Rails (Rolling Roof)Everyone knows a dome is the classic shape for an observatory, but I really like to see the entire open sky when I observe. The purpose of the observatory is really just to keep my equipment set up, and to shield me from the bulk of any wind that might be blowing, otherwise I'd like maximal exposure to the sky. There are dome products that would meet this criteria, notably the Astrohaven Clamshell, but I can't afford the cost of one large enough to give me space for my Dob.
So I decided to have a roll-off. I split the roof in two, to reduce the weight of individual pieces that would have to be pushed, and cantilevered the rails out to prevent me from having to put posts in. The roof is covered in lightweight steel panels -- strong and durable without the mass of wood panels and shingles. The roof splits in half, and runs on rails to the southeast and northwest. I didn't want extra posts in my yard, and the rest of the observatory is free standing, so I cantilevered the rails out from the observatory walls. Each rail is two 12 foot long 4x4 posts, joined at the middle, and secured to the top of the observatory walls.
I wanted the roof to roll easily without having to resort to a winch; I also wanted it to be relatively free of problems like binding in the rails. I settled on a simple enclosed rail design. Two boards at form right angles on the northeast and southwest walls of the observatory. The roof is attached to another pair of right anlge boards at the top and bottom of each roof section. The roof angles have the mounted rollers. I settled on a roller design with pairs of casters at right angles to each other at two points on each end of the roof sections, for a total of 8 pairs of casters for the entire roof section. The right angle between the casters is supposed to keep the roof from binding by twisting in between the two rails as it is moving.
Roof trussesBecause the roof has to roll, the big endeavour is to solidly mount the trusses to the rails! I also was challenged by the shape of the roof -- because of the steep pitch, I couldn't put cross members between the diagonal rails of each truss to make them rigid. In the end, I put plywood gusset plates at the peak of each truss, which seemed to make each one plenty rigid. Each truss was screwed into a vertical 2x4 beam mounted directly onto the sliding rail; diagonal 1x4 boards were screwed into the interior of the truss network to help make it rigid, and intersticial 2x4 beams were also placed to provide support for the metal roof. Ultimately it worked out reasonably well, though neither roof section is perfectly square due to warps in the long 12 foot 2x4s that make up the main part of each truss; despite this fact, the roof still rolls quite easily.
In order to make the roof as light as poosible, it was sheathed in metal sheet roofing rather than plywood and shingles. The roof was not designed to support my entire weight, so getting out to the seams between metal panels to put screws in was a challenge! Because the roof splits in half, there is a seam in the center which will run right over the telescope. Two pieces of flashing, one a right angle and one a "z" shape sit on the seam and overlap in an effort to prevent leaks on the centerline. We'll see how that works.