<p>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.</p>

<p>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.</p>

<p>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. </p>

CC BY-SA 3.0
2017-07-01T16:03:23.527Z
2017-07-01T16:03:23.527Z
JerryTheC
<p>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.<br>
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.</p>

<p>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.</p>

<p><a href="http://today.cofc.edu/2017/06/13/solar-eclipse-viewer-diy/" rel="nofollow noreferrer">http://today.cofc.edu/2017/06/13/solar-eclipse-viewer-diy/</a>
<a href="http://www.surplusshed.com/pages/item/l14766.html" rel="nofollow noreferrer">http://www.surplusshed.com/pages/item/l14766.html</a></p>

<p>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:</p>

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

<p>$H$ 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)</p>

<p>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.</p>

<p>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.  </p>

<p>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.</p>

CC BY-SA 3.0
2017-08-21T12:33:44.017Z
Equation cited is wrong. This is the correct one:
H = 2 * tan( S/2 * L1/L2 * pi/180) * D
Paul
CC BY-SA 3.0
2017-08-21T14:45:35.753Z
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?
J. Chomel
CC BY-SA 3.0
2017-08-25T06:46:34.017Z
You are correct, it should be pi/180, not the way it was originally written.
nflemming2004
CC BY-SA 3.0
2017-07-11T12:50:21.693Z
2017-08-25T06:47:13.603Z
2017-08-25T06:47:13.603Z
nflemming2004
<p>This might be an astronomy.stackexchange.com question, but I think it is more in line with a camera obscura.</p>

<p>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.</p>

<p>I'd like to plan ahead more this time.  </p>

<p>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.)</p>

<p>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.</p>

CC BY-SA 3.0
2017-07-10T20:45:41.270Z
Projecting with a telescope, even if it's a "lousy toy" works better than a simple lens. http://astrosociety.org/edu/publications/tnl/05/stars2.html More resources: http://www.skyandtelescope.com/2017-total-solar-eclipse/2017-eclipse-resources/
Florin Andrei
<optics>
Projecting an image of the sun / eclipse
rrauenza
# Projecting an image of the sun / eclipse

(Asked by: username_0 on 2017-06-29)

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.

**Tags:** optics

> **Comments:**
> **username_1**: Projecting with a telescope, even if it's a "lousy toy" works better than a simple lens. http://astrosociety.org/edu/publications/tnl/05/stars2.html More resources: http://www.skyandtelescope.com/2017-total-solar-eclipse/2017-eclipse-resources/  

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## 2 answers

### Answer by username_2 on 2017-07-01. Score: 1

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.


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### Answer by username_3 on 2017-07-11. Score: 1

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.

> **Comments:**
> **username_4**: Equation cited is wrong. This is the correct one: H = 2 * tan( S/2 * L1/L2 * pi/180) * D  
> **username_5**: 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?  
> **username_3**: You are correct, it should be pi/180, not the way it was originally written.  

---

