I didn’t find out about last Sunday’s solar eclipse until just a few days prior, and I didn’t have any equipment that I could use to safely view the eclipse. Not wanting to resort to the pinhole projection method for viewing the eclipse, I consulted my 40-year-old copy of Sam Brown’s classic (and extremely informative) book All About Telescopes for some other ideas. The book showed a design that would fit over the front of my 8″ Newtonian telescope, stopping the aperture down to 2″ and using a lens from a welding helmet to knock down the sun’s intensity to a manageable level. All I needed was the welding lens.
Not having enough time to order something and have it delivered in time, I needed to find the welding lens locally. I was pleased to discover that both Home Depot and Lowe’s carried a 4.5″ x 5.25″ lens that would work perfectly (Lincoln Electric model KH618). This is a shade #10 lens, which my book said would be okay for up to 2″ aperture. I bought two of them since they were fairly cheap, thinking that one could be used to view the sun directly (which worked quite well) in case the solar filter didn’t work out so well.
Now all I needed was to figure out how to construct the solar filter. I noticed that the lens itself was about as thick as a piece of corrugated cardboard, and I quickly devised a plan to sandwich the lens between two layers of cardboard. I used a science fair board like what you can buy at any hobby craft store. Here’s the outer layer (the part that will face the sun). The diameter of this layer is large enough to completely cover the end of my telescope, so that no light can reach the telescope’s main mirror. The four tabs were left in place to aid in attaching the filter to the telescope. You can also see the 2″-diameter aperture cutout, off axis in order to avoid being obstructed by the telescope’s diagonal mirror.
The next layer holds the welding lens. You can see the cutout for the lens. This layer was sized so that it would fit snugly inside the telescope tube.
Here’s the first two layers with the welding lens in place.
The final layer is the same diameter as the previous layer and has another 2″ cutout. This layer holds the welding lens in the cardboard sandwich. It’s obviously important to make sure that the 2″ cutout in this layer is aligned with the 2″ cutout in the outer layer.
The layers were glued together using Elmer’s Wood Glue, taking care not to get glue on the welding lens (so it can be removed and reused at some point if needed). Here’s the whole sandwich.
Here’s the solar filter installed on the front of the telescope:
The filter worked reasonably well. I elected to use a moon filter at the eyepiece to further cut down on the brightness, but I could have gotten by without it. With a 25-mm eyepiece, the sun filled most of the field of view. The edges of the image weren’t totally sharp, which I attributed to some diffusing of the light by the welding lens, but it was sharp enough to observe the eclipse without any trouble.
Watching the eclipse was a hoot! Clouds to the west were threatening to hide the sun from us, but the sun emerged just in time to see the start of the eclipse, and we were able to watch for about 45 minutes before the clouds wiped us out for good. Several neighbors stopped by to take a look, and a lady from up the street brought her kids over for a look. The second welding lens that I had bought was passed around from person to person and used to observe the eclipse directly, and that was as popular as watching through the telescope. Luckily, I had managed to get my equatorial mount pointing close enough to north that my mount’s clock drive tracked the sun quite well–I never once had to touch the telescope tube to re-aim.
I’m planning to use my homebrew solar filter again to observe the transit of Venus on June 5th. With any luck, I’ll have some pictures to show, as well.