The tube is a subsystem of the telescope that supports the mirrors, the Cassegrain focus and the folded Cassegrain foci (and their respective instruments). It is made up of the primary mirror cell, the tertiary mirror turret, the elevation ring and the secondary mirror assembly (ring, spider and support structure).
The primary mirror cell, an intricate assembly of welded metal supports, will hold the primary mirror’s 36 segments in place, keeping them aligned and adjusting their position to counter the effects of expansion and contraction. It will use 168 onboard sensors and 108 primary mirror ‘actuators,’ or adjustment systems, to do this. The primary mirror cell will also support the tertiary mirror turret and the Cassegrain focus – a total of 40 tonnes.
The tertiary mirror turret, no more than 1.8 m in diameter, will be 7 m tall and will support the tertiary mirror. Its total weight, including the mirror and its mechanical parts, will be 2.5 tonnes.
The elevation ring is a hollow, circular casing that will weigh 40 tonnes. Structures on either side will attach the ring to motors so that the tube can rotate about the elevation axis. The elevation ring will carry the full weight of the tube, transferring it to the mounting. It will also support the folded Cassegrain foci.
The movement and support structure for the secondary mirror will weigh 650 kg. The mobile part (this includes the mirror itself) will weigh 200 kg, measure 1 m in diameter and 1.6 m across, and will have a range of movement of approximately 30 mm, accurate to 0.1 microns.
The ring, one of the structures in the secondary mirror, will attach the upper part of the tube to the spider. The spider is made up of six pairs of metal supports, which will carry the weight of the secondary mirror assembly (mirror and ring). In other words, 6,500 kg will hold up 2,500 kg, 20 m high, yet the stress will compress the spider by just 300 microns or less! To reduce the shadow cast by the spider, its supports will be narrow and positioned over the gaps between the segments of the primary mirror.