How to Make Solar Filters for Binoculars and Such
If you haven't already bought ready-made filters for you binoculars you are going to have a hard time finding them. Most ready-mades have disappeared from Amazon due to them being sold out. If you have solar filter mylar film and want to make you own this page may help you. The way I make them is a little time consuming, mostly due to having to let the glue dry between steps. However, they are made to last. If you want a quickie method there are simpler ways to do it. The simplest way is to cut two circles of mylar about an inch bigger in diameter than the outside of your binocular lenses, fold them over the outside of the lenses, and attach with rubber bands! The mylar will deform and wrinkle but they will still work ok. If you want to build something a little more solid but still simple take a look at the websites linked below for ideas. Since you may not use these filters again after the eclipse you may not care about longevity!
Box type filter: AstroSolar.com
Rubberband attached: Instructables.com
Materials you will need:
1. Poster board, color of your choice
2. Felt sheet
3. Solar filter material wide enough to cover the binocular lens x 2
4. Elmer's glue or equivilent
5. Contact cement
6. Masking tape
NOTE: Do not handle the mylar directly any more than necessary. By keeping the mylar sandwiched between the protective cardboard you should be able to avoid touching it with your fingers at all. You also want to avoid scratching it even while between the cardboard so handle with gentle pressure only. DO NOT wipe the mylar at all. Do not worry about dust on the mylar during assembly. Once assembled you can use gentle air pressure to blow off dust. A small amount of dust will not affect use of the filter and will go un-noticed by the user.
Step 1: Cut Ring Parts
Cut strips of posterboard. I made mine 1/2" wide. The strips must be long enough to wrap around the end of you binocs with at least an inch overlap. More is better! For my 50mm binocs I made mine about 14 inches long. I recommend a minimum of 3 strips per lens or 6 total. In this case I used 4 per lens. Also cut two strips of felt. I cut mine at 7/16 inches wide, slightly narrower than the posterboard strips, and just long enough to wrap around the lens with some overlap. You will trim the felt to fit in the next step.
Step 2: Cut Felt to Size
Wrap the felt strip around the binoc lens, mark where it overlaps and then cut it. I didn't mark mine too accurate but a small gap, a quarter inch or less is not a problem.
Step 3: Glue Felt to Strip
Lay the felt strips over a pair of posterboard strips, mark where the felt ends and then coat the posterboard from your mark to the end of the strip with a generous amount of Elmer's. Place the felt onto the posterboard starting at your mark and gently press together as you go. You don't want to stretch the felt too much. If you end up with a little overlap at the end of the strip wait for the glue to dry (15 minutes at least) and trim with scissors.
Step 4: Build First Ring Layer
Time to glue the first layer of the ring! I placed aluminum foil on the desktop to reduce the chances of gluing the assembly to the desk. First do a "test wrap" without glue to get a feel for how the posterboard wraps around the lens. This will also help pre-curl the strip. Have a strip of masking tape ready to hold the strip together while it drys. Coat the bare part of the strip with the felt attached with Elmer's glue, I found it worked best to add the masking tape to the end of the strip after you coat the strip with glue and before wrapping the strip around the lens. Helps you keep from getting glue on the tape and makes it easy to finish off the wrap. Now carefully wrap it around the lens pulling it nice and snug as you go. Try to keep the edge of the strip against the foil so the edges of the posterboard remain aligned. Doesn't have to be perfect! Allow this to dry for a minimum of 30 minutes. You can repeat this process for the second lens while you wait!
Step 5: Add Remaining Ring Layers
Once the first ring layer is dry it is time to glue the remaining layers of the ring! Do another "test wrap" without glue with each strip added to pre-curl the strip. Coat the entire next strip with Elmer's glue, then add the masking tape to one end. Starting at the end of the previous wrap and wrapping in the same direction carefully wrap the new strip around the outside of the previous one pulling it nice and snug as you go. Again, try to keep the edge of the strip against the foil so the edges of the posterboard remain aligned. Allow each layer to dry for a minimum of 30 minutes before adding the next. When done you should have a pair of fairly stout cardboard rings that fit snuggly over your lenses!
Step 6: Cut Outside Cover Disks
Now we need to make the outside filter/cover pieces. Start by drawing two circles on posterboard that are the same diameter as the outside dimension of the rings you just made. You can measure the outside diameter and use a compass to draw a circle or trace with a pencil using the ring itself. You then need to draw another circle centered inside the first that is approximately the size of the actual lens of your binoculars. Cut out these rings using scissors and/or an Xacto knife. They should look like the ones in the pictures below.
