Prairiehill Farm Observatory - Part 1 Building Design & Construction

These web pages detail the construction of a Dome observatory to house my 10 inch f/6.3 LX200 classic. The building site is surrounded by sprawling corn and soybean fields with only a few trees to the north and east. On clear evenings the Milkyway band stretches from horizon to horizon. Looking up on these evenings is a tremendous sight that can leave you feeling as though you are standing in space! Even so the location has risks as the building is exposed to blasts of wind that to date have reached 50-60 mph during thunderstorms. Steady breezes can persist at 35-40 mph for 24 hours or more during seasonal changes in the fall. A sturdy structure is essential for this location.

I researched and weighed the many options between a roll-off roofed versus domed observatory. The Internet offered a wealth of opinions on the topic but I was partial to domes. In high school I designed and built a domed building for my 8 inch Meade reflector. I loved that building and I've always missed it. My observatory had to be domed. Once the decision was reached the product search began. One can spend a little to a lot of money on domes. I ended up with a Technical Innovations ( HD-6S. It's a 6 foot short, meaning it's the dome plus an adaptive rectangular skirt (6 foot 6 inches sides) to mount on some structure beneath it. Technical Innovations sells cylindrical wall assemblies as well as domes (i.e., HD-6Tall). The HD-6S is a sturdy fiberglass dome rated for 100 mph so it has some margin of error for the building site. I chatted with the owner of Technical Innovations by telephone to be sure the LX200 10” (or even a 12”) would fit comfortably in the HD-6S. A 12” might be a bit tight depending on how much equipment is attached to the observing end of the scope but the 10 inch Cassegrain is a comfortable fit. I ordered the dome in April 2002. It shipped overland in a large crate arriving at my door on May 11, 2002. A very curious truck driver, my wife and I disassembled the crate inside the trailer and stacked the dome sections in the garage.

With the arrival of the dome I was ready to get started with one exception. I had promised my wife that I would put priority on our home renovation work which began in earnest that same spring. So the dome would stay put in the garage for a couple months. At the end of July I took a break from the house to dig the footing for the observatory pier. For several reasons unrelated to astronomy, this pier had to be “portable” in the event the observatory might be moved some day. So I was not able to pour the classic n-ton concrete pier. Instead I shoveled out a 40 inch deep 2-3 foot wide hole. Our heavy clay soils abruptly turn to sand at approximately 35”. I set a footing on sand using about 10-60 pounds sacks of concrete mix. I reinforced the footing with short lengths of rebar in a horizontal grid pattern with a few stuck vertically in the center. Then I centered a 24 inch long 12.5 inch round Quik-Form Building Tube on the vertical reebar to bring the pier up to near ground level. If the observatory was ever moved a few inches of dirt would cover the whole pier. Then I needed to move quickly on the next step.

My first engineering challenge involved setting the anchor bolts for the imagined pier. I planned to use 4 heavy bolts positioned at the points of the compass. Ideally they would align with true north-south to make polar alignment of the telescope easier. I used LX200 base compass to determine magnetic north and compensation dial to locate true north. These were anxious moments for me. No matter how much you plan, when you put a lot of wet concrete in a form on a hot day you need to work fast. I’m clearly not a professional at this sort of thing but it turned out okay.

The next engineering challenge would be constructing a building, centered over this pier and oriented roughly true NSEW. I convinced myself that if I could find Messier objects I could figure this one out as well. First I had to spend more time on the house. The concrete would cure for 4 weeks as I worked my other projects. I finally returned to the observatory construction the weekend of September 3rd.

I planned a 10x10 foot building. The dome would be off center towards the west to make room for an east side entry way. This meant positioning the building sub-floor accordingly over the pier. Fortunately through the summer I had many evenings to think through these contingencies. In keeping with the portability constraint I decided to build a raised structure that allowed air to circulate underneath. Additionally the interior of the building needed to vent well to avoid moisture problems and counteract the intense summer heat this area is prone to. This part of the design has worked well for me. Work began the weekend on September 3rd.

The raised structure is set on 4-4x6” treated posts inset from each corner of the building. Two 2x8 inch by 10 headers were stretched across, leveled and bolted to the east and west pair of posts in the north-south direction. The headers acted as the foundation for the sub-floor. The 4x6 posts are set in ~36 inch holes using concrete footings and backfilled with sand. I worried about the raised foundation’s ability to withstand the winds so I didn’t skimp on wood. The outside edge of the sub-floor consists of 4-2x10 inch by 10 foot lumber with 2x8 inch by 10 foot floor joists running perpendicular to the foundation headers. Each joist is attached with metal hangers. Before fastening the floor to the header I double checked the position of the concrete pier to be sure it was where it should be. The floor joists are fastened to the foundation with metal ties. I used 4x8 foot 5/8ths treated plywood to cover the joists. It would take an extreme weather event to dislodge this structure and there’s no doubt that Mother Nature will try!

I learned how to frame buildings in high school and working summer jobs in college. The next phase of the construction on September 7th was anxiety free weekend. My next challenge would again come from my wife. She found a picture of a domed observatory with the dome resting on a hip roof. It didn’t matter that I’d never framed a roof before. My first roof would be a hip roof! Also required by my wife was a “normal” looking building. The walls had to be normal 8 foot height. During this phase I’m arguing that the telescope is going to be some 10 feet above ground level to rise about the sub-floor, 8 foot walls and hip roof! She confidently told me, “You’ll figure it out.” So up went the walls…

The Technical Innovations rectangular skirt recommended a 2x6 inch top plate to rest on. After some study I came up with 2 2x12 inch headers tapered on each end to the expected rise and run of the hip roof. Heck, I didn’t know if it would work at this point. It looked feasible on paper. I notched each header (see photo), leveled and nailed the 2x6 inch top plate on top of the headers to make the desired 6 foot 6 inch square frame for the skirt. Before nailing the 2x12 inch headers to the wall structure I plumbed a center line to the concrete pier. This was a critical step to assure that the pier was centered within the dome. I wrapped up the day by completing the siding of the walls with inch USB board and framing the hip roof.

The following weekend, September 15th, started with a day in the garage. Technical Innovations strongly suggested assembling the dome on the ground to learn how it is done. Then the dome could be “quickly” disassembled and reassembled on the building. They recommended having two people for the assembly. My father-in-law joined me for the garage assembly. We worked cautiously and had it assembled in an hour or two. Satisfied it was easy we disassembled it. My father-in-law had one question for me, “Who’s going up on that roof with you to put it back together?”. Suffice it to say the second assembly was a one person job.

My impatience to see the dome on its throne got the best of me. I spent the remainder of the afternoon setting the rectangular skirt on the 2x6 frame, leveling and re-leveling to make sure it was exact.

Once I had the skirt fastened to the building I was anxious to set the dome on top. Bad weather was expected the next day. Although it was getting late in the day, it was now or wait another week. There was also that home remodeling project being threatened by the changing seasons. I decided to set the dome. My wife was pressed into service. She’s afraid of heights but she was a sport. She stood on a step ladder helping pass parts and pieces. She was just as excited as I was. There were a couple rough moments when I set the dome halves in place. The dome slot slides up over the top of the dome and rests on a fixed section of the slot cover. I struggled to get the fixed section in place. In hindsight this is where the second person would have made things much easier. In the end I managed to set the dome halves in place. The scene was captured by my wife. The skies were calling but the work was far from complete.

I added a small window on the north, siding and finished the roof with cedar shingles. The home improvement work included a new front door. The house's existing steel door was in decent shape so I transferred it to the observatory the weekend of October 22nd. I declared the observatory building complete. Now I had to figure out the pier...

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