Above about 250mm a solid tube becomes an object that two people move, and the truss design exists to solve that. It replaces the tube with poles that come apart, so a large instrument travels in pieces and assembles in ten minutes.
How a truss works
The mirror box sits at the bottom, the upper cage with the secondary and focuser sits at the top, and a set of poles holds them at the right separation. Take the poles out and the instrument becomes two manageable boxes. Collimation is preserved by the poles seating in the same place each time, and a well made truss holds alignment between sessions.
Why the largest amateur instruments are trusses
Aperture beyond 400mm in a solid tube is a shipping problem and a storage problem before it is an optical one. Trusses make apertures of half a metre and beyond practical for one person with an estate car, and almost every very large amateur telescope built in the last forty years uses the design.
The shroud, which is not optional
An open truss lets stray light and dew reach the optics, so a fabric shroud around the poles is part of the instrument rather than an accessory. Without one, contrast falls noticeably under any sky that is not perfectly dark, which undoes some of the aperture you paid for.
Questions people ask about telescope
What is a truss telescope?
A large reflector whose tube is replaced by demountable poles, so the instrument breaks down into two portable pieces.
How big can an amateur telescope get?
Amateur trusses of 400mm to 600mm are common at star parties, and instruments beyond a metre exist. The practical limits are transport, storage and the ladder you need to reach the eyepiece.
Are large telescopes for sale second hand?
Yes, and the large end of the market moves slowly, so bargains exist. Check the mirror coating, which dulls over decades and is expensive to renew.