In my last post, I mentioned that putting in the 4x4s presented a challenge. Here's why:
The corner earth anchors sit about a quarter inch above the level of the floor. I'm not sure why Arrow designed them this way, but it's the only solution that will work with a roll-off shed (the other anchor system has a cable to runs through the roof rafters. As you can see, we had to cut the floor in the corners and I thought I would have to cut the 4x4s to allow for the anchors. However, a few blows with a hammer was enough to flatten them down (!) and we were able to mount the 4x4 corner braces successfully.
Below is a pic of the 4x4s in place:
The metal braces work well, but they are a little springy(which I knew going in). They will be further reinforced by 45-degree 2x4s attached to the 4x4 and the floor.
The final pic shows what part of the completed frame will look like:
I had planned to mount a horizontal 4x4 by screwing it to the vertical timber (it's just laid on in the pic above) and also by screwing it to the screw holes in the top frame of the wall (where the roof would normally attach). However, on further thought, I think I will make the top frame from 2x4s (which I will probably brace in a couple of places--a kind of stud wall). This frame will attach on three sides (back and 2 sides); the door frame prevents mounting on the front wall. I will mount 2 12 foot 4x4s on the sides as the tracks for the roof castors. This solution will be more solid and will give me greater wall height for the scope. I'll fit a weather board from the roof frame with cutouts for the rails on the rear of the roof. I'll also put weather boards on each of the other sides, which should give the observatory a nice, finished look.
NOTE: I can't take all the credit for the ideas in this build. I relied on many sources, but most prominently the posts of 'tulit' on the Cloudy Nights forums, and also the Linnhe Observatory pages. Thanks to everyone who (knowingly or unknowingly) contributed to this project!