wpo - rare 17.5-inch Coulter optics set - now sold!
I've rebuilt my 1980's equatorially mounted 17.5-inch f/4.5 Newtonian optics as a Dobsonian telescope.
Twin detachable aluminium spars support the diagonal head and eyepiece rackmount weight a manageable 30lbs. The 17.5-inch main mirror in an 18-point flotation cell weighs another 35lbs - grossing all up under 70lbs. Classic Teflon pads used for side bearings with smooth castors to ali ground board. The whole scope designed for one-person erection and fits into a very small UK car eg my GM Vauxhall Corsa.
Dob first-light proved a great successful with in theory 5-times the light grasp of my classic Celestron C-8 allowing for the central obstruction in each case. Both scopes are 80-inches [2m] focal length. Apart from a paint job and realuminising mirror set, new finder and eyepiece - the project is complete [and now sold!]
Traditionally the side bearings of a Dobsonian comprise two pairs of Teflon pads [as used here] but three Teflon pads or castors to the ground board around a central pivot point.
Unconventionally I've used four castors - one located towards each corner on the bottom board near the side boards that carry the total 70lbs weight of the telescope. Tension is applied to the central bolt to slightly bow both the bottom board, but preferentially, the heavy duty aluminium ground board so all four castors remain in contact for a smooth ride.
The minimum bracing used for the side boards but racking or twisting as the telescope is turned horizontally in azimuth seems minimal.