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Telescope:
The GMT is a segmented mirror telescope that employs today's largest
stiff monolith mirrors as segments. Six off-axis 8.4m segments surround
a central on-axis segment, forming a single optical surface with a
collecting area equivalent to a filled aperture 21.4 meters in
diameter, and the resolving power of a 24.5-meter (80 foot) primary
mirror. The focal length is 18m; its focal ratio is 0.7. The mechanical
stiffness and short thermal time constants
of the honeycomb borosilicate mirrors produce outstanding images on the
Magellan telescopes in Chile, the Multiple Mirror Telescope (MMT) and the Large Binocular Telescope (LBT) in Arizona.
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The GMT secondary mirror is composed of seven thin adaptive shells,
with each segment mapping to a single primary mirror segment. The
adaptive secondary will provide diffraction-limited performance over
modest fields of view and ground-layer adaptive optics over a field of
ten to twenty arcminutes in diameter. The final focal ratio at the
straight Gregorian focus will be f/8.4 with an image scale of one
arcsecond per millimeter.
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The telescope mount is based on two C-rings
connected with a series of
K-braces. The mount has high stiffness with a lowest resonance of 4.5
Hz. The secondary mirror is supported by a hybrid hexapod support
system that is composed of carbon fiber members attached to stiff steel
legs. The secondary support is light weight yet stiff. The structure of
the secondary support and the primary mirror geometery ensures that
there will be no extraneous sources of diffraction or thermal emission
within the aperture of the six outer segments. For this reason the
telescope will have both an exceptionally clean point-spread function
and low emissivity in the thermal infrared. The moving mass of the
telescope is just over 1000 tons.
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Enclosure:
The baseline GMT dome is cylindrical in shape and allows
for maximum ventilation. A series of louvers can be opened or closed to allow maximum
airflow while shielding the telescope from wind-loading. The dome shutters open in two
pieces, one below and one above the aperture. These shutters can also be used as wind
screens and moon-shades.
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Instruments:
Instruments will be mounted behind the central
primary
mirror segments. Two modes are planned: Large instruments will use a
straight-through light path and will utilize a large (6m x 5m) volume
behind the central segment. This focus will likely be used for the
wide-field optical and near-IR spectrographs and mid-IR imagers and
spectrographs. Smaller instruments can utilize a rotating instrument
platform fed by a set of folding flats. This focal station will support
smaller field of view instruments, rapid response instruments and
systems that require a high degree of stability (e.g. MCAO system).
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Site:
The GMT will be constructed at Cerro Las Campanas, Chile. World-renowned for its excellent seeing,
the outstanding image quality, dark skies and high fraction of
clear nights make Chile an ideal location for large telescopes. The
existing infrastructure at the site (roads, water system, connection to
the electrical grid, etc.) will reduce the cost of both construction
and operations for the GMT. The
scientific opportunities afforded by locations near the Large Atacama Millimeter Array (ALMA)
and other current and proposed large astronomical facilities make Chile
even more attractive as a site.
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