Solar Projection

N.B. This page was thrown together the day before I left for Oregon. I may not clean it up until 2024. You do know what's happening in 2024, don't you?

Solar projection

There are four good ways to look at the sun:

  1. Through eclipse glasses.
    In 2017 there was a flap over how many conformed to the ISO standard. These are typically cheap and inconvenient to use. Plus, if they're good, the sun is the only thing you can see through them.

  2. Through a telescope or other optical instrument with a proper filter.
    The filter must be on the objective (the big lens) because the objective concentrates sunlight close to the eyepiece and filters mounted near the eyepiece have gotten so hot that they break. Not good if someone is looking through it at the time!

  3. Projected through a pin hole camera.
    I hope to write more about this. For now, find a reference on the web or simply climb a tree during a partial eclipse. (Preferably a silver maple because they have deeply lobed leaves and make a lot of great pinholes.)

  4. Projected through an unfiltered telescope or other optical instrument.
    That's what I do here. People can use a smallish telescope, heliophysicists have some very large (and filtered) telescopes to make table-size images. If the instrument is small enough it will pass enough light and conduct away the rest without getting damaged itself.

Projection has some big advantages. The biggest is that multiple people can look at it at once. You can even track the progress by drawing on the target. With magnified optics like the monocular here, the aim needs to be adjusted frequently (you can see the sun move across the target). It's not much hassle to make the small adjustments to keep the image on target.

I expect that my group of five or so will use it more often and for more time than our eclipse glasses.

Monocular projection

This was taken during a light overcast. The light from the clouds washed out the image of the sun on the target.
The mounting piece is from a 1x2 piece of trim. From rear to front are:

  • The monocular.

  • Two magnets for attaching the sun shield.

  • 1/4"x20 T-nut to attach to tripod.

  • A U-bolt to get the aiming dowel away from the mount.
The monocular fits very nicely in a 15/16" hole.

The monocular is 8x20, I don't remember where I got it, but they've been advertised in many places. The 20 is for 20 mm, the size of the objective. This controls the amount of light let into the device. 20 is too small for most applications, but it works well for this one. I left it aimed at the sun for a couple hours and nothing broke or melted. The 8 is the magnification, how much wider the projected image is compared to a pinhole projector.

7x50 binoculars are good for nighttime use, 8x30 are good for daytime birding, 10X magnification starts getting hard for handheld use, etc.

A common way to aim a telescope at the sun is to look at its shadow and point so the barrel makes a circular shadow. This setup does just as well. the U-bolt gets the dowel away from the mount, though it and the rubber bands get in the way a bit. I generally use one hand to display the shadow of the dowel and aim things with my other hand.

At the very least, it gets the sun through the monocular and I can use my hand or the target to pick up the image near the eyepiece and follow it down to the ground.

The target is just a piece of matte board attached to a backpacking tripod and approximately aimed at the sun.
This what you'd see if you looked from the target at the sun. There's no need to do so. Well, maybe to see where obstructions are, but you can see their shadows. Or perhaps to see how bright the overcast is.


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Thrown together 2017 Aug 16, last updated 2017 Aug 16.