As the name implies, the "tube" of this design is actually composed of an upper 'cage assembly', which contains the secondary mirror, and focuser, held in place by several rigid poles over a ‘mirror box’ which contains the objective mirror.

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The Newtonian telescope has flat secondary mirror and a paraboloid or spherical primary mirror
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As the name implies, the "tube" of this design is actually composed of an upper 'cage assembly', which contains the secondary mirror, and focuser, held in place by several rigid poles over a ‘mirror box’ which contains the objective mirror.
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https://en.wikipedia.org/wiki/Dobsonian_telescope#The_truss_tube
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Collapsible "truss tube" Dobsonians appeared in the amateur telescope making community as early as 1982 and allow the optical tube assembly, the largest component, to be broken down.
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