As part of our ongoing efforts to build a small classroom planetarium at our school, we have opted not to go with a package deal from a single vendor and instead assemble components from different vendors.
Along the way we’ve discovered some interesting gear that might be of interest to some other developers of immersive projection systems for flight simulators, gamers, photographers, and others, so I thought we’d share the current state of our project as a sort of preview, and I’ll follow up later this year with a report on the actual installation when it is up and running.
The Screen
The short version for the non-astronomer is that we are basically building a giant hemispherical screen with a 180 degree computer projector in the center. The projected image we plan to use will be a planetarium program such as Celestia or Stellarium, but if you succeed in installing a setup like ours you could also use it to project Quicktime panoramas, flight simulator screens, and, of course, video games.
Our “screen” is a hemispherical fiberglass dome manufactured by Astro-Tec of Canal Fulton, Ohio. They make one standard size of such a dome, and it is fifteen feet in diameter. For the purchase price we paid, approximately $22,000, we expect to get a complete dome with a mounting frame underneath to allow us to tilt the dome forward instead of the more traditional horizontal arrangement. The dome consists of gores like the slices of an orange with a cap for the top, which can be later sealed and painted to conceal them when in use. Astro-Tec has been very good about communicating with us about the dome design, and we will hope to take delivery of the dome in a couple of months.
There is another manufacturer of small planetarium domes, MMI Corporation. They were unable to assist us in mounting the dome in a tilted position, but otherwise they offer a similar product. Their dome is slightly larger, but as far as I know there is no provision for “sealing” the seams. We do plan to purchase a projector from MMI in the near future.
One thing we learned from this process is that you cannot use a blank white dome for this sorts of project; glare from any bright projected object such as a full moon would light up the whole dome and spoil the effect. Although the dome is light-tight, we’re going to paint the surrounding room black to absorb stray light and then paint the interior of the dome some darker shade of off white, perhaps 40% black paint mixed with 60% white, so the glare is reduced. As we’ve noted in our inflatable Starlab planetarium, a gray surface actually looks great under dim lighting conditions – you simply cannot tell it is anything other than black when not lit, white when stars are projected.
As far as domes or “spherical screens” go, there are at least two other alternatives to the route we chose. You could go with an inflatable dome, for which the industry standard is the well-known Starlab dome. A dome with blower and no star projector can be purchased for about $6,500.
If you are trying to be even more budget conscious, you could build a hemispherical projection screen from cardboard using our instructions posted at A Planetarium for Every Classroom. It would be a relatively simple matter to create a dome nearly the size of most living rooms. The cardboard triangles of the geodesic dome can be sealed with tape and painted to be nearly seamless. We painted our original dome white, but we would use a darker shade than what we originally used given what we know now.
The Projector
Finally, for us, there is the primary electronic projector. For this project I got several interesting bids for all-dome digital projectors. Based on a tip from a reader of this column who works for Stanford University, we selected a projector from Elumenati, LLC. They make a variety of wide-angle projectors with a base price of $7,900 for the most basic model, which we purchased.
The only other bids we received were on the order of $150,000 for a new projector – obviously for a higher quality device. Unfortunately this exceeded our entire budget.
We tested the projector the other day in a darkened room, and the results seem promising. The projector is called a HAL-1200a and has the following specifications: XGA (1204 x768 resolution), 400:1 contrast ratio, 7.8 lbs. Our model is not meant to be moved around much; the model 1200 (sans a) is more portable but somewhat more expensive.
The important part for planetarium construction is that you need to have your dome people talk to your projector people so you can find out what specific shade of grey you want the dome to be painted. We’ve pretty much completed that process and will be awaiting our dome to be constructed and shipped later this year.
I’ll write a followup with pictures as the project progresses, but one thing occurs to me – the dome can be used for other purposes. I intend to load a QuickTime VR of the Parthenon in the projector and see how it looks; there will probably be some distortion, but it ought to work as a proof of concept.
It’ll be interesting to see some video games in the dome. That would be an interesting way for traditional planetariums to increase foot traffic over the traditional rock-music-and-laser-show crowd, wouldn’t it?
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