Observatories on small footprints (i.e. 100 square feet or less), whether roll-off or dome, are typically designed for small telescopes (ex: short focal length refractors under 6-inches or SCTs 10-inches or smaller). The observer with a medium or large aperture reflector (12-inches and larger) typically has to go with a larger building to deal with the reflector’s large “sweep” radius, thereby increasing costs, and, if located within a typical community, subject to building code regulations.
To avoid these issues, I chose to house my 22" f/3.3 reflector in an observatory I call a Split A-frame which was built on a 10'x10' platform. I am not the originator of this style of observatory. The first (and only) example of this type of construction that I have seen was built by Greg Mort about 30 years ago to house a large, pier-mounted refractor. Photos of the observatory can be seen on pages 22-23 of John Hicks’ “Building a Roll-Off Roof or Dome Observatory” and an article with photos describing the observatory was originally published in the March 1990 issue of Sky & Telescope, pp. 329-331. Though we followed the design very closely, a few improvements were incorporated to make the observatory more functional and easy to use. Because I have little or no talent when it comes to building things (and, in any case, do not have the tools available to do a proper job), I hired an experienced local carpenter to execute the build.
Two rows of cement footings, five on each side and three feet deep, were poured. Each row was twenty feet long, separated by ten feet. The footings support two twenty-foot fence posts, 6"x6" in size and laid flat on the footings. Aluminum 1/8" plate strips were attached to the upper surface of the posts to provide a smooth, low resistance rolling surface for the observatory halves (see photos with this post). The 10'x10' floor uses 2x6s in a standard frame construction, overlaid by 3/4" plywood. On the north side of the floor, 500 pounds of cement was poured in a 30"x30" frame to support the telescope.
The roof slopes at a 60° angle. Each half of the observatory uses 2x4s in a simple frame construction, overlaid with 1/2" plywood sheets. Roofing underlay was then applied, followed by standard asphalt shingles to complete the “roof” portion. For the walls, 1/2" plywood sheets were overlaid with standard issue white vinyl siding. The observatory halves rest on standard in-line casters. There are three casters under each wall, 12 casters in all. Ratchet straps (four in all) are attached to the interior of the building... these serve to secure both halves of the building when closed and also serve to bring the halves together to perform a secure and tight seal. Standard hooks and eyebolts secure the rolling halves to the observatory floor when closed. A sub-sized door provides access to the observatory when closed.
The high walls (7'2" on the west side, 8'5" on the east side) provide good wind protection as well as blocking illumination from local street lights. Though the walls obstruct the sky somewhat to the east and west, as I am a deep sky observer, I typically do 99% of my observing within a couple of hours of right ascension either east or west of the meridian, so any object will eventually make it into this observing window. When opened, the observatory has an “airy” feel with very good sky access and none of the claustrophobic feel of the typical, high-walled roll-off or domed observatory. Movement around the telescope is very free and even if several observing companions are invited over, there is room for everybody and easy access to the platform and telescope.
I am following this initial post with a series of posts illustrating the build for those who are interested.
Overall I am very pleased with the results... the design would allow, I think, for a 24" f/3 reflector to be housed, although I think this would be a tight fit. Naturally, this observatory could be scaled up to accommodate larger telescopes but this would begin to make the walls difficult to open and close by hand.