TMT is a telescope designed for optical and infrared observing,
with the wavelengths of interest ranging from 310 nanometers in
the ultraviolet to ~30 microns in the infrared.
The telescope optical design is a folded Ritchey-Chrétien.
Both the primary and secondary mirrors are hyperboloidal, and
together they form a well-corrected focus. The tertiary
mirror is used to fold and steer the light path so that the
science beam can be delivered to any of eight instruments that
will be mounted on the two main Nasmyth platfoms. The
image is formed 20 meters from the center of the tertiary mirror. The
focal ratio of the telescope is f/15.
The field of view of the telescope is 15 arc minutes (fully
illuminated), or 20 arc minutes with slight vignetting at the
edges of the field. At f/15, the focal length of TMT is
450 meters (1476 feet)! This means that the 20 arc minute
field of view measures 2.618 meters (8.6 feet) in diameter.
The primary mirror focal ratio is f/1. This short focal
ratio was chosen to make the telescope compact, which helps
to keep the telescope structure and the enclosure affordable. As
the name implies, the primary mirror is 30 meters (98 feet)
in diameter, and because it is f/1 it has a focal length of
The primary mirror is segmented, following the lead of the
highly successful Keck 10-meter telescopes. By dividing
the aperture into segments of manageable size, many of the difficulties
involved in the construction of large telescopes are reduced,
including fabrication, testing and transportation of large
mirrors and mirror cells. The need for large handling
equipment, high-capacity handling cranes and large vacuum coating
chambers is also greatly reduced.
Some risk issues are also mitigated. For example, breakage
of a single segment would not be nearly as catastrophic as breakage
of a traditional telescope primary mirror.
Moderate-sized segments can be fabricated at moderate cost
and can be mounted on support systems of moderate complexity. It
is also possible to keep the glass in the segment thin, which
reduces the overall mass and thermal inertia and allows the
glass temperature to follow the changing ambient temperature
to minimize mirror seeing effects.
The TMT primary mirror includes 492 hexagonal segments, each
about 1.44 meters (56.6 inches) across corners. The segments
are closely spaced, with gaps between the segments only 2.5
mm (0.1 inch) wide.
The segments will be made from zero expansion glass or glass
ceramic. Depending on the material choice, the glass will
be between 45 and 50 mm thick (about 2 inches).
Each segment has a support system that holds it in position
without distortion from gravity. Twenty-seven thin flexures
are attached to the back of the mirror and the weight of the
segment is reacted by a "whiffletree" lever system that spreads
the load in the correct proportions to avoid distorting the
shape of the segment. The lateral support (required when the
telescope points towards the horizon) is provided by a central
metallic diaphragm recessed into the glass. The segment
support system is illustrated in the figure below.
Design concept for the segment support assembly.
Adjacent segments will fit together as shown in the following
figure, which views the mirror from below.
Several adjacent segments on their supports,
shown from below (cables are not shown).
The secondary mirror reflects the light from the f/1 primary
mirror and converts it to an f/15 beam for the science instruments.
The mirror is 3.1 meters (10 feet) in diameter, as large as
the primary mirrors of many telescopes currently in use.
It will be mounted in a steel mirror cell that contains the
axial and lateral supports for the mirror. The mirror
supports are active and can correct the shape of the mirror,
for example correcting errors that may be caused by the changing
zenith angle and temperature.
The mirror cell is held in alignment in the telescope by a
hexapod positioning system that can move and tilt the secondary
mirror in all degrees of freedom.
The secondary mirror assembly is illustrated in the following
TMT secondary mirror assembly.
(The laser guide star facility has been removed from this illustration.)
The tertiary mirror is a large flat mirror,
located in the center of the primary mirror, that is used
to direct the telescope image to the instruments on the Nasmyth
platforms. The mirror is elliptical in shape, 3.5 x 2.5 meters
(11 ½ x 8 feet) across.
The tertiary mirror must be able to switch among
the science instruments rapidly and precisely, and it must
be able to track in two axes to keep the beam aligned with
the instrument as the telescope changes zenith angle. One
of these axes (the "rotation" axis) is coincident with the
primary mirror optical axis, and the other (the "tilt" axis)
is perpendicular to that axis.
The tertiary mirror assembly is illustrated in the following
The tertiary mirror sits in the middle of the primary mirror.
The telescope structure is designed as an altitude
over azimuth (alt-az) mount. This allows the telescope
to be very compact (relatively speaking) and provides direct
load paths from the telescope down through the structure
to the pier and foundations.
