Anybody following this blog will know that I have been collecting information for two dome projects: a mini-dome authoring system, which I describe as one geek at his computer with a 5 foot bowl over his head; and a 30 foot planetarium/digital dome that is under construction as part of the Center for Community.

The choices for projection in a dome boil down to three, in ascending order of expense:

• Paul Bourke's method of using a standard projector and bouncing the image off of a section of a spherical mirror to warp it into the desired hemispherical projection. 
• A single fisheye lens projector in the middle of the dome, or sometimes toward the back if using truncated fisheye projection.
• Multiple projectors at the periphery of the dome, splitting the image between them and requiring blending of the pieces.

For the mini-dome system, I have received an NEH digital humanities start-up grant, promising that I would create a low-cost system based on commodity components, and would share the design of the system. I expected that due to budget limitations (under $10K for the computer, projector, dome, DSLR, lenses, 3D modeling and game engine software) I would have to go with the spherical mirror projection system.

But then I found the Lhoumeau DIY fisheye lens hack online. Since it used DSLR lenses that would be part of the camera kit anyway, it looked promising. But I did not have the hardware or mirrors to rig up the lenses in the configuration I first saw. 

I found an old macro bellows designed to put a camera on one end and a lens on the other. It was an Olympus still camera bellows that once belonged to my Dad. After looking into adapters to my Canon lenses, and taking it apart to hack it, I realized it was not going to be an easy fit.

But I found a cheap Canon bellows online - the Fotodiox macro bellows for Canon EOS cameras. For $40 I could take the chance that it would work. To attach a lens where the camera would normally go, I could use a double end cap, sold as a lens storage solution, and cut through the middle to let the image through. Or I could connect two replacement Canon rear lens caps. Since today was Thanksgiving, I couldn't run to Adorama camera to get the only double lens cap I could find online. So I assembled the fisheye projection lens rig using the two cap method. Those caps are more readily available.

You can see in the above photo the two lens caps. I had a circular cutter used to cut a hole in a stanless steel sink top from Ikea (you can probably get one at any hardware store) which made a neater cut than I could make any other way. I put the caps back to back and used two small screws and nuts to attach them together.

You can also see above the finished lens assembly (third in the gallery).

From left to right are the Canon 50mm lens, the 2 lens caps I assembled, the bellows, close to complete compression, and the Sigma 8mm circular fisheye lens.

And the last photo is the orientation of the lens assembly and the projector, supported on magazines for a rough test. The projection cone on desktop projectors tilts upward, so the lens has to follow that axis.

How does it work? The 50mm lens reduces the projector image back to the size of a sensor in a camera. Then the fisheye lens takes it and spreads it out to approximately 180 degrees. In both cases the lenses are used backwards from their normal orientation. The bellows lets you easily adjust the distance between the lenses - my improvement on the Lhoumeau system, along with the easy lens mounting.

I'll keep posting photos as I rig up a mount for it all, and as I figure out the odd posterous photo gallery routine.