The tube of a telescope is the tube in which the optical system, comprising mirrors or lenses, is housed. These tubes are made of steel, aluminium or carbon fibre. Only very inexpensive telescopes are made of plastic.
Particularly large telescopes do not have a solid tube, but instead are constructed of a lattice of tubes, known as a truss tube. In this case the technical property "tube material" then refers to the material from which the truss tubes are made.