Portable Planetarium Projection Options

2024-01-25 / Julia Jansen / Buyer Guide

Introduction

Digital portable planetarium systems are a great choice for educators looking to reach a wider audience with their planetarium programming, since they can be packed up for easy transportation and set up anywhere an inflatable dome will fit. However, if you are looking to build, purchase, or upgrade a portable planetarium system the variety of options to choose from can be daunting.

One of the choices you’ll need to make is whether you want a projection system that uses a fisheye lens or a spherical mirror. As a planetarian who has operated both kinds I’ve compiled some of the essential differences between these projection systems and my experiences to give you a better idea of what will work best for you.

Fisheye Projection Systems

Fisheye lenses are lenses that bend incoming light through extremely large angles to project over a wide area like the surface of a dome.

A fisheye lens system is easy to set up in a mobile dome since the projector is one piece. Once the dome is inflated, the projector and lens combo is set up near the center of the dome so that the projection will be above the heads of the audience. If the projection is a little out of focus or tilted these can be quickly and easily fixed by adjusting the projector.

Another benefit of a fisheye lens system is that the projection quality is very uniform across the dome, so audience members can get a good view of the sky wherever they look. This wide range of good seats, combined with a projector that sits near the center of the dome, makes a concentric seating plan ideal for a portable dome with a fisheye projection system.

If a low-quality fisheye lens is used in the projection system, it can distort the color of the projection. This creates an effect called “chromatic aberration,” where different colors of light drift apart in the final projection because the lens bends each color by a different amount. This effect can be jarring, so just be aware that not all lenses on the market are created equal and quality does vary.

A fisheye projection lens is expensive to build because it is a challenging design that requires multiple high-quality optical elements, microscopic tolerances, and high-tech anti-reflective coatings. Though more expensive, a fisheye lens system is, in my experience, far easier to set up and therefore well worth the cost.

Spherical Mirror Projection Systems

Instead of bending incoming light through a lens, a spherical mirror reflects light that hits its surface so that the image from a projector is spread out over a larger surface like a dome.

In portable domes the projector in a spherical mirror system is usually pointed at a flat mirror which reflects light from the projector onto the spherical mirror, and the spherical mirror then reflects that light onto the dome. The flat mirror helps prevent a shadow of the projector from appearing on the dome, and also allows for a more compact system.

Unlike a fisheye projection system, it’s possible to build your own spherical mirror projection system from scratch, which is less expensive than a commercial system if you have the time and skills to build one. The components of a spherical mirror system are less expensive than a high-quality fisheye lens used for dome projection. A spherical mirror system is therefore often a cheaper option for portable dome projection, although it comes with several unique challenges that you should consider.

One challenge is that spherical mirror systems are prone to distortion and inconsistent brightness, which is caused by the projector’s light being spread out differently depending on where it reflects off of the spherical mirror. This is addressed by using a digital map to warp the size, location and brightness of the output from the planetarium software before it gets sent through the projector. However, this map requires a precise alignment of the mirrors, projector, and dome to work. The map takes a long time and a lot of know-how about computer graphics outputs to recalibrate, making it impractical to do every time your planetarium is setup.

One time I worked with another volunteer to try and recalibrate the map for our portable spherical mirror system. It took about an hour and a half of pouring over the graphics output of the computer and online forums for how to recalibrate the map to realize that we were in way over our heads, despite having a few years of planetarium experience and two physics degrees between us.

Without recalibration, the projection quality will depend on your ability to set up the parts to match how they were set up during calibration. In a mobile dome, where each part gets packed separately for safe transport, this can be especially difficult to do.

Another big challenge with a spherical mirror system is bringing the projection into focus. This is because projectors are designed to focus an image onto a flat surface like a projection screen, not a curved surface. This means that it can be impossible to bring the entire projection into focus at the same time.