Step 7: Cut Inside Cover Disks
Time to make the inside filter/cover pieces. Start by drawing two circles on posterboard that are slightly less than the inside diameter of the rings you just made. You can measure the inside diameter and use a compass to draw a circle or trace with a pencil using the ring itself. You then need to draw another circle centered inside the first that is approximately the size of the actual lens of your binoculars as you did in the previous step. Cut out these rings using scissors and/or an Xacto knife. You should now have two outer and two inner disks.
Step 8: Cut Mylar Film Disks
Next we cut the mylar solar filter material pieces. Be careful handling the film! You do not want to scratch it! Do NOT use a compass to draw the circles as the point may penetrate through the cardboard and damage the film! It is recommended that you do not handle the film directly if possible. Since the film comes sandwiched betwen two cardboard sheets this is not difficult to accomplish.
Take a look at one of the inside disks you already cut out. You want the mylar to be bigger than the inside hole and smaller than the outside diameter. Select a diameter for the mylar that is about half way between the inside and outside diameters of your posterboard disk. Using this diameter cut out a template from your posterboard and use it to draw the circles on the mylar cardboard protector and then carefully pinching the mylar between the cardboard seets use a scissors to cut out the mylar through both layers of cardboard.
You should now have pieces as show below.
Step 9: Mark Disks For Assembly
To aid in assembly take the inner ring, lay it on top of the outer ring and center it, then use a pencil to trace arounfd the outside. This will help you align the two rings when gluing. Similarly, lay your mylar template you cut out before and do the same thing on both the inner and outer rings.
Step 10: Apply Contact Cement
NOTE: Wilst applying contact cement to the disks be aware that the cement likes to create fine filments of glue as you lift the brush during application. You can end up with hair like filaments across the inside open area of the disks. After applying the cement I carefully use my fingers to run around the inside edge of the disks to insure that I remove any of these that I may have created. Discovered them the hard way! They won't really affect the filter's performance but they look bad!
Coat the inside and outside disks with a thin layer of contact cement. We use contact cement not Elmer's here because Elmer's will not stick at all to mylar. The mylar can be peeled off of the contact cement but once sandwiched between the two disks it will not come out or loosen up. You only need to coat the portion of the outer disk that will be covered by the mylar and inner disk. You DO NOT want to cover the outer part of the outside disk as you will want to use Elmer's glue to attach the disks to the rings in a later step! You can see this in the picture below if you looke closely, the contact cement has a yellow tint. Coat the entire inner ring. This will allow you to glue the mylar first to the outer disk, then when you place the inner disk on top it will be glued to both the mylar and outer disk. Allow the contact cement to dry for 15 minutes before starting assembly! Do not wait for more than a hour though as the glue may be too dry.
Step 11: Assemble Mylar to Outer Disk
I used tweezers to grasp the myar and using the pencil marks I carefully align the mylar and simple lay it on top of the outer disk. No pressure is required at this point, just lay the mylar down in place. You only have one chance at this alignment because the contact cement will not let you move things around once the mylar is placed. Once again perfection is not required. What is important is that the mylar completely cover the hole in the outer disk so that no light can pass by the edge, and blind you!
Step 12: Assemble Inner Disk to Outer Disk
Now carefully locate and place the inner disk on top. Again no pressure need be applied during placement. Once the inner disk is in place I used a piece of cardboard placed over the assembled disks and then I applied VERY GENTLE pressure to press the parts together. Beware that if you are sloppy with the contact cement and get it on the outer disk that is NOT covered by the inner disk you may glue this cardboard to your assembly, so use some judgement here and just use you finger to GENTLY press the inner disk to the outer instead of cardboard. This is if you have appreciable contact cement still visible on the outer disk surface.
Step 13: Apply Glue to Ring Assembly
Apply a generous bead of Elmer's glue to the leading edge of one of the ring assemblies as shown in the picture below. Some excess on the inside of the ring is desirable but avoid excess at the outer edge of the ring. Not a big deal but will look better!
Step 14: Glue Disk Cover to Ring Assembly
Place one of the the disk assemblies on a piece of aluminum foil with the inner ring on top. Carefully place the ring assembly with the glue bead facing down over the disk aligning the outer edges. Press together gently. If there is excess glue on the outer edge and you want to clean it off use a damp cloth and while carefully pressing down on the ring wipe away the excess. In the picture below you can see excess glue on the inside of the ring at the edge of the felt. This is entirely desirable as it will serve to further maintain the inner disk in place. Repeat steps 14 and 15 for the second ring and disk. Allow the completed filters to dry for at least 5 hours. Best leave them overnight!
Completed!
You should now have a solid pair of filters that will last a long time! I still have the ones I made for my binoculars for the eclipse I saw in Baja in 1991 and they were made with one layer of thin cardboard and are taped together! These should last a lot longer! Just store them in a manner that protects the mylar from abrasion!