Thirty Meter Telescope

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.

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 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 × 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.