There is another solution that will help, and that is to use an auto-focus projector. When the slide pops out of focus due to heat from the bulb, the projector will instantly re-focus and maintain sharpness over the whole image.
Back when I projected slides, using two projectors with a professional controller for lap dissoves/cuts/etc and synchronized with sound track, I used a pair of Kodak Ektagraphic AF-2 (auto-focus) projectors with zoom lenses. They were set up to project onto a 70x70-inch lenticular screen that could be tilted at the top toward the projectors (to eliminate the "keystone" distortion of the image), and the zoom lenses could be adjusted so that the projected images were perfectly superimposed. While one projector was showing a slide, the other projector had a slide in position being pre-warmed by its bulb, so that any slide being projected had already been "popped" before being seen on the screen.
Last edited by silveror0; 02-17-2012 at 04:20 PM. Click to view previous post history.