With the upcoming solar eclipse, I decided to add a set of solar filters to my astronomy binocular rig.

My binoculars are a 12x70 set, which means that they magnify 12x and have a 70mm objective lens. Most binoculars have a 50mm objective lens, so it is more difficult to find filters and accessories that fit them.

Because of this, I decided to build my own!

Step 1: Safety and Materials

Viewing the sun is extremely dangerous. Using the wrong equipment, or even carelessly using the right equipment, can cause serious injury or blindness. Use extreme caution when doing anything related to solar observation. Carefully read all of the documentation available for the filter material that you are using, and heed all of the warnings and suggestions in that documentation.

Recently, there has been an influx of mislabeled and non-certified solar filter materials on the market. Please use a material that is safe for your application and that is from a trustworthy vendor who has sought out the proper testing and certification for their product - your eyesight depends on it! The American Astronomical Society keeps a list of reputable vendors of solar filters on their eclipse website: https://eclipse.aas.org/resources/solar-filters

I chose to use a solar filter material made by the Baader Planetarium in Germany. The material that I selected is not rated for direct / naked-eye solar viewing! The "solar safety film" is only safe to use in conjunction with additional optics like a telescope or binoculars. This is because the filter material itself does not filter out all of the harmful light from the sun. The coatings on the lenses, mirrors, and prisms inside of your optics are needed to shed the remaining UV and other harmful spectrums before the light is safe to enter your eyes. Read the documentation to be sure!

Needed Materials and Tools:

-Solar Filter Material

-Double Sided Tape

-Hobby Knife

-Wax Paper

-Tissue

-Scissors

-3D Printed Parts

Step 2: 3D Printed Parts

I designed 3D printable parts for this project. The filter ring has an aperture for 70mm objective lenses, and the filter cap is designed to fit over Celestron Cometron 12x70 binoculars.

You'll need to print 2 of each file, and should make sure to clean up any print artifacts that might effect how well the pieces fit together. Test fit the filter rings in the holders, and test fit the holder on your binoculars.

Use an opaque printing material, and check for voids and gaps that might allow unfiltered sunlight to enter your optics. Make sure that you're safe at every step.

Step 3: Prepare Filter Rings

Apply double sided tape to the flattest/smoothest side of the printed filter rings. You want to cover as much of the ring as you can with adhesive, while being careful not to overlap the tape or include wrinkles. Care at this step is critical, because this tape is the main thing that will hold the solar film in place.

After applying tape to the rings, turn them over and set them on wax paper to trim the excess adhesive from around the edges.

Step 4: Adhere the Rings to the Filter Material

After you've got a nice layer of adhesive on the filter rings, you're ready to stick them to the filter material.

The instructions for my filter material said not tot touch the material with your bare hands, so I wore gloves. The instructions also suggested laying it out on a soft tissue so that you didn't scratch or damage the film.

Remove the wax paper, and adhere the rings to the filter material. Be careful to avoid creases and inclusions, and try to keep the film as flat as possible. Do not stretch the material - some waves and ripples will be unavoidable and are considered normal.

Step 5: Trim the Excess Filter Material

Using a sharp pair of scissors, trim the excess filter material from around the outside edge of your filter rings.

To avoid touching the filter material, I held the rings by their outside edges during this step.

Inspect both sides of the film before proceeding. Any scratches, inclusions, or pinholes should be rejected.

Step 6: Final Assembly

Next, apply some adhesive to the filter holder caps. If you did a nice job of securely applying tape to the filter rings, the adhesive that you apply here only needs to hold those rings in place... Apply the tape, and trim the excess with a hobby knife.

When you're satisfied with the tape in the filter holders, insert the filter rings that you assembled in the last step. You'll want the filter material to be sandwiched between the two printed parts when you're finished.

Carefully press everything together, making sure that you don't damage the film in the process.

Step 7: Be Safe and Enjoy!

Once you have the assemblies completed, you're ready to slip them on your binoculars and use them! Be sure to press the filters all the way on. You can visually check this through the slots on the side.

Be careful not to scratch or damage the filters when you're handling them, and store them in a safe place when they're not being used.

Be careful, and plan your viewing safely. Remember to read and follow all of the warnings that come with your filter material, and take some time to browse the many astronomy websites and forums online for rules, tips, and tricks to safely observe the sun.

Enjoy!