Solar binoculars

Buy or make?

Due to the last great solar eclipse in America, some of the big renown astronomical companies decided launch new models of solar binoculars and probably many people decided it was a good time to spend their money on a pair.

Those models of solar binoculars are exclusively dedicated to solar viewing because they were built on a non-removable solar filter glass in front of the objective lenses. I feel it is a safe way to look at the sun, but also a good way to waste money, especially if one had already owned one or more pair of binoculars. Other than the nice looking glass, there is nothing really special in these binoculars that one can’t achieve by placing a binocular solar filter in front of the objectives.

You can buy a dedicated solar filter for your binoculars or you can made a solar filter, using a solar filter film. I chose the second option and for less than 20 US dollars I achieved not only solar filters for my 15x70s binoculars, but also one for my telescope.

Here is my “quick and easy” tutorial on how to made good and nice looking solar filters for your binoculars:

Materials

1. First and most important:  a solar filter sheet.  I used  a 140mm x 155mm (5.6” x 6.1”) sheet of Baader Solar Film.

2. Cardboard. Choose a cardboard thickness that can easily bend around your binoculars without breaking or folding. The kind of cardboard that comes with the cereal boxes is a good one for small binoculars.

3. Tape. In the picture I put duck tape but for my filters I used blue masking tape.

4. Glue. I strongly recommend wood glue. It glues the cardboard very well, at the same time it fills in the gaps between the different layers.

5. Double sided tape.

6.  Measuring tape, ruler, pencil, a box cutter, scissors and a drawing compass.

7. Paint and a good paint brush. Be sure you have a little and smooth paint brush. I bought mine in the dollar store and it was a disaster. I ended up using a sponge to paint the cardboard.

8. Your binoculars (or small telescope). 

Ring Construction

Once you gather all the necessary  to make your solar filter, it is time to get down to work.  Making

them one at a time:

1. Cut about 5-6 cardboard strips of about 2cms wide (or 1 inch).   The first one’s length needs to be the circumference (C) of the outer edge of the objective.  You can measure the diameter (d) of the exterior edge and use the formula C= πd or measure the C with a soft measure tape. Cut the first strip with the C measure, then the next ones at least 5 cms longer than that.

2. Once you have protected your lens, wrap the C length strip around the objective and use some tape to hold it there. Keep it tight to the objective.

3. Measure the new circumference including the first ring to know and cut the length for the second strip.

4. Put some glue around the strip and use a piece of cardboard to spread it even. There should not  be clots of glue, especially close to the edges of the cardboard. Once you finish spreading the glue, place tight the second strip to the first one.

5. Repeat steps 3 and 4 to add more layers until you have reached a good ring thickness (like in the last picture) then you can paint the inside of the ring. Save one strip for the final step in the Donut Assembly. 

Special advice on the ring construction: when you add a new layer, be sure to NOT start placing the strip in the same end that the last one. Rotate the strips at  least 45 degrees clockwise in relation to the previous one. This will give more strength to the ring. Also be sure to use cardboard think enough to make a good layer thickness, but not so thick that it will fold while you are placing it around the binoculars (like in the picture). 

Donut Filter Construction and Assembly

1. With the ring done, you’ll need to make a donut to support and attach the filter.  Measure the diameter of the outer edge of the ring. 

2. Use that diameter (the ratio, actually) to trace the outer circle of the donut.  For the inside circle select the best width for your FOV and your double sided tape.  I used 1.5 cm for my donut width giving me an inner diameter of about 5.5 cm or 9” (that is for using with my  7cm or 2” of aperture binoculars).   

3. Trace two donuts and cut them out.  Be sure they match.  Paint the exterior sides. 

4. Cut the filter with scissors. Cut a square where the side is the measure of the diameter of the outer circle.

5. Put double sided tape on one of the unpainted sides of one circle and drop it carefully onto the filter, trying to cover the whole donut. Do not stretch the filter and once it touches the double side tape it has to be left that way.

6. Going back to your cardboard ring, put glue on the ring edge and drop the other donut (the one without the filter) on to match the edge of the ring. Then put three strips of tape from the ring to the donut (like in the picture) and put some double side tape in the donut.

7. Cut the corners of the filter sheet off and place it carefully on the other donut. Be sure, you match the borders before dropping it completely. Once it touches the surface of the sticky tape, there is no turning back.

8. Finally, put another strip of cardboard on the ring. In this way, you make sure the tape won’t slip out and carefully paint the outside of the ring. At this point, carefully avoid touching the filter or dropping paint on it. 

The filter is complete! You just need to repeat the whole procedure for the other objective unless you are doing both filters at the same time.  I built the whole filter first and then painted it, but it is better to paint as it is being constructed to avoid the risk of dropping paint on the filter.  

With a 15x70 binoculars it is possible to see sunspots. I am not a big fan of solar observation, yet I have seen couple of those amazing earth like size, solar areas.  Here is an sketch I made of the Sun (no sunspots where visible though), on the 18th of November on a partially clear morning in the city of Scott’s Valley, CA.

Now, if the sky is giving you some trouble at night, it is time to put some DIY solar filters on your binoculars (or telescope), get a chair and enjoy some solar observation.

Happy Thanksgiving!

LG

Edited by: Jennifer Steinberg (editor in chief)