The view is looking slightly to the east of north. The roof opens slightly to the west of north.
The original roof was covered in plywood and sheathed in fiberglass and weighed in at 460 lbs. The weight was too much for the mechanism to handle. The roof opened well enough at that weight, but when fully open with the observatory roof sitting on the house roof, there was too much weight at the end of the long moment arm of the mechanism. When we first tried to close it, it needed a boost to get it started closing. It would also accelerate to the fully open or closed position once it got past the midpoint. The first time we closed the roof, we twisted both 1" stainless steel drive shafts by 90 degrees! We didn't believe that was possible, but it happened. Fortunately, the rest of the mechanism was so well built, nothing else broke. It's hard to beat a 1:75 dual worm gear reducer for generating lots of torque, which we obviously had plenty of <g>.
We tried several things to alter the weight seen by the drive arm. First we replaced the drive shafts. Then we moved the pivot point of the passive cantilever arms (on the left in this animation) farther away from the observatory to alter the geometry so less weight had to be lifted by the drive arms. We also limited the amount the roof could open so the drive arms would not go below horizontal when fully open. This sacrificed being able to see the horizon where the roof rests, but it seemed necessary. The roof was still too heavy and we twisted both drive shafts again, but only about 40 degrees this time by the time we were done messing around.
Once again, we replaced the drive shafts. We then removed the passive cantilever arm and made a temporary rolling mechanism out of wood for the north (left) side by putting wheels on the shaft that goes through the observatory. We built a track for each wheel to roll in as well. The idea was for the track to carry the majority of the roof' s weight with the drive arm just needing to push or pull the roof into position. The geometry of the drive arm would still lift the south (right) edge of the roof to clear the scopes in any position. This didn't work either, there was just too much weight at the end of the drive arms.
By now, we were fairly frustrated, but we were determined to make this design work. We considered converting to a slide off roof, but I knew we could make it work if we could lighten the load. What we did was to strip all the plywood and fiberglass off the roof and replace it with several layers of lightweight woven nylon. This is the same material used to cover airplane wings. In fact, the material was applied by a local craftsman that restores old planes for a living. The covering is extremely tough and is very taught. It withstands very high wind speeds on a plane and is impervious to water. There are four coats of UV protectant on it and it should last for ten years or more. It can easily be patched if needed. The final weight of the roof is 160 lbs. The lifting mechanism doesn't even strain now and high winds don't affect its operation. There are springs both inside and outside which serve to stabilize a slight wobble as the weight shifts from lifting to falling with the drive arm straight overhead and to counteract any tendency of the roof to accelerate toward either rest position.
Limit switches shut off the motor in the fully open or closed position. The metal arm on the south side of the roof contacts one of the limit switches when the roof is in the fully open position. The speed of opening or closing can be adjusted from a sedate maximum of 70 seconds to a fairly swift 10 to 15 seconds. The animation below covered 30 seconds in real time.
