Projection Dome

This is an awesome dome projection system that we built for a camping/arts festival. A single projector can shine an image across the entire dome by bouncing off of a spherical mirror. The surface of the dome is translucent, so the images can be viewed from either inside or outside.

The spherical mirror dome projection scheme was pioneered by Paul Bourke, who has posted lots of information about how to do it.

The dome structure is a PVC dome using hub connectors from SonoStar Hubs. Many domes have coverings on the outside, draped over the structure, however we wanted to have the inside form a screen, so we created a covering that hangs on the inside instead.

Animation in main image is by Automatagraphics and Featherfurl

Dome

2v Geodesic Dome hub kit $220 (predrilled, comes with nuts and bolts)

1/2" Schedule 40 PVC pipe 33 x 10 ft. @ $1.67 = $55

Covering

6 mil plastic sheeting 10 ft. x 100 ft. $60

Heavy Duty Shipping Tape 6 rolls @ $11/3 = $22

Vinyl flooring for templates ~ $20.

Tarp clips 30 @ $1 = $30

48" Bungees 5 @ $2.67 = $14

36" Bungees 10 @ $2.32 = $24

24" Bungees 10 @ $2 = $20

3/4" Elastic strapping 36 yd $32

3/4" Triglide slides 50 $11

Projection

26" diameter Half Dome mirror $38

Short Throw Projector $900 (a cheaper projector would probably work fine. See Paul Bourke's advice about selecting a projector)

Projector Mount $31

Plastic Shelving for platform

Flooring (Optional)

20 ft x 20 ft Heavy Duty Tarp $53

Foam tiles 69 tiles 2 ft. x 2 ft. @ $1.19 = $330

Screen Windows & Doors (Optional)

Screen Mesh 72 in x 25 ft $55

Tools

Chop saw for cutting PVC

Long rulers

Cutting mat

Scissors

Drill/screwdriver 5/8" bit

Jar Openers

You need to figure out how big a dome you want to make. Useful calculators:

• Radius to Strut Length
• Strut Length to Radius

A 2v dome has two different strut lengths. We wanted to have a fairly large dome, and to minimize the number of cuts we'd have to make in the PVC pipes, so we chose our longer strut length to be 5 ft, exactly half of one of the 10 ft pipes. The SonoStar hubs add 2.28 inches to the strut length, so that results in the following sizes:

• Dome Radius: 100.75 in = 8.4 ft
• Strut Length A: 60 in = 5.0 ft
• Strut Length B: 52.78 in = 4.4 ft

Many other sizes are possible. Another one that tempted us was to make Strut Length A + Strut Length B = 10 ft, so we'd only have to make one cut per pipe, which would have created a 108 in radius dome. This size worked a little better with our covering plan.

I've attached spreadsheets showing various calculations.

You're going to be making a lot of cuts, so I strongly suggest using using a chop saw. It'll make each cut in seconds (you'll spend more time lining up the cuts than cutting). We used clamps on either side of the blade to hold the pipes in place while we cut.

The lengths should be close, but they don't have to be super-precise. There's enough flex in the pipes to make up for a fraction of an inch of size differences. Also, 10 ft PVC pipes from a hardware store are probably not exactly 10 ft, so unless you want to make extra cuts, there will be a little variation anyway.

You can mark the two different lengths with different colors of tape or markers, but because a 2v dome only has two strut lengths, it's not too hard to keep them straight. We only tried to put a long in where a short belonged once while assembling our dome!

It's probably worth making a few extras of each length, just in case.

It is helpful to make a model to understand how the coverings fit together. If you print two copies of the 6+6 PDF and one copy of the 5+6 PDF, you'll have all of the parts you need to make a paper model.

Cut out the outlines of the shapes. The hexagonal shape from the 5+6 page has some labels and colors to help you identify which lines are which. You'll want to lightly crease along the solid lines radiating from the center, and make folds along the dotted lines. For all of the edges except the free edge, tape them down. These will be the sleeves that hold the PVC struts around the outside of the sheet. Repeat this for the other sheets. Because I was lazy and reflected the hexagons on the 6+6 sheets instead of rotating them, two of the dotted line foldovers will be in the wrong direction. This doesn't really matter. Don't fold over any of the edges of the pentagon. This will be the roof of the dome.

Assemble the dome by starting with the top pentagon. Tape an edge of the pentagon to the at the Top edge or one of the hexagons. Add another hexagon and overlap the free edge of the first hexagon on the other side and tape it down. Work your way around the circle.

