3
$\begingroup$

This might be an astronomy.stackexchange.com question, but I think it is more in line with a camera obscura.

During the last eclipse I didn't plan ahead and grabbed some lousy toy "spy" telescope we had (about 2 inches long), set it up on a tripod and pointed it at a white foam board.

I'd like to plan ahead more this time.

What can I use to project a decent image of the sun onto a foam board? What kind of lens do I need? Would a magnifying lens work? How about a close-up diopter filter? How about a 50mm lens? (I'd like to use something I already have, or is cheap -- and I don't want to use a pin hole.)

My first thought was that a magnifying glass only focuses the sun down to a pin point (and would burn the board), but I'm recalling now that past that focal point you can project an image.

$\endgroup$
1
$\begingroup$

I've done this using two lenses. In my case they were a 300 mm and a 50 mm. I built a jig to hold the lenses in-line with each other, to allow for small adjustments in the placement, and for a tripod mount.

Put the long lens, in my case the 300 mm, in front normally. That will form a focused image behind it. Put the short lens backwards behind that. This lens is used in reverse. It takes the small image created by the long lens and projects it out what is normally its front.

The ratio of the front lens focal length to the back lens focal length is how much angular "gain" you get from this setup. In my case, the cone of project of the sun was 6 times wider coming out of the 50 mm than going into the 300 mm. This means you don't have to get as far away from the overall setup to get a decent size picture.

I'm not where this jig is now. I'll try to remember to take a picture of it and post it here within the next day or so.

One thing you'll find is that the earth rotates annoyingly fast when you are looking at a magnified picture of the sun. Use a telescope mount if you can.

Another issue is blocking out as much ambient light as you can. The projected image of the sun is reasonably bright, but so is daylight. Some sort of baffle over the top and the front except for a hole for the front lens helps a lot. People can then view the projected image from the sides.

$\endgroup$
1
$\begingroup$

Safety: make absolutely sure nobody looks onto the light path. The concentrated light/heat/UV can cause eye damage. One way to do this is to set things up in a box, which (a) shades the viewing screen, and (b) lets you make the viewing opening too small for kids (or thoughtless adults) get their heads in - or make sure that your screen is close enough that there's no room to squeeze a head in.

The size of the prime focus image is determined by focal length. To get a decent size prime focus image needs a LONG focal length. That's why a telescope style projection setup as in olin's answer can give you a bigger image from shorter length lenses. (and is less likely to just ignite your viewing screen). However, remember that you're using focused raw sunlight, so there's a lot of heat involved. That means that for a real telescope, it's safest to use only an all-metal style refractor, where there's nothing to melt. You probably also want to avoid using expensive eyepieces - cheaper ones usually have fewer elements, and it's less of a financial hit if the heat damages something.

If you're using a scope with a 90 degree star diagonal, that means that you're projecting things side on, which makes it easier to shade the screen. Or if you're going straight through, you can add a card across the front with a hole for the lens to provide shade.

$\endgroup$
1
$\begingroup$

Like the two above, I'm building a solar projector. There's actually some pretty well-laid out plans and instructions (and lens kits) available for this. That said, it's basically a telescope with the eyepiece slightly refocused, and a projection screen some distance away.
You can use a pre-built telescope, spotting scope, or binocular to build this. However, I would NOT recommend using one you like, as explained in the cautions at the bottom.

Check out the following link for some info, and the second link is to a lens kit that costs $11 after shipping. It's the one used in the instructions.

http://today.cofc.edu/2017/06/13/solar-eclipse-viewer-diy/ http://www.surplusshed.com/pages/item/l14766.html

If you want to pick out your own lenses, here's an equation to help you figure out how big the projected image will be:

$$H = 2D\tan (\frac{S}{2} \times \frac{L_1}{L_2} \times \frac{\pi}{180})$$

$H$ is the image diameter $S$ is the angular size of the sun (0.53 degrees) $L_1$ is the focal length of your first lens (called an objective) $L_2$ is the focal length of your second lens (called an eye lens or eyepiece) $D$ is the distance between your eyepiece lens and your screen (in whatever units you want H to be)

Note 1: your calculator should be set to radians (not degrees). Google uses radians by default. Note 2: if you have a cheap telescope, binocular, or spotting scope where the eyepiece is built in, you can substitute $\frac{L_1}{L_2}$ for the magnification.

As an example, if you have a 20x setup (say 500mm objective and 25mm eye), and the distance to the screen is 1 foot, your image will be just over 2" in diameter.

Cautions: NEVER look directly at the sun or worse, directly through magnifying optics at the sun. Due to risk of permanent damage to your telescope, never point a quality telescope at the sun. Optics can melt, and plastic parts WILL melt given time.

$\endgroup$
  • 1
    $\begingroup$ Equation cited is wrong. This is the correct one: H = 2 * tan( S/2 * L1/L2 * pi/180) * D $\endgroup$ – Paul Aug 21 '17 at 12:33
  • $\begingroup$ Someone without rep. said: equation is wrong, and should be one: H = 2 * tan( S/2 * L1/L2 * pi/180) * D . Are you able to confirm and fix it? $\endgroup$ – J. Chomel Aug 21 '17 at 14:45
  • $\begingroup$ You are correct, it should be pi/180, not the way it was originally written. $\endgroup$ – nflemming2004 Aug 25 '17 at 6:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.