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
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.
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.
- Lhoumeau's projector
- More posts on the building process at C4C
- 8mm circular fisheye lens
- 50mm lens
- Macro Bellows
- Radial Grid for Testing
- Projector stand