If you live in North America, you know that this upcoming Monday is a big day for aficionados of astronomical phenomena: it’s a solar eclipse. And if you haven’t already, you need to get some of these glasses that let you look at the sun. But, the sun’s angular diameter in our sky is only half a degree. It’s smaller than your thumbnail at arm's length, and that’s pretty small. I won’t be in the path of totality this time, but I will be at an elementary school here in San Antonio enjoying the eclipse with students. And as usual when I talk to kids, I’m feeling compelled to try and make this experience a bit more visceral. Like mythbusters with explosions, I figure the best way to increase the excitement of a solar eclipse is to make it bigger.
The one job of a telescope is to gather a large amount of light and concentrate it so that you can see astronomical objects more clearly. That’s perfect for faint subjects like planets or nebulae, but not ideal for our nearest star, the sun. Even looking at the sun with no magnification can damage your vision, and magnifying those harmful rays is going to make the damage worse. So how can we get a bigger view of the sun safely? There’s two common ways: first you can use a solar filter that works just like eclipse glasses, blocking out the majority of the light so that you can safely see the sun through your telescope. But this way has a disadvantage: only one person can see at a time. Not ideal if you’re trying to share the excitement with a crowd. The second way is to use the telescope to project an image of the sun on a screen, and that’s the basis of the sun funnel.
Telescopes and binoculars use lenses or curved mirrors to concentrate light. The optics are usually aligned to focus the light onto the retina in your eye. But, if you move your eye out of the way, the light keeps going. Put a screen in front of it, and now you’ve got a projection. It really is as simple as holding up a screen in front of the eyepiece, but the sun funnel solves two practical problems: (1) it automatically keeps the projection screen in the correct orientation to the eyepiece even as you move your telescope around to follow the sun, and (2) it keeps anyone who doesn't know better from looking through the telescope at the sun and accidentally blinding themselves.
My design is based on a guide published on NASA’s website that I’ll link down below. They used a plastic funnel, but I decided to make my own using this thin-gauge aluminum sheet. I used pop rivets to fasten the edges. You can size the funnel based on your telescope, eyepiece, and how large you want the sun to be using a fairly simple formula. The eyepiece to my telescope slipped into the small end of the funnel and I connected it with a hose clamp. On the other end, I stretched a piece of vinyl shower curtain to act as a rear-projection screen. The last part of the project is to stop down the aperture on my telescope. Most reflector telescopes aren’t meant to focus the strong rays of the sun , and I didn’t want to overheat my secondary mirror. I’ll also be covering up the aperture altogether at regular intervals to make sure I don’t damage my telescope.
Here’s a test I did in my backyard. The sun is plenty bright enough to see, and big too! I think the kids are going to be impressed. I’ll post some pictures of the real eclipse in the sun funnel on my instagram next week but for now I can show a simulation of what it will probably look like. Even if you’re not looking at an eclipse the sun funnel is a fascinating way to observe an actual star up close and personal. I hope you liked this quick project, and for those of you in North America, I hope you get a chance to see the eclipse next week. Thank you for watching, and let me know what you think.