Designing a Planetarium Dome
In almost fifty years of designing things, I’ve noticed that design starts out with a simple idea that becomes more and more complex. At some point complexity gets out of hand and you have to back up and look at the problem again. If you do this, a much simpler solution usually appears. It was no different designing the planetarium dome.
The Digitalis projector came with an inflatable dome, but the dome didn’t work well in the meeting room at the observatory. We needed a more permanent solution that didn’t cost a fortune. A dome for the meeting room had to be light enough to hang from the ceiling and leave the room usable for other purposes. Since the room is 14.5 feet high, I set the horizon at seven feet so that most people could walk under it. This left me with an elliptical dome, 7 feet high and 19 feet in diameter. My first idea was to use a geodesic dome. I went to the Internet and found a design. Since the dome is elliptical, each level of triangles had to be different. It took a lot of figuring, but I worked it out. My plan was to construct a frame of aluminum tubing, cut acoustic ceiling panels to fit, and attach them beneath the frame. The gaps could be taped and filled with sheet rock mud. The weight came out to about 600 pounds.
Looking through the aluminum framing of the dome. Photo by Bill O’Neill
Well, it was time to step back and look at the problem again. How do I make it lighter? How about cloth? So I called my son, who is in the theater business, and asked him how to use cloth for a lining. He suggested using a cloth screen similar to the type used on stage for a projection screen called a ‘Cyclorama’ and recommended some theatrical supply companies. I found a company named Sew What? and gave them a call. Within five minutes, my geodesic dome design turned into an aluminum frame umbrella with a cloth lining attached with Velcro straps.
Attaching the cloth to the frame. Photo by Bill O’Neill
I calculated the shape of the ribs and gores on a spreadsheet. Using the graph from the spreadsheet, I was able to make scaled drawings of the dome, gore shape and rib shape. All I had to do was to send Sew What? the spreadsheets and graphs.
The cloth dome is made of 20 gores just like a parachute. Since sewing 20 points together is a mess, they put a two-foot diameter circle of cloth at the top and sewed the gores to it. I sent the shape of the ribs to Jack Flemming and he made an accurate template for bending the aluminum ribs.
The completed dome. Photo by Bill O’Neill
Eugene Cooper, Bill O’Neill, Nels Johansen and Greg Bajuk bent the tubing over three weekends. Nels painted the ceiling dark blue. Then we assembled the dome and hung the cloth. I knew that there was trouble as soon as we started putting the cloth on. The cloth dome was a foot too big. Fortunately, it was deliberately made small so that it would stretch. In the end, it turned out all right. The extra cloth at the bottom was folded around the outside and the dome fit perfectly. Aside from the seam that runs across the flat screen, and the funny looking fillets at the side of the flat screen, it looks fine.
Three Easy Ways for Members to Get More Involved in BPAA:
1. Attend a BPAA Star Party with your own telescope, or a club telescope, and share the universe with visitors. (To borrow a club telescope, contact Russ at (206) 842-8758, firstname.lastname@example.org)
2. Learn how to run the Big Scope by attending Ritchie Telescope tutorials. Contact Malcolm at (206)780-1905, email@example.com