A couple of weeks ago I spend 4 days in a local elementary school with an inflatable planetarium.  I did shows during the day for all of the classes in the school (20 shows in all) and then did brief, 12 minute public shows in the evenings(18 of those).  In all, 600+ people saw a show in the dome.

After a couple of the public shows, I gave the audiences a peek behind the scenes. Here's my Mac desktop on the dome, warped by the mirror.

I have to say, I had an amazingly good time – the teachers and students were wonderful – and the kids had amazing questions.  But I’ll talk about that another time…  What I want to talk about today is the geeky end of the whole thing – and how it’s possible to set up a portable, inflatable digital planetarium.

The Dome

The dome, made by Go-Dome, is inflated much like a bounce house at a fair, except that it has no floor – it just sits on the floor of the room it is set up in.  You roll it out on the floor and it inflates in a couple of minutes.  The kids (and adults) really enjoy squeezing through the airlock to get inside.

The dome set up and ready for an audience.

The Projection and the Mirror

The projection technology I used is very cool.  It was originally developed by Paul Bourke, an Australian academic who works on 3d, dome projection and related ideas. The problem with digital projection in domes traditionally is that it either requires many projectors, blended together, or a very very expensive custom fisheye lens.  Paul realized that a single projector aimed at a 1/2 hemisphere mirror would cover almost the entire dome.  He then figured out the warping process that’s necessary to pre-distort the images to be projected, so when the are distorted by the curvature of the mirror, they come out right on the dome.   If you’d like to know more about this process in detail, check out Paul’s pages on spherical mirror dome projection.

I used a commercial implementation of Paul’s technology from Discovery Dome.  It encloses the  (fragile) mirror in a case which opens up.  The projector goes under the mirror facing forward, the light bounces off a flat mirror, and then off the spherical mirror on to the dome.  This keeps the projector out of the way….

Here's a look at the back of the dome - the funny looking thing towards the upper right is the spherical mirror setup. I'm to the right behind the computers.

The Computers

All of the images were generated from my MacbookPro, and I used a Macbook for control of certain things on the projection computer – more on that later.

The software I ran is an open source project called Nightshade, which is a fork of Stellarium specifically for use in domes.  By using a combination of live keyboard commands in Nightshade and running scripts written in Nightshades scripting language that I wrote, I had a very powerful and flexible planetarium that I could use interactively to take the shows where ever the kids questions lead.

That interactivity is where the 2nd computer came in – running scripts in Nightshade requires going several levels deep in a menu and taking more time the I wanted.  That is for all the scripts except the demo which runs from a keyboard shortcut.  So I wrote a shell script to give me a menu of my scripts and rename whichever one I wanted to run “demo.sts.” so I could run it with the hotkey.

The only problem is that when Nighshade is running, it takes over the displays completely – no way to have a terminal open to run my script in.  So I brought the Macbook – I used it to SSH into the Nightshade computer and run my shell script.  So I used the Macbook to select scripts, and the MacbookPro right in Nightshade to do everything else.

It’s kind of a hack, but it worked great.

Try it yourself

By the way – If I’ve gotten you interested in trying to do something like this on your own, you can home build your own planetarium.  I particularly like the instructions on the Microsoft WorldWide Telescope site. You can use this with Nightshade, or with the MS WorldWide Telescope – which I highly recommend.  In fact I would have used it in my shows if I had known about it at the time – it’s not a replacement for Nightshade (in my view) but a great compliment to it, and an amazing piece of work.  Lots of fun on your desktop to – you don’t need to run it in a planetarium.

Here I am in front of the dome at the end of a long day of shows.

On to the Planets

Now I’m moving on to another astronomically related projection project – I’m creating giant projections that will accompany the Portsmouth Symphony Orchestra’s performance of Holst’s suite The Planets.  The Performance will be on June 6th at The Music Hall in Portsmouth, NH.  I’m going to be blogging about my design process along the way, so keep an eye out for that, coming up soon.

Planetariums are magical places for many of us – I’d love to hear stories about your first or most memorable planetarium visit in the comments!