Telescope

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.

Optics

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 30 meters.

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 figure.

The 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 figure.

The tertiary mirror sits in the middle of the primary mirror.

Structure

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 are located.

The Nasmyth platforms are located 7 meters (23 feet) below the elevation axis, to accommodate the largest instruments that will be mounted on TMT.

Telescope Structure

The total moving mass of the telescope will be about 1430 tons. This includes:

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 figure.

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.

Controls

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 enclosure.

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 control systems.

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 both rockers.

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.