|A DIY planetarium model with its Environmentaland crew, posted at Treehugger.com.|
I've always enjoyed working with planetariums and have recently taken them up as a hobby. As I'm also a member of the Yahoo Groups Small Planetarium blog, I thought it would be fitting to create a blog on my project, documenting the steps for anyone who would like to replicate them. Most recently I constructed a 5-meter diameter Gore Dome inflatable Planetarium out of a thin, lightweight tablecloth plastic.
There are several methods for building a planetarium, but some methods are obviously better than others. A typical amateur approach to planetarium construction is the geodesic dome, constructed of cardboard pentagons. Although this is a perfectly valid approach, it does a poor job of representing a hemisphere as there are many flat sides.
A better approach to planetarium design is a pattern called Gore Domes. Although there is little literature on Gore Domes, the design is used in most planetariums around the world. I am a native of Colorado and live close to the Denver Museum of Nature and Science who recently renovated their planetarium. Their new all digital Gates Planetarium is constructed from a thin sheets of aluminum using the Gore Dome design. Gore domes are unique in that they take flat objects and stitch them together to create a three dimensional dome. The dome itself is constructed from many bulging triangular shaped pieces called 'Gores.' A good representation of this that many of us have done is the elementary orange peel trick; trying to make a 3D object flat (the opposite of what we want to do).
The Gores in this project will look almost exactly like these below:
The following materials will be needed when constructing this planetarium:
- Ruler (Metric)
- 40'' wide roll of white tablecloth plastic
- Masking tape (1.25'' wide)
- Duct Tape
- Blue Painters Tape
By using the pattern provided by the July 1973 Popular Science Article (here) on Gore Domes, scale all measurements to fit the diameter of the dome being built. The diameter of the dome in the article is 25 Feet, so in order to make my dome (5 meters), I had to multiply all of the dimensions by a scale factor of 0.656167979. This multiplier scaled all dimensions down to a 5 meter diameter, but kept units in English measurement. This is fine if you're comfortable with English units, however I find metric units to be easier to work with on a project like this. I simply set up an excel spreadsheet to calculate the proper dimensions at each height level. A link to a PDF of my calculations can be found here.
Setting up a solid template for cutting out the Gores is crucial when aiming towards a 'perfect' dome. After calculating the dimensions of my Gores, I found it easy to make a template out of tape on the floor. This made cutting and fabricating the sections easy. I laid out plastic over the pattern, put a few books on the top to hold things in place and just ran scissors around the edge for a nice clean cut.
Taping together the Gores is tedious work. If you're taping on the ground (easiest) I would recommend a pair of knee pads to combat the hours of brutality they would otherwise endure (I learned the hard way). Taping the gores together can be done on a table, but is difficult because they keep sliding around. I haven't found a cut and dried method for taping together the sections, but small strips at a time seem to work well as the curvature of the gores is always changing. In my procedure, I started with the top of the gores and worked down to the bottom. On the last section, I brought around the ends and taped them together. After the main body of the dome has been taped together, the pieces can be spread out to reveal the circle left in the top of the dome. Simply cut out a circle to fit in the top (see dimensions PDF) and tape it in.
The final step of the planetarium process is the fan/inflation procedure. For my setup, I created a pipe that fitted onto a common everyday house fan with the excess plastic. I taped one end to the fan and the other to the ground. Taping the bottom of the output pipe to the ground and narrowing the top (taping parts of the top edges to the ground) creates a very steady non-turbulent airflow ideal for planetariums. After I had the inflation system in place, I laid out the planetarium on the floor so that the inflation pipe was sticking under one of the edges of the dome. The edges were held down with soup cans, but I recommend anything from tape to sandbags (the smaller the footprint the better).
My basement was too small for this dome to fully inflate. When I head back up to CSU next week I will be testing this dome in full capacity in one of the ballrooms, complete with a mirrordome projection system. Until then, questions and comments are welcome. I will keep this blog updated as this project progresses. My email is firstname.lastname@example.org. Thanks for reading and happy building!