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The Giant Magellan Telescope. AAS San Diego January 11, 2005. Matt Johns. The GMT Institutions. Carnegie Observatories Harvard University Smithsonian Astrophysical Observatory Massachusetts Institute of Technology University of Arizona University of Michigan University of Texas, Austin

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the giant magellan telescope

The Giant Magellan Telescope

AAS San Diego January 11, 2005

Matt Johns


The GMT Institutions

Carnegie Observatories

Harvard University

Smithsonian Astrophysical Observatory

Massachusetts Institute of Technology

University of Arizona

University of Michigan

University of Texas, Austin

Texas A&M University


the gmt organization
The GMT Organization
  • Memorandum of Understanding
  • Conceptual design phase funding.
  • Work toward GMT incorporation agreement
  • Governing bodies
    • GMT Board: each institution has two members
    • Science Working Group
    • Project Scientists’ Working Group
    • AO & Instrumentation Groups
    • Project Office
gmt design
GMT Design
  • Alt-az structure
  • Seven 8.4-m primary mirrors
    • Cast borosilicate honeycomb
    • 25.3-m enclosed diameter
    • 24-m diffraction equivalent
    • 21.5-m equivalent aperture
  • 3.2-m adaptive Gregorian secondary mirror
  • Instruments mount below M1 at the Gregorian focus
gmt optical design
GMT Optical Design

Primary Mirror

D1 = 25.3 meter

R1 = 36.0 meters

K = -0.9983

f/0.7 primary mirror overall

Gregorian secondary mirror

D2 = 3.2 meter

R2 = 4.2 meter

K2 = -0.7109

Segments aligned with primary mirrors

Combined Aplanatic Gregorian focus

f/8.2 final focal ratio

Field of view: 24 - 30 arc-min.

BFD = 5.5 meters

M2 conjugate = 160 m above M1

gmt structure
GMT Structure

Design goal: Compact, stiff Structure

Low wind cross-section

Maximize modal performance

Minimum swing radius -> cost

Model parameters

Analysis includes telescope structure, optics, & instrument load

Height = 36.1 meters

Moving mass = 991 metric tons

Lowest vibration mode = 5.1 Hz

exploits 8 4 m experience

8.4 m, f/1.14 LBT surface, 24 nm rms

Exploits 8.4 m experience
  • Large 8.4m diameter subapertures of well-corrected wavefront.
    • Co-phasing not needed for seeing-limited imaging at l<5 mm
  • Thick cross section (0.7m) resists surface deflection under wind loading.
  • Developed technology
    • Active supports maintain figure accuracy & alignment in the telescope.
    • Thermal Control  Settling time: 1/e < 1 hour
  • Existing production facilities & technology exists within the consortium at SOML.
preparations for casting gmt 1 8 4 m off axis segment
Preparations for Casting GMT 18.4-m off-axis segment
  • Primary mirror production
    • Pacing item for GMT completion
    • Requires development of off-axis technology
    • Modification of test tower
  • Prototype mirror
    • Casting contract signed December 2004
    • Projected casting date: July ‘05
stressed lap polishing machines at soml
Stressed Lap Polishing Machines at SOML

Test tower



Stressed lap

3 2 m segmented adaptive gregorian secondary mirror
3.2-m Segmented Adaptive Gregorian Secondary Mirror

64 cm MMT AO secondary mirror

Technology developed for MMT & LBT

7 ~2-mm thick facesheets aligned with Primary mirror segments attached to a single reference body.

~4700 voice coil actuators total

Laser projector rides on top.

adaptive optics modes
Adaptive Optics Modes

First Generation AO Capabilities

  • Ground layer AO (GLAO)
  • Laser tomography AO

Second Generation capabilities

  • Extreme (high contrast) AO (ExAO)
    • Ref. J. Codona, SPIE 5490-51.
  • Multi-conjugate AO (MCAO)

Adaptive secondary mirror is the first deformable element in all AO systems.

ground layer ao glao with gmt
Ground Layer AO (GLAO) with GMT
  • Emerging technology.
  • Low altitude turbulence correction.
  • Secondary conjugation at 160m above primary.
  • Natural guide stars or lasers.
  • Performance goals:
    • l> 0.8 m
    • Field of view: > 10’
    • Factor of 1.5-2 reduction in image size.
  • GLAO test at Magellan (A. Athey, SPIE 5490-179)
  • GLAO at MMT

Modeled using Cerro Pachon turbulence profile. (M-L Hart 2003)


Figure 89. (Left) Five beams projected on a 1 arcmin radius from a single 15–Watt laser using a custom hologram. The beams are seen here on the bottom of cloud. (Center) Images of the Rayleigh beacons gated between 20 – 30 kilometers without dynamic refocus. The streaking is caused by perspective elongation as seen through the off-axis 1.5-meter telescope. (Right) With dynamic refocus, the images become nearly circular.


