© 2003 Loch Ness Productions.

NOTE: To see the dome images in this article with finer detail, click on them for enlargements.

Panoramas

The horizon panorama is one of the most effective forms of planetarium imagery, and is a staple of planetarium show programming. It sets the scene, establishes location, and by stretching across an audience member's peripheral vision, adds that surround element so essential to creating that "you are there" environment. The pan can also function much like a picture frame, enhancing the night sky above it. To be effective, the pan must remain low, to help keep the illusion of a distant horizon.

Most planetarium panoramas are made using multiple slide projectors shooting across the dome, aimed on the horizon.

While there are, unfortunately, "billions" of variations, the average horizon pan uses 4-inch (100mm) lenses on the slide projectors. The projected 3:2 rectangle stretches up 25 degrees, and spans an effective area 30 degrees across on the horizon (40°, actually, with 5° on each side for overlap). With 6 projectors, you cover 180 degrees -- half the dome. 12 projectors will get you a full 360 degrees all around.

While one could simply align the projectors so the edge of one frame abuts the adjacent one, the curvature of the dome prohibits a perfect matchup.

So, to prevent gaps between frames, the projectors are aimed so that the screens overlap slightly.

This creates another problem, however. Hitting the same area of dome surface with two projector beams naturally causes a brightness "hot spot" in the overlapping areas.

In years past, planetarians used gradated soft-edge masks sandwiched in the slide mounts to smooth out the seams -- to varying degrees of success.

One limiting factor: commercially-available masks don't compensate for the curvature of the dome.

Unless the projector placement just right and the masks (or the density of the image containing the soft-edge blending) are just right, you're still likely to get either bright or dark bars at the overlap spots, marring the seamless effect, and simply looking ugly and unprofessional.

Fortunately, these days we have computer tools -- such as Sky-Skan's DigiDome or Spitz's PolyDome -- to manipulate and resize the panorama imagery, adjust for the curvature of the dome, and incorporate soft-edge masks as well. Here is an example:

This computer-generated image of one slide out of our 6-panel pan example incorporates both curvature correction and soft-edge blending. You'll notice that virtually the entire image fills the frame; where it fades on the left and right edges will be compensated for by the adjoining frames, of course.

All-skies

In the late 1970's, the 6-projector all-sky system was developed by Jim Sharp, Dan Zirpoli and Ray Villard. Using extremely wide angle (35mm) lenses on the projectors, the entire dome is blanketed with a single scene. All-skies can provide a visually stunning effect -- overwhelming at times -- with imagery completely filling the peripheral vision of the audience member.

All-sky slides are projected vertically. Each projected rectangle spans 70 degrees across and 105 degrees high.

Like panoramas, all-skies use overlapping images too.

Within the image area of the slide, a triangular "pie-wedge" shape is inscribed, with the bottom corners covering 60° at the horizon, stretching and converging 90° up, to a point at the zenith.

Careful soft-edge blending is needed to avoid seeing seams at the overlap area boundaries, of course.

Here is a typical all-sky soft-edge mask.

If the original image is a polar-projected circle
(often called a dome master or dome original)...

it's sliced into 6 "pie-wedge" shapes for each of the 6 projectors.

Using An All-Sky As A Panorama

The question is often raised, "Since the all-sky is covering the same area as a panorama (and more), can't it be used as a panorama system too?"

It's the compromise in image quality we think is paramount for consideration. A slide projector with a 35mm all-sky lens on it simply spreads out an image so far and wide across the dome that, when compared to a 100mm panorama lens projection, any all-sky will look dim and fuzzy with washed-out colors.

It's just the nature of the beast -- that's the way all-skies are. Perhaps the all-sky's "saving grace" is that, when it's used as a complete all-sky, the entire dome lighting up with imagery can be so visually impressive. But it's just not a very high-resolution form of projection.

When you compare a panorama slide and an all-sky slide, the reasons become more obvious. Let's say the image we're projecting is 1000 pixels by 667 pixels. In the pan slide, virtually the entire frame is used, so we're concentrating onto a dome area 30° by 25° a rectangle projection of some 667,000 pixels. And fully 100 percent of the projector's lamp is used to illuminate our image.

But in an all-sky slide, covering the same 30° by 25° dome area as the panorama utilizes only 1/8th of the film chip. In our example, the actual image area is 240 by 360 pixels, or only 86,400 pixels out of the available 667,000. And 7/8ths of the available light from the projector's lamp is uselessly illuminating the black area! A 300-watt lamp effectively is transformed into a dim 40-watt bulb.

So -- if your purpose is to project a panorama, it's pretty obvious to us the best way to do that is to dedicate a set of projectors for the task.

An all-sky works best for covering all the sky ("well, duh!" we hear the wag in the back saying). But when you use only a thin slice of one, it's not so hot.

(Yes, we know we're mixing media with our illustrations here, trying to convey using computer graphics how reflected-light image projections might look. You'll just have to bear with us.)

In case we haven't convinced you of the superiority of a pan system for projecting pans instead of an all-sky, there's one more consideration: if your star projector is not on an elevator to lower it out of the way, it's going to cast six large ugly shadows of itself on the horizon of every all-sky you project. While the "wow" effect of the whole dome lighting up can ameliorate the obnoxion of the shadows somewhat, if you're trying to use the all-sky as a horizon pan, the projector shadow will end up blocking much of the thin strip available for the pan image.

With panorama projectors, there's a workaround -- you aim the projectors at an offset angle (22.5° left is recommended) to shoot around the projector. Your dome image processing software will compensate for the keystoning when you work the image in the computer.

But there's no such workaround for all-skies -- they MUST project straight across the dome, and naturally need to cover the full dome area. There's no place for the projector to hide, except out of the way!

Without an elevator for the mechanical beast in the center of the room, a planetarium simply cannot use an all-sky as a panorama. And since all-skies themselves are compromised too, the whole situation isn't viable in any event.

Summary

As in much of life, "use the right tool for the job" is always good advice.

Panoramas and all-skies are two different animals, and each has its own place and proper usage in the planetarium programming vernacular. If you're just starting out, it shouldn't be a question of "which one system should I install?". Determine your purpose, and install the system that meets the needs of the task at hand. You may find most planetarium show packages use panoramas much more often than all-skies -- ours certainly do. Decide which system will produce the best images for your audiences, and then use it effectively!