What to do if you're caught without optics on eclipse day.
In preparation for this month’s total solar eclipse, my last two columns were about making solar filters and solar finders so you could observe the partial phases with a telescope. But what if you don’t have a telescope or didn’t get your filter material in time? Are you totally hosed?
Not at all! Observing the eclipse safely can be as easy as poking a hole in a piece of aluminum foil. No, don’t look through the foil at the Sun! That would still damage your eyes. Use the hole to project an image of the Sun on a screen a few feet away.
A pinhole acts like a tiny lens, creating a focused image. It doesn’t let much light through, so it only works well for bright objects like the Sun, but it works great for that. With a pinhole projection you can watch the Moon slide in front of the Sun on eclipse day, taking a bigger and bigger bite until mid-eclipse, then slide away again.
The biggest problem with a pinhole projector is that the image is small and dim. Moving the screen farther away from the pinhole will make the image larger, but because the light is spread out more, it will get dimmer. Making the hole bigger will let more light through, but the image will get blurrier.
How small is it? Pretty small. The Sun is only half a degree across, so the projected image will only be half a degree across, too, from the perspective of the pinhole. You’ll get about 1/10 of an inch of image diameter per foot of distance. So if your screen is three feet away, your image will only be about a third of an inch across. If you want the image to be an inch across, you would need to put your screen 10 feet away. That’s just not practical.
Fortunately there’s another way: Use a mirror to reflect an image of the Sun onto a shaded wall. If you mask down the mirror to half an inch or so, it’ll act like a pinhole and create a fairly good image. It’ll be blurrier than an actual pinhole but much brighter, so you can cast its light a lot farther and get a bigger image. For viewing the crescent shape of partial eclipse phases, it will work just fine.
If you want a sharper, brighter, bigger image yet, there’s one more good trick: You can use a pair of binoculars to project an image of the Sun onto a screen. The binoculars act as a complete optical system, using the entire front aperture to gather light and focusing it into a collimated beam that comes out the eyepieces. You can put a screen anywhere behind the eyepieces and project a crisp, clear image onto that screen, and the image will be much larger and brighter than with a pinhole or small mirror. You might have to adjust the focus of the binoculars, but you can get it crisp enough to see sunspots if there are any big ones.
You need to be careful about a couple of things if you use binoculars. First of all, don’t ever look through them at the Sun without a proper filter, not even for a second. And make sure nobody else can, either. That means setting them up in such a way that no curious children can get their heads between the screen and the eyepieces.
Second, don’t use a good pair of binoculars. While you’re aiming them at the Sun, the intense beam of focused sunlight will be dancing around inside the eyepiece housing, heating up the field stop and melting any plastic parts it stays in contact with for too long. This is a project for a pair of Goodwill binoculars, not that fancy pair of Celestron 15×70s.
If you have none of the above methods at hand, nature gives you one for free: Just stand under a tree and look down at the dappled pattern of light filtering through the leafy canopy onto the ground. The gaps between leaves act as tiny pinholes, casting hundreds of crescent-shaped spots of light during the eclipse’s partial phases.
And during totality? Look up! It’s safe to look at totality (but only totality) directly. Remember to close your mouth or flies will buzz in!
This article originally appeared in print in Sky & Telescope's August 2017 issue.