I had purchased a 12" fresnel lens from Edmund Scientific about 30 years ago, for a tracking solar furnace that ran a small steam engine.
solar steam engine 1972
The old tracker had been stuffed in the attic and forgotten, so I decided that this lens was to be the biggest lens for the system. I fabricated an aluminum lens holder and used silicone caulk to hold the lens. The lens would need to be quickly focused up and down in order to draw with the assembly, but the 1.5 lb lens lifting against gravity proved to be a bit much for the motor to do quickly, and the current required to lift it was nearly at the limit for the servo amp and solar power source. Another issue was that the well balanced multitracker vertical axis would only remain balanced at one position in its 7" of travel, which made the vertical axis also draw a lot of current as the lens moved away from that balance point. I solved both issues by mounting a second linear stage on the other side of the lens mount arm. A steel block that weighed the same 1.5 lbs as the lens was mounted to this stage, and is driven by the other side of the drive cable, so as the lens moves out the weight moves in by the same amount. This configuration balanced the lens weight with the steel weight at any pointing angle from horizontal to straight up, so the drive motor only has to provide the motive force, and not have to lift any weight. The vertical axis of the tracker was able to be balanced with a fixed counter weight on the stage side. This keeps the vertical axis balanced no matter where you focus the lens.