You'll notice a few things: at each joint, one edge is folded over, and the other is loose and layered on top. This is so if it rains, the water will mostly stay outside the dome. Also, the gaps that you folded over on the bottom edge are a little too long. They can safely be trimmed.

For the first covering we tested, we measured everything out with rulers. It turned out to be a huge time sink. So we made templates to make measuring easy. Our local Home Depot had small pieces of vinyl flooring for cheap (something like 8x8 ft for $20). It was ideal for the templates. It's stiff enough to provide a good edge, but rolls up easily and likes to lie flat. There are four shapes to cut:

1. AAA Equilateral Triangle: 62.26 in on all sides
2. ABB Isoceles Triangle: 62.26 in on one side, 55.06 in on the other two
3. Edge Flap A Rectangle: 6 in x 57.58 in
4. Edge Flap B Rectangle: 6 in x 50.38 in

The dome covering is made of plastic sheeting. We chose 6 mil sheeting. For past projects we've used lighter weights, but we wanted something a little more durable for this one. It's available in 10 ft x 100 ft rolls. One of those is more than enough to cover a dome of the size we made.

For the dome covering, there will be five hexagonal-ish side sheets and one pentagonal-ish top sheet. The side sheets consist of four ABB triangles and two AAA triangles. The top sheet consists of five ABB triangles. Because the sheets will not be flat, those shapes don't tile together perfectly on a flat surface. There will be a gap. In order to not cut the sheet in the middle of a side, we dealt with the gap by folding it over. There will be more detail about that in the later steps.

You'll need a large, flat area to cut the covering. About 12 ft x 12 ft is the absolute minimum. Use the templates to measure out a sheet.

You should trace in at least two colors, so you can keep track of which side is which. We used blue for the A side (long) and red for the B side (short).We found this process worked well:

1. Cut a 10ft x 11ft length of sheeting off of the roll and lay it out flat
2. Line up 6 inch rectangle A with the 11ft edge of sheet near the middle
3. Line up AAA triangle with rectangle. The center of an edge of the triangle should line up with the center of the rectangle
4. Line up ABB triangle with AAA triangle
5. Line up 6 inch rectangle B with ABB triangle
6. Make sure all parts are on the sheet with a minimal amount of extra
7. Trace out all edges of the rectangles
8. Trace out the outer edge of the triangle and a small mark at on each side of the center point, for alignment
9. Trace the entire edge of the AAA triangle that doesn't touch the ABB triangle. This will be one side of the gap.
10. Move the ABB triangle and B rectangle
11. Trace
12. Move the AAA triangle and A rectangle
13. Trace. Note that part of the A rectangle will be off of the sheet. That's fine.
14. Move the ABB triangle and B rectangle
15. Trace
16. Move the ABB triangle and B rectangle
17. Trace. Trace the entire A edge of the last ABB triangle. This will be the other side of the gap.

You should end up with a hexagonal-ish pattern with a gap between one of the AAA triangles and the A side of an ABB triangle.

If you built the model, this should be familiar. You'll want to fold the gap cloth out. The easiest way to do this is:

1. Lay the sheet flat so that the inside is up. We chose to use the side that we didn't write on as the inside.
2. Find the two gap lines and line them up.
3. Tape along the lines. Start from the center and run one long piece of tape to the outside edge, centered on the lines.

Next flip the sheet over and start working on the outside edges. For every edge except the free edge do this:

1. Mark along the edge, 1 inch from the triangle. This creates a little extra fabric in the sleeve to make the sheets not quite as tight and give the poles a little space. It's possible to skip this, but it turns out to be very difficult to assemble the dome without this ease.
2. Reinforce the corners of the edge. Put about 8 inches of tape along the short edge of the rectangle and cross it with a few inches from the corner along the edge of the triangle. Repeat on the other side of the sheet. This will help prevent the cover from tearing at the places where it's stressed.
3. Fold the edge over so the 1 inch marks you made line up with the edge of the triangle.
4. Tape the edge in place. One long strip along the whole edge should do it.T

For the free edge, reinforce the corner with tape and also the entire outside edge. We'll attach tarp clips to this edge, and it will tend to tear if it's not reinforced.

Finally, take the gap fabric and fold it over towards the bottom of the dome. Tape it down and trim the excess.

