I bought out of curiosity a Samyang MZ 5000 telephoto lens, adjustable focal length from 650 mm to 1300 mm, diameter 82, F / D 8 to 16. The focal length is adjusted by sliding the rear tube, locked in place by a clamp ring (it behaves like a chuck).

Although suitable with the lens on a tripod and the camera held in hand, this system is not suitable for long exposures (it moves and it turns !).

So I considered mounting it on a screw carriage, on a Vixen dovetail to then fit it on my mount.