Usually the solution is to bring the “front” of the dome—the side opposite the mirror—into the sharpest focus and seat the audience facing the front so the blurrier edges are mostly out of view. This also helps draw attention away from the “back” of the dome near the mirror where the projection cannot reach. Overall, this means a spherical mirror system tends to work best with unidirectional seating.

When I volunteered in a portable dome with a spherical mirror system, I was always frustrated trying to achieve a decent projection. If the flat mirror or the projector was even a centimeter or a few degrees away from the “ideal” alignment, it could cause noticeable distortion on the dome. Even once a passable alignment was set between the parts, I often ran into the problem that the mirrors were just slightly too far from the projector to be within its focal range. Since the alignment of the mirrors and dome in relation to the projector affect the focus of the projection, it was often necessary to adjust both the alignment and the focus of the system more than once to get a decent projection. This meant that even after the dome was inflated, I often spent a lot of time going back and forth between minimizing the distortion and focusing the projection, only to end up with a projection that was only just “good enough”.

Another challenge with spherical mirror systems besides projection quality is that they are more fragile than fisheye lens systems. Because there are more moving parts, it’s more likely that a small bump will knock the system out of alignment. The mirrors are also more susceptible to damage than a fisheye lens or even a standard household mirror.

Most household mirrors are “second surface” mirrors which have two layers: a reflective layer, and a transparent layer that helps to protect the reflective layer from damage or degradation over time, while still allowing light to bounce off of it. To achieve a high-quality projection, you’ll need to use a “first surface” mirror, or a mirror made without the thick protective layer.

These mirrors are significantly more prone to damage so it’s important to leave extra space on all sides of the projection system to avoid people bumping the mirror by accident. This takes up valuable seating space in portable domes where space is already limited.

Concentric vs Unidirectional Seating

I’ve mentioned that it often makes the most sense to use concentric seating in a portable dome with a fisheye projection system, and unidirectional seating in a portable dome with a spherical mirror system. Different planetarians have different opinions on the matter, but I personally prefer a portable dome with concentric seating for a few reasons.

Most importantly in my experience, concentric seating tends to create a more immersive experience than unidirectional seating. Rather than facing towards one particular view, which can feel a bit like watching a movie, the audience sees the projection covering a full hemisphere in the dome, which is much more true to the experience of being outside. This allows a planetarian to share tricks for star-gazing that the audience can practice in the dome, and then easily replicate outside.

You can also fit more people in a portable dome with concentric seating, since the widest views available are at the outer edge of the dome. Even as the audience gets closer to the projector near the center, they are still able to see a large portion of the dome. The only space that can’t be utilized for seating is right around where the projector sits, where it could take up a significant portion of someone’s view.

In contrast, in domes with unidirectional seating, there are fewer “ideal” or “great” seats. The quality of view for an audience member decreases quickly the closer they get to the “front” of the dome as their angle of view gets narrower. In my experience, the front quarter to a third of the dome needs to be left empty since viewers sitting in those areas can’t see much of the projection. This leaves a lot more unusable space than a dome with concentric seating meaning you need a larger dome for the same audience sizes.

A spherical mirror system presents a unique challenge to unidirectional seating: the system has to be set up at the back of the dome where the majority of the seating is located in a unidirectional dome.This leads to the difficult task of trying to maximize the amount of seating in the dome while still leaving enough room around the projector system to prevent accidental bumps and damage to the system.

Summary

Both spherical mirrors and fisheye lenses can be used to project to a domed surface in a planetarium. The best option for your planetarium will depend on your budget, how you intend to use your mobile planetarium, and how much experience you have with computers and projection equipment.

See the table below for a quick summary of the differences between a fisheye lens set-up and a spherical mirror set-up to help you determine which system will work best for your needs.

At Digitalis Education Solutions, we only use fisheye lenses in all our portable planetarium systems as they produce the most consistent projection quality, dome coverage, ruggedness, and ease of use. Our proprietary lenses are the highest quality and best value on the market and only available from Digitalis.