How to Make A Solar Viewer
This blog details everything needed to make a Safe Solar Viewer (SSV). Two types of projection viewers are described, a simple one costing less than $1 and a more elaborate one that can be made for twelve to eighteen dollars depending on the items you have on hand. The viewers depicted on these pages are part of the Eclipse Science Ambassador Project, a public outreach effort based at the College of Charleston Department of Physics and Astronomy. Before we write about viewers, the next section is a reminder about eclipse eye safety.
Eclipse Eye Safety
At the time of a solar eclipse it is important to avoid eye damage by using proper observing techniques during the partial phases of the eclipse. Many people know it is unsafe to look through a telescope at the partially eclipsed Sun. Some do not realize that it is also dangerous to look directly at the partially eclipsed Sun unless a safe solar filter or some other technique is employed. The late partial phases of the eclipse magnify this danger. One of the safest ways to view the partial eclipse phases is to project an image of the Sun onto a white surface. Below we give the directions for constructing two types of solar projection viewers.
The 5-Minute 55¢ SSV, a 1-Lens Safe Solar Viewer
It is surprisingly easy to make a simple solar viewer and this one is similar to the really neat pinhole viewer I was taught to make as a child but it is even cooler because it provides a brighter, sharper image at an affordable price. The SSV described in this section is less complex than the one in Figure 1 above. Constructing this viewer takes no more than five minutes after you have gathered the materials. I have made dozens of these viewers over the years as has my astrophysics colleague, Dr. Laura Penny.
All that is needed for the optics is a +1 diopter lens and this item is readily available locally. In fact you might already have one and not know it. A +1 close-up camera filter is just a +1 diopter lens and +1 reading glasses contain two such lenses. (Don’t use the reading glasses of your parents or grandparents! Buy your own.) A pair of such glasses can be had at the discount stores for as little as $1 plus tax, providing the lenses for two viewers. We got ours at Dollar Tree. These lenses have a focal length of about 39.4 inches (1.0 meters) and produce a magnified image of the sun almost half an inch across, easily large enough to show the partial phases of a solar eclipse.
There is some manufacturing variation in the focal length of these lenses so no matter which kind you use, the first thing is to measure the actual focal length of your particular lens. When the Sun is nearly overhead go outside with the lens, a white card, and a measuring tape. Hold the lens about 40 inches above the ground and project an image of the
Sun on the card. Move the lens toward and away from the card trying to make the Sun’s image as small as possible. Record the distance between the lens and the card when the Sun’s image is the smallest. That is the focal length of your lens.
Next find a box or tube long enough to hold the lens at one end and a piece of white paper at the other end exactly one focal length away from the lens. Stores like Kinko’s have dumpsters that are good places to find such items that have been discarded. In the past we used a cardboard tube but a box (or cardboard you fold into a long skinny box) is easier to make and use. (See Figure 2a.) Attach the lens to an opening cut in one end and tape white paper inside the box at the other end. Once you have done that your SSV is finished and ready for use.
With this SSV made
with a +1 lens you can simply lean the viewer against
a bench or other object at the correct angle so that the image of the Sun falls on the screen. We also make a shorter 1-lens SSV with a +2 lens that produces a bright quarter inch image of the Sun and the device is only about 20 inches long. If you stand with your back to the Sun and the SSV on your shoulder, you can practice using the shadow of the SSV on the ground to aim your viewer at the Sun. When the shadow of the tube or box is the smallest, the SSV is pointed at the Sun and an image of the Sun will appear on the viewing screen.
The Image Size of a 1-Lens SSV
We have made these SSVs from reading glasses from +1/2 to +2. The fractional power reading glasses are more difficult to find (though they do produce large images) and the viewers are very long (often more than 6 feet) and so we have ignored them in Table I below. This table shows the lens power, the length of the viewer and the solar image size.
Table I 1-Lens SSV Sizes (approximate values)
Power Focal / Length Solar / Image
+1.00 39 inches 0.40 inches
+1.25 32 inches 0.31 inches
+1.50 26 inches 0.26 inches
+1.75 23 inches 0.22 inches
+2.00 20 inches 0.20 inches
The instructions for the 2nd type (Super Safe Solar Viewer) are a bit more complex. If you are interested, they can be sent on request for full details.
Have Fun! BE SAFE!!