(Posted 25 October 2015) I received the below email from a teacher about making Tin Can Planetariums. Given my Tin Can pinhole planetarium is a very popular download, I wanted to share my response.
Her email (with name and location deleted) is as follows:
Hello Paul,
I am a 4th grade teacher at a project based learning charter school in XXXXX. My students are currently working on a project that is going to involve creating a tin can planetarium, much like the one I found on your website while searching for ideas. I am currently attempting to make the project myself before I send the kids off to do it and I was wondering if you can offer some advice. What size can is best? So far I’ve tried some smaller cans, but the star projection on the ceiling was dull and fuzzy. We are hoping to wire an LED circuit inside to light the can and have the constellation project onto the ceiling. Would a larger can provide a clearer image? How bright does the light source need to be? I am assuming that the brighter the light, the clearer the stars. One LED was much too weak. Those are my main concerns right now, but any help you are able to offer would be greatly appreciated!
Kindly,
XXXXX
My response is as follows.
Tin can pinhole planetariums can only ever work so well. To have sharper star images, you need smaller holes. This equals fainter stars. Unless of course you use a brighter light source. Unfortunately, that usually means your light source (i.e. the light’s filament in the case of old fashioned light bulbs) is physically larger. Your pinhole acts as a camera obscura and projects an image of your light source. This means you see an image of the light bulb filament projected (rather than the pinpoint star you want). Ideally you want a infinitely bright, very small light source and your problems would be solved! A really small Star perhaps (insert humour here!)? In practice, the best affordable solution for students I ever found was to use a Mini-Maglight torch bulb (for a 2 AA battery torch), and to only project the star images about one meter.
Hope that is of help.
Regards,
Paul Floyd.