6.4 arcsec H

2.7 arcsec V


2.8 arcsec

15w laser

20-30 km DR off

20-30 km DR on

  • Laser Tomography AO
    • Single conjugate AO with the AO secondary mirror & multiple lasers.
    • Diffraction limited imaging over full sky in the NIR.
    • Fields of view limited by tilt anisotropy
  • Prototype systems under development at 1.5m telescope & MMT
    • Rayleigh beacons with dynamic re-focusing (DR) (Stalcup,SPIE 5490-29).
    • Sodium lasers will be required to scale up to GMT (Angel, SPIE 5490-31).

Extreme AO

Radial average of GMT diffraction-limited PSFs with a bandpass of 1.57 to 1.73 microns. Blue dash is the normal profile. Red line is with apodization of individual segments. Green line is 150-degree average of the PSF formed by phase compensation applied to the adaptive secondary.

candidate first generation instruments
Candidate First Generation Instruments

Concepts under development

gmt instrument platform ip
GMT Instrument Platform (IP)


GLAO Guider

Folded port



NIR AO imager

NIR Echelle

Small-intermediate sized intstruments

Rapid exchange




6.4 m Dia.

7.6 m high

25 ton

Optical MOS


Mid-IR Spectrograh


Enclosure Structure

Height: 60 m

Diameter: 54 m

Structure design & cost study complete 12/04

Thermal & flow studies

On-site Facilities design mid-2005

M3 Engineering

site testing

Magellan (Manqui)

Campanas Pk.

Alcaino Pk.

Ridge (Manquis)

Site Testing

Northern Chile location

  • GMT conducting tests at 4 LCO sites
  • Coordinate/share data with other projects

Test equipment

  • Differential Image Motion Monitors (DIMM)
  • Multi-aperture Scintillation Sensor (MASS)
  • Meteorological stations
decadal survey key problems
Decadal Survey Key Problems
  • Large-Scale properties of the Universe, Matter, Energy, Expansion History
  • First Stars and Galaxies
  • Formation and Evolution of Black Holes
  • Formation of Stars and Planetary Systems
  • Impact of Astronomical Environment on the Earth

“Astronomy & Astrophysics in the New Millennium”

gsmt key science areas
GSMT Key Science Areas*
  • Origin of Large-Scale Structure
  • Building of the Milky Way and Other Galaxies
  • Exploring Other Solar Systems

*Frontier Science Enabled by a Giant Segmented Mirror Telescope

gmt science priorities
GMT Science Priorities*
  • Physical Studies of Exoplanets
  • Star Formation & the Origin of the IMF
  • Stellar Populations & Chemical Evolution
  • The Nature of Dark Matter and Dark Energy
  • Galaxy Assembly
  • Black Hole Growth
  • First Light & Reionization of the Universe

*The Giant Magellan Telescope: Opening a New Century of Cosmic Discovery


GMT Science & Technology in Context

The GMT Scientific priorities and capabilities:

  • Address the key decadal survey goals
  • Are aligned with the GSMT science priorities

The GMT design will readily

  • Adapt to new discoveries & evolving priorities
  • Enhance value of ALMA, JWST, & other existing and planned facilities

Reaching the diffraction limit of the GMT with adaptive optics

The central peak of the GMT PSF contains 65% of the total incident flux, compared to 84% for a filled circular aperture.

FWHM is the same as for 24 m filled aperture:

40 mas FWHM at 5 mm

8 mas FWHM at 1 mm

ground layer measured with laser at mmt 9 28 04
Ground layer measured with laser at MMT 9/28/04

Telescope measurement of ground layer seeing

(Michael Lloyd Hart et al)

5 Rayleigh beacons in 2 arcminute circle

30W 532 nm YAG laser

Centered around natural star in 0.7” seeing


Rms wavefront error summed over all 6 orders. bavg = average of all 5 LGS signals


GMT cophasing of 7 segments (Lloyd Hart)

Quarter-wave piston error




  • phase information for closed loop operation will come from a natural star.
  • A single NGS can sense the 6 relative pistons in addition to regular tip/tilt. (Need 8 modes from 7x8.4 m mirrors; better than 2 modes from 1x8.4 m on current large telescope with LGS, so should not at all compromise sky cover.)
  • Absolute piston measurement
  • Piston misregistration has unique effects on the MTF of three partially non-redundant arrays made from the full pupil.
    • Each M1 segment is used exactly twice.
    • Ideal PSFs are shown in second column.
    • A quarter wave of piston on either an edge or center segment will affect two of the three MTFs.
    • (N.B. MTFs are shown at much higher resolution than would actually need to be sampled.)
new test tower at mirror lab
New test tower at Mirror Lab

* Needed for 8.4 m off-axis segments

* Long 36 m radius of curvature (LBT = 20 m)

* Requires diffraction limited 4 m folding spherical mirror at top