How to Make a Solar Filter

By Captain James R. Watson

The sun is magnificent to view through a telescope—but only with a filter. Building a solar filter for a telescope is pretty straight forward.

To build one for my telescope, I sawed two 1″-wide rings from 1/4″-thick plywood. The inside diameter of the rings matches the outside diameter of my telescope’s optical tube (10 3/4″). I bonded the two larger rings together with an identically-sized ring of foam between them. The laminated ring slides over the tube.

I cut a third plywood ring with an inside diameter that matched the inside diameter of the optical tube (10″).  <D>I laid a piece of solar filter material over the foam/plywood laminated ring. I placed the smaller plywood ring over the filter material, and bolted the rings together with the filter between them. I glued felt to the inside of the rings. For viewing, I slip the filter over the open end of the optical tube. The friction of the felt holds the filter in place.

WARNING! Never view the sun through a telescope without a filter. It can instantly result in permanent blindness.

For more information about building telescopes, see Building a Ladder to the Heavens.