Because the sheets will hang on the inside of the dome, they tend to droop a bit. Wrinkles on the projection surface don't look good. We solved this problem by making sleeves similar to what you see around flexible tent poles in modern tents. It's handy to mark one of the PVC struts 6 inches from the end.

1. Cut a 6 in x 3 ft piece of sheeting
2. Mark off 1 in from each side
3. Fold in half so the marks line up
4. Fold open so that 1 in is exposed on each side
5. Tape along the fold
6. Thread a PVC strut through the sleeve
7. Place the strut at one of the center marks you made when tracing the cover and the corresponding corner.
8. Adjust the sleeve so the center of the tape is under the pole and the end of the sleeve is 6 in from the center
9. Roll the strut to one side and tape down the other edge of the sleeve
10. Roll the strut to the opposite side and tape down the remaining edge of the sleeve

You'll need six sleeves on each of the side hexagons and five on the top pentagon.

Dome assembly works by taking each cover sheet, putting a hub of the appropriate shape in the middle and sliding struts through the sleeves to the hub.

You can pre-drill holes for the bolts, but in order to get the angles right, you'll need to assemble the dome before doing that, then take it all apart, and finally put it back together with the covering on. We found it easier to just drill it as we were constructing the dome. Because you'll want to drill through the struts once they're in place, you'll need to insert the last two without their sleeves, drill them, remove them, and finally insert them through the sleeves.

The PVC struts tend to be pretty snug in the hubs, so to insert and remove them takes some twisting. We found that soaking the hub in soapy water and dipping the ends of the struts made it much easier. When removing, a spray bottle full of soapy water also came in handy. It's very useful to be able to get a good grip on the struts. We used rubber jar openers and "Gorilla Grip" gloves, which worked well.

The assembly plan that worked well for us was:

1. Assemble the hub and 5 short spokes of the roof, but don't attach the 5 long edges
2. Build the ring that sits on the ground (with the coverings attached to every other bottom strut)
3. Build the 5 bottom (long/long/long) triangles of the hexagons with the coverings attached
4. Prop up each of these triangles with a pole from the inside of the dome to the center of the hub
5. Build the 5 upper (long/long/long) triangles of the hexagons with the coverings attached. These 5 join up at their top hubs. Once those are connected, the structure is moderately stable.
6. Connect the roof
7. Build out the pentagons around the sides

This will require some tall people to help wrangle the top. Luckily, we had some tall people lying around.

You should now have a freestanding covered dome with five loose flaps hanging down from the roof and five more where the panels join. There may be pretty big gaps where the free edges are.

Attach three tarp clips to each loose edge, one in the center and one on each corner. Use bungees or elastic to connect them to the appropriate hubs. We found that a good pattern was:

• Top of free edge to top of adjacent panel: 48" bungee
• Center of free edge to 6-hub of adjacent panel: Elastic
• Bottom of free edge to adjacent floor 4-hub: Elastic
• Center of top edge to 6-hub beneath it: Elastic
• Corner of top edge to 5-hub beneath it: 36" bungee

After cinching down the dome, the gaps should be covered by the flaps and the dome should be very close to its proper alignment. We found that the covers were pretty taut (they make a drumlike sound when you tap them) and that they shed rain pretty well.

Water can definitely enter the dome at the five corners of the roof, and at the tops of all of the doorways, so you'll probably want a 20 ft x 20 ft tarp to cover the whole thing if it rains.

We decided to put down a 20 ft x 20 ft tarp as the base for the ground cover. On top of that, we used foam tiles that fit together securely. The tiles are advertised as 2 ft x 2 ft, but because of the jigsaw pattern they're actually slightly smaller, so a 16 ft diameter dome doesn't quite fit on an 8 tile x 8 tile square. 69 tiles should be sufficient. When it threatened to get really rainy we decided to use only tiles that fit inside the dome, which only took 32 tiles.

The mirror needs to be slightly inside the dome to shine on the whole thing. We built a platform out of some plastic shelves that we had around. It was nice to be able to screw the reflector and the projector mounts to rigid objects that were still moveable.

The reflector that we used has three screw holes: one in the middle and one on each side. This made it easy to align the projector horizontally by projecting a centered line (L mode in the Dome Lib software we wrote). We could then center the projection on the top of the dome ceiling by projecting a polar grid (G mode in the software).

Focusing was slightly tricky. We had a short throw projector, but even so we were all the way at one end of the focus range.

Discussion of software to distort images.

Github Processing code

Hit up GIF artists for content.