The structure consists of two major components: the
azimuth and elevation structures. The elevation structure
provides a mounting for the telescope optics and the laser
guide star facility (LGSF). The azimuth structure supports
the elevation structure and two large Nasmyth platforms
where the observatory instruments and adaptive optics systems
The Nasmyth platforms are located 7 meters (23 feet)
below the elevation axis, to accommodate the largest instruments
that will be mounted on TMT.
The total moving mass
of the telescope will be about 1430 tons. This includes:
|Instruments and adaptive
The telescope elevation
axis is located above the primary mirror. This enables the
articulated tertiary mirror to direct the science light
to the instruments, and the structure has been designed
to provide the necessary clearance for the light path from
M3 to the Nasmyth instrument stations, as shown in the following
The drive motors used to move the telescope in azimuth and
elevation will be direct drive "linear" motors, curved to
match the large radii of the drive arcs. Position feedback
will be provided by linear tape encoders.
Because TMT is a large structure, elevators, stairs
and walkways will be included to provide access to all subsystems
on the telescope. All the utility lines (power, cooling
water, compressed, air, refrigerants, etc.) required by the
telescope subsystems will be built into the telescope structure.
TMT is the first ground-based observatory
to be designed from conception with fully integrated active
optic systems, adaptive optic systems, laser guide star systems,
and instrument systems. As a result, TMT is a complex
system-of-systems. The telescope controls are responsible
for over 30,000 input/output channels and nearly 12,000 controlled
degrees of freedom. This includes controlling the pointing
and tracking of the mount, the position, tilt and active optics
shape control of the primary, secondary and tertiary mirrors,
the alignment and phasing system, the wavefront sensors built
into the instruments, and coordinating the motions of the
The telescope controls include:
- Telescope Control System
- Primary Mirror Control System
- Mount Control System
- Test Instrument Control
- Telescope Safety System
- Engineering Sensors
- Commissioning Acquisition and Guiding System
- Power Lighting and Grounding
The Telescope Control System is responsible for the coordination
and control of the various subsystems that make up the telescope,
responding to commands received from the Observatory Control
System and from expert user interfaces.
The Telescope Control System consists of a sequencer and
status/alarm monitor, a pointing kernel, a corrections module,
and several adaptors. The sequencer and status/alarm monitor
provide high level control of the mount, the primary, secondary
and tertiary mirrors, and the enclosure (including the cap,
base, shutter and vents).
The pointing kernel converts target positions (right ascension
and declination) into pointing and tracking demands in the
appropriate coordinate systems for the telescope mount, instrument
rotators, atmospheric dispersion correctors, instrument and
adaptive optics system wavefront sensor probes, and the enclosure
cap and base.
The corrections module is responsible for the creation and
management of the look-up tables that control the position
and shape of the primary, secondary and tertiary mirrors as
a function of zenith angle and temperature. It will
also process data from the telescope global metrology system
and provide appropriate position information to the other
The Primary Mirror Control System maintains
the overall shape of the segmented primary mirror despite
structural deformations caused by temperature and gravity,
and disturbances from wind and vibrations (observatory-generated
and seismic). It can be considered a stabilization system
that works to maintain the shape of the primary mirror based
on previously determined set-points, which vary as a function
of zenith angle and temperature. The Alignment and Phasing
System uses starlight to make measurements from which the
control set-points can be determined.
Three out-of-plane motions (piston, tip and tilt) of each
segment are actively controlled by the Primary Mirror Control
System via three high-precision actuators per segment. Nanometer-level
feedback is provided by two sensors per inter-segment edge. In
total, the Primary Mirror Control System contains 1476 actuators
and 2772 sensors.
The Primary Mirror Control System also controls the segment
active optics warping harnesses, based on measurements made
by the Alignment and Phasing System.
The Mount Control System provides servo control of the mount
azimuth and elevation axes by closing a position loop around
the telescope mounted motors and encoders to follow pointing
and tracking commands from the Telescope Control System. The
Mount Control System will do pointing and acquisition, open-loop
tracking, closed-loop tracking (i.e., guiding), offsetting,
and nodding motions.
Telescope motions are driven by azimuth and elevation torques
supplied by linear motors mounted in the telescope structure. The
elevation axis motors will be located on both the left and
right elevation rockers. Position feedback will be via tape
encoders mounted around the base of the telescope and along
Because TMT has been designed as an active telescope, virtually all of its functions depend on the performance of these control systems. Accordingly, the TMT Project is giving considerable attention to this aspect of the Observatory, particularly in the system engineering area.