The distortions from projecting a flat image on a curved dome surface are a given in this medium. To avoid worsening the situation by adding even more distortion, the ideal is for projectors to shoot straight across the dome.

Projecting at off-axis angles produces the warping effect called "keystoning", wherein none of the four corners of the rectangular image is a right angle.

Perhaps the one thing the 1990 L-H-S Level Specification codified in many people's minds was the concept of Left-Center-Right dissolve screens. Images appear in front of where the majority of the audience is facing.

But simply setting up 3 pairs of projectors shooting straight across the dome doesn't quite work. Because the dome curves in as you move up toward the zenith, the side images can look like they're leaning or falling over. The more verticals there are in the image, the more obvious the effect.

To correct for this, the side projectors need to be tilted, so that the side edges adjoining the center frame are parallel.

If this is done, the images look more natural; verticals are perpendicular to the floor, even though the projectors themselves are tilted to achieve the effect.

With tilted domes, the effect is even more pronounced. The center screen faces straight on to the most seats in the house. The wing screens somewhat resemble the windows of a jetliner cockpit.

A center-edges-aligned array is what we recommend for all 3-screen dissolve arrays. Of course a planetarium may have more screens than this; but we feel it should be a minimum base from which to build.