The TMT telescope optical design is a folded Ritchey-Chrétien. Both the primary and secondary mirrors are hyperboloids, and together form a well-corrected image. The tertiary mirror folds and steers the science beam to any of eight instruments, mounted on the two Nasmyth platforms. The image is formed 20 meters from the center of the tertiary mirror, with the telescope focal ratio being f/15.
|Number of segments
|Paraxial Radius, m
|Segment thickness, mm
|Mass, metric tons
|Segment gaps, mm
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.
Following the lead of the highly successful Keck 10-meter telescopes, the TMT primary mirror is segmented, with 492 hexagonal elements, each measuring 1.44 meters (56.6 inches) corner-to-corner. The segments are closely spaced, with segment gaps of only 2.5 mm (0.1 inch).
The segmented primary design provides a number of advantages. Primarily, with the segments being of manageable size, many of the difficulties involved in the construction of large telescopes are reduced, including fabrication, testing, risk mitigation 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 reduced.
The segments are made from Clearceram, a zero expansion glass-ceramic and are 45 mm thick. Manufacturing of the primary mirror blanks is very advanced with over half of the segment blanks already fabricated. Polishing of the blanks will be distributed amongst three partner countries, with efforts to polish and machine the blanks already started in the USA. In addition, roundels have been polished in Japan and new facilities in India are nearing completion.
Each segment has a support system (Segment Support Assembly - SSA) that holds it in position with minimal gravity distortion. Twenty-seven thin flexures are attached to the back of the mirror and the weight of the segment is reacted by a “whiffletree” pivot system that spreads the load in the correct proportions to minimize the surface shape distortion of the segment. The lateral support (required when the telescope points towards the horizon) is provided by a metallic central diaphragm, recessed into the glass. Construction of a segment support assembly and a completed Primary Mirror Assembly are shown in the figures to the right right and SSA manufacture is taking place.
Testing of the Primary Mirror Support System and the Primary Control System is being carried out with the TMT Multi-Segment Integration and Test (MSIT) facility in Monrovia, California.
The TMT board of directors in front of the TMT MSIT. The MSIT will be populated with 7 Primary Mirror Assemblies consisting of dummy aluminum segments, SSAs and primary mirror actuators.
The TMT Primary Mirror System has successfully passed the Final Design Review. Shown are TMT optics and system engineers and the international review committee in Pasadena, 7/26/2018. Image Credit: TMT International Observatory/Fred Kamphues
The secondary mirror reflects the light from the 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 observatory 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 cell is held in alignment in the telescope by a hexapod positioning system that can move and tilt the secondary mirror in five degrees of freedom.
The tertiary mirror is a large flat mirror, located near the center of the primary mirror, that directs 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.
The Tertiary mirror system is in the design phase and a 1/4 scale prototype has been built.