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.
|Number of segments||492|
|Paraxial Radius, m||60|
|Segment thickness, mm||45|
|Mass, metric tons||121|
|Segment gaps, mm||2.5|
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 including a total of 492 hexagonal elements, 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.
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, 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 greatly reduced.
The segments are being made from Clearceram, a zero expansion glass and are 45 mm thick (almost 2 inches). Manufacturing of the primary mirror segments is very advanced with over half of the segment blanks already fabricated. Polishing of the blanks will be distributed amongst four partner countries, efforts to polish and machine the blanks about to start in the USA, have been taking place in Japan and new facilities in India and China are nearing completion.
Each segment has a support system (Segment Support Assembly - SSA) 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. Construction of a segment support assembly and a complete unit are shown in the figures to the 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.
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 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.