related technologies    Authors: Goodwin, M. et al.
Proc 8446-058 The GMACS spectrograph for the Giant Magellan Telescope    Authors: DePoy D. L et al.
Proc 8446-293 Conceptual optical design for GMACS, a wide-field, multi-object, moderate resolution optical spectrograph for the Giant Magellan Telescope    Authors: Barkhouser, R. H. et al.
Proc 8446-294 Optomechanical design concept for GMACS: a wide-fi eld multi-object moderate resolution optical spectrograph for the Giant Magellan Telescope    Authors: Smee, S. A. et al.
Proc 8446-052 The GMT-CfA, Carnegie, Catolica, Chicago Large Earth Finder (G-CLEF): a general purpose optical echelle spectrograph for the GMT with precision radial velocity capability    Authors: Szentgyorgyi, A. et al.
Proc 8446-053 The GMT integral-field spectrograph (GMTIFS) conceptual design    Authors: McGregor, P. J. et al.
Proc 8446-059 NIRMOS: a wide-field near-infrared spectrograph for the Giant Magellan Telescope    Authors: Fabricant, D. G. et al.
Proc 8446-060 TIGER: a high contrast infrared imager for the Giant Magellan Telescope    Authors: Hinz, P. M. et al.
Proc 8447 Adaptive Optics Systems III
Proc 8447-043 Design of a truth sensor for the GMT laser tomography adaptive optics system    Authors: van Dam, M. et al.
Proc 8447-054 The Giant Magellan Telescope adaptive optics program    Authors: Bouchez, A. et al.
Proc 8447-057 Wavefront sensor design for the GMT natural guide star AO system    Authors: Esposito, S. et al.
Proc 8447-135 The Giant Magellan Telescope Laser Tomography Adaptive Optics System    Authors: Conan, R. et al.
Proc 8447-136 Optical designs of the LGS WFS system for GMT-LTAO    Authors: Wang, M. et al.
Proc 8447-137 Design and Predicted Performance of the GMT Ground-Layer Adaptive Optics System    Authors: Hinz, P. M. et al.
Proc 8447-138 The Giant Magellan Telescope phasing system    Authors: Bouchez, A. et al.
Proc 8447-187 A prototype phasing camera for the Giant Magellan Telescope    Authors: Kanneganti, S. et al
Proc 8447-202 GMT AO System Requirements and Error Budgets in the Preliminary Design Phase    Authors: Trancho, G. et al
Proc 8449 System Engineering
Proc 8449-005 Systems Engineering Implementation in the Preliminary Design Phase of the Giant Magellan Telescope    Authors: Maiten, J. et al.
Proc 8450 Technology Advancements
Proc 8450-031 SCOTS: a large dynamic range reverse Hartmann test for Giant Magellan Telescope primary mirrors    Authors: Su, P. et al.
Proc 8450-090 Production of 8.4 m segments for the Giant Magellan Telescope    Authors: Martin, H. M. et al.
Proc 8451 Modern Technologies in Space and Ground-based Telescopes and Instrumentation II
Proc 8451-131 GMT software and controls overview    Authors: Filgueira J. M. et al.
Proc 9145-47 Overview and Status of the Giant Magellan Telescope Project    Authors: Bernstein R. A. et al.
Proc 9147-341 The MANIFEST fibre positioning system for the Giant Magellan Telescope    Authors: Lawrence J. S. et al.
Proc 9147-78 A preliminary design for the GMT-consortium large Earth finder (G-CLEF)    Authors: Szentgyorgyi A. et al.
Proc 9148-31 The Giant Magellan Telescope Adaptive Optics Program    Authors: Bouchez A. H. et al.
Proc 9148-38 Wide field adaptive optics correction for the GMT using natural guide stars    Authors: van Dam M. A. et al.
Proc 9152-58 End-to-end observatory software modeling using domain specific languages overview    Authors: Filgueira J. M. et al.

Copyright Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.