This is a alt/az vertical fork mount I built some thirty years ago, actually my original ATM project which was used back then with a 3.5" homemade rft refractor. Since that time it has evolved considerably, but I spent a lot of effort adding rigidity and stiffness to the entire structure, and all that work did pay off because it is one of the more solid mountings of all the ones I own. I had started with a very high quality bearing I found to use for the azumuth bearing, perhaps a former computer tape drive unit, that coincidentally was a close fit into an aluminum flange I had, and it just grew from there, including a DIY tripod with the necessary leveling screws at the leg tips- and with a fork that tall you're going to need really accurate leveling.
This one is a bit more than 16" high from the altitude pivot point to the top surface of the bearing, or more than 400mm, and I think you would have problems going a lot taller than that (depending on your tolerance for vibration and stability), and 900mm is fairly extreme. In part, I was forced to take this approach by the size of the maple lumber I originally had on hand to make the tripod legs out of; I needed to add some height and the vertical fork was a solution. As you see here, the one I built has aluminum x-bracing incorporated into the fork and lots of hardwood inside and out, and I incorporated friction brakes for altitude and azimuth into the mix. The 80mm triplet it is pictured with here is quite heavy mounted in its cradle but with these proportions this is a robust, convenient, and portable mount for it. A final aesthetic touch was some experimental inlay work I did (the wood is not as fluorescent colored as the camera pictures it here).
Edited by figurate, 28 February 2019 - 05:25 PM.