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Where should I sit?

January 8, 2009

There are four typical seating arrangements in a planetarium theater:

Ÿ Unidirectional. These are seats like what you see in a movie theater- -chairs facing forward in straight rows.

Ÿ Epicentric. Probably the most commonly seen arrangement, these are seats in rows that are curved, to allow for the best use of space in a round theater. This also points every audience member in roughly the same direction (a point in the front of the theater) which is good for focusing attention during shows.

Ÿ Co-centric. A common sight in older theaters, co-centric seats are often actually benches, arranged in circles around the house floor. This sort of seating is good for live presenter-led demos, but not necessarily for automated shows, because the audience often needs to move their heads around (neck-craning) to follow the action on the dome. Co-centric seating is rare nowadays, as most theaters are going to unidirectional or epicentric.

Ÿ Floor. Why, oh, why anyone would make their audience sit on the floor to watch a show is beyond me. Sure, this makes sense for very small theaters (like 2-3 meters, or portable inflatable domes that you’d take to a school) because size is an issue, but in my opinion, not having actual chairs in a big theater with the floor space to do it is unforgivable.

Most of the planetaria you’d visit will have epicentric seating. Even very large theaters with tilted instead of flat domes, and thus stadium seating (also called “raked” seating, to use an ol skool term) will often have slightly curved rows to maximize space and forward-facing foci.

However, no matter what kind of seating a theater has, there is one fundamental problem with all projection on a planetarium dome: hemispherical distortion.

Imagine you’re under a dome (let’s say a 60 foot dome, about 18 meters) with a starfield projected on it, standing in the very center of the theater (directly under the apex of the dome).

No matter which direction you look, you’re focusing your eyes at about 30 feet away. The horizon will be 30 feet from you, the dome apex will be 30 feet from you, and all in-between points on the dome will be 30 feet from you.

If you tilt your head back and look straight up, the zenith of the starfield (the spot in the sky that is directly above your head at that moment) will be at the apex of the dome.

In this spot in the theater, the starfield will look pretty good in every direction. What you see is just about what your field of view would be outside. The stars will appear to be in all the right places, and constellations will look correct in regards to shape and separation. This size of the constellations may seem smaller than normal- -primarily because the dome isn’t as big as the outside, and also because you’re not focusing your eyes to infinity here inside the theater, but besides that, the night sky should look fairly accurate.

Now, keeping your eyes on the dome (and imagining that there aren‘t any chairs to stumble over in my little thought experiment here), walk toward the “south” theater wall. As you move away from the center of the theater, the starfield changes drastically, thanks to the shape of the dome. Suddenly, the Northern quadrant of the sky is 60ish feet away from you, and the Southern quadrant is very close to you, so close that a small portion of the sky fills your field of vision, and you have to crane your neck to look around it.

Think of it like this: when you’re outside, the “dome” that is the sky always stays with you as you move. The “apex” of this dome (the zenith) is always directly over your head, no matter where you are on the Earth.

In a planetarium, there is typically only one spot where the zenith is directly over your head: the very center of the theater. When we tell people to look at the zenith, it’s typically in a spot that’s not directly over their heads, because not everybody gets to sit in the very center of the theater. Actually, in most planetaria, nobody gets to sit in the very center of the theater, because usually there’s a projector occupying that space!

Spherical screens cause all sorts of other visual problems too. For instance, imagine a thin, straight line (say, drawn by a laser projector) from due east across the dome to due west, right through the dome’s apex.

If you were sitting directly under this line, it would appear as pretty much straight, across the dome from one side to the opposite, 180 degrees away.

However, if you were sitting in front of or behind this line, it would appear to be curved, following the circle of the dome. The farther forward or backward you get, the more distortion is applied, and the more curved the line seems.

This also affects images projected on the dome. Imagine a rectangular image on a flat movie theater screen. For the most part, no matter where you are in the theater, the projection will look like a rectangle. Unless you are very far to the side of the screen or very close to it (in which case the projection would look skewed, like a trapezoid), it will look fine.

Now imagine the same image projected on a dome. If you’re sitting directly in front of the image, in essence between the image and the projector making it (like you would be in a movie theater), its distortion in your frame of reference will be small. However, if you’re very close to the dome, essentially under the part of the dome that the image is hitting, it will take on a strange shape, stretched on the bottom and at the top, with severely pincushioned sides.

We have to take this into effect when projecting round images, which as you can imagine, are projected quite often in a planetarium. Most of the time, for very big circular images being shown, a hemispherical correction is applied to the image. This makes it look funny on a flat screen, but look right on a spherical one.

A good example of this is a commonly-seen planetarium image: a chord.

A chord of Mercury

A chord of Mercury

Now, obviously, when you’re looking at a chord, you’re essentially seeing the horizon of the object, therefore it’s curved significantly (since it’s shaped like a sphere!), and the bottom of the image, where the object is cut off, is flat.

But if you were to just project this kind of image in a planetarium without correcting it, it wouldn’t always look right. The curvature of the dome would add to the curvature of the planet/moon/whatever, making it appear to bulge at the top and fall off unnaturally at the sides.

You can fix this by applying hemispherical correction to the chord (there are a handful of programs that do this for you, for instance, there’s one called PolyDome, but you can also finagle PhotoShop or Blender to do it too). The result doesn’t look right on a flat screen; the horizon of the object, which was originally curved, is now almost straight, and the bottom is curved. When projected onto a hemisphere though, it looks right.

Speaking of horizons, another annoying effect of this distortion is that stars just above the theater’s horizon that is furthest away from you (the other side of the theater) will appear to be lower than it actually should be, and the horizon that is closer to you will appear much higher than it should be.

Besides image correction with software, there are a handful of little tricks that we use to minimize the problems with hemispherical distortion. The one that makes the most sense is to focus the show’s action on one specific area of the dome (the “front” of the theater, or the direction that your unidirectional or epicentric seats are facing).

So for instance, if you wanted to show off Orion in a mid-winter sky show, optimally you would spin your star projection to southeasterly heading, placing the Hunter front and center. Personally, I would also subtract about 10 degrees to your latitude north, so that Orion would be just a little bit higher in the sky than people are normally used to seeing him. This helps to alleviate the problems with hemispherical distortion by putting the subject into a position where the majority of the audience sees it from an optimal distance, and also cuts back on necessary neck-craning.

We’ll also make sure that images being projected aren’t so big that the viewer needs to tilt his or her head to take in the entire thing. Again, if you’re close to the image, this is going to be more of a problem than if you’re far from it.

Thus, almost always, the best place to sit in a planetarium is in a row in the “back” half of the theater, and in a seat in the middle of the row. Seats that are close to the edge of the row, or seats too close to the front of the theater, will always have some kind of distortion because of the dome, and unfortunately there’s really no getting around it.

My advice: arrive long before your showtime, so that you have pick of seats. Go to the back of the house, maybe not the very last row, necessarily, but definitely the back half. Sit in the middle seats, so that the apex of the dome is in front and ahead of you.

(And turn off your cell phones! Put away that PSP! Kids, quit bouncing your flashy shoes! What are you, heathens or something? We’re trying to make it dark in here!)

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