Fisheye Projection Lens

The fisheye projection lens is the crucial element in this workstation. Without a creative, DIY solution, a commercial lens would make the system too expensive for the applications we were targeting. 

After a number of experiments with normal projection into hemispheres and projection using mirrors, it was clear that the distances required for straight projection and the difficulty of getting close enough to the hemisphere for a clear view would require a fisheye lens of some sort. A search on the internet turned up a French planetarium enthusiast, Yves Lhoumeau, who had assembled a projection lens out of two normal camera lenses in an unorthodox configuration. Lhoumeau's lens used a circular fisheye lens and a 50mm lens, placed in front of the projector.
One advantage of this assembly would be that the 8mm fisheye lens could serve double duty as a normal fisheye lens on a full frame DSLR, for capturing 180 degree images that could be projected into the mini-dome. In fact, this is the simplest method for creating imagery that fits into a dome, and it is often combined with time lapse and long exposures to create dome programming (see the separate section on this site for Fisheye Photography).

Finding that we did not have the fabrication and lens assembly skills of Lhoumeau, we looked for an alternate way of connecting the lenses in a back-to-back
configuration. A cheap macro bellows seemed to have promise, but it is normally used with a lens and a camera, not two lenses. A custom coupling would be necessary for the second lens, and it was constructed using two lens caps with an opening cut through them, and then bolted together. Another solution would have been a commercial double-sided lens cap, made for storing and transporting two lenses. If this were to be used in another system, the opening between them would still have to be drilled and sawed out.

The first version of the lens mount was constructed out of wood, in imitation of Lhoumeau. It had to be mounted at an angle to the Optoma projector, since it had no controls to adjust the angle of the beam.
When we upgraded the projector, a simple piece of thick plastic (from an Ikea lap table for a taptop) was cut to shape and screwed onto the bracket mounting holes in the bottom of the projector. It was found that the plastic flexed a small amount, and a set of small braces were added, attached with black duct tape. The final mount is limited in adjustability - ideally, the lens would be adjustable in distance from projector as well as side-to-side - but between bellows movement, bellows pivot (around the normal tripod mount), projector focus, and the focus on each lens, good results can be acheived. 

The projector sits on an amplifier stand (QuikLok) available through music equipment suppliers. A radial grid is useful in aligning and locating the projector in relation to the dome. Overall, adjustability is sufficient to align, and focus the system to fit the particular dome height and tilt.