DIY Solar Filter (for Telescopes and Cameras)

Solar astronomy can be a very rewarding (if often overlooked) area in astronomy, especially with events like the Great American Eclipse (August 21st) around the corner. However, because of the inherent danger or looking in the direction of the Sun, special precautions have to be taken – using a less than satisfactory method is not a very bright thing to do (pun intended :) ).

This instructable gives an idea of how you can prepare cheap, simple, and effective solar filters for use with a telescope or camera lens (e.g. a telephoto zoom), using a commercially available filter sheet. Happy building!

Equipment

The following pieces of equipment were useful:

• Polymer solar filter (bought on Amazon: https://www.amazon.com/Solar-Filter-Telescopes-Bi...
• Thin cardboard sheeting (ideally 2mm or less - once flexible it should work)
• Scissors
• Hot glue gun + glue
• Instant glue (cyanoacrylate adhesive)
• Duct tape (every project needs duct tape), or other sturdy tape.

The telescope this filter was made to fit is a Galileoscope - a small refractor with 2 inch aperture; the same design could be used for larger ones as well, but becomes more difficult to construct safely at larger sizes.

Step 1: Preparing Cardboard Housing (pt 1)

The first step is to prepare a sleeve out of the cardboard, to fit snugly around the end of the telescope. Using a tape measure or thin piece of cord, determine the outer circumference of the telescope. Using this measurement, cut a thin rectangular sheet of cardboard about two inches wide, and as long as the circumference you just measured.

Take this sheet and wrap it carefully around the end of the telescope, curving it gently to avoid any sharp bends or kinks in the surface. Once satisfied that the sheet is long enough (you can trim or add additional cardboard sections here if necessary), fasten the two ends together with the tape.

Alternatively, cut the cardboard strip long enough that the ends overlap when held around the telescope, and fasten with tape or hot glue as above (this method may be slightly more sturdy). Either way, ensure this fits securely against the telescope body; this is important to prevent the filter from coming off while in use.

Next, using the cardboard sleeve as a guide, cut two small circular shapes out of the cardboard sheeting, and then cut out most of the material in the center of these pieces, in a circular shape. These pieces will sandwich the filter sheet, allowing it to be safely attached to the telescope.

Step 2: Preparing Cardboard Housing (pt 2)

Take one of the cardboard circles from the previous step, and trace the outline on the outer sheet housing the polymer filter (if this came in a cardboard or paper housing). Using this outline, cut out a circular section of the filter sheet - cutting through the outer housing; this protects the filter itself. If the filter doesn’t have a cardboard outer housing, you can also cut it against the circular sheets from above, taking care not to wrinkle or tear the layer.

Once this is done, carefully remove the filter sheet, and using instant adhesive (NOT hot glue - the filter can be damaged by high temperatures), sandwich it between the two cardboard circles as shown. Once set, and after trimming one end of the cardboard sleeve to a smooth surface, carefully hot glue the filter window you just created, to the sleeve from the earlier step, taking care to seal any gaps between the two pieces (note: in some cases, the filter assembly is directional –one side needs to always face the sun; in my case, this was the shiny side, so this side was glued facing outward). You can also fix up the appearance of the outside with more tape or with paint. Once the glues are set, inspect the filter carefully by holding it up to a strong artificial light (NOT THE SUN!), and checking for any pinholes or bright spots. Once none are detected, you’re ready to start observing!

Step 3: Constructing Camera Lens Filter

If you don’t have a telescope, another way to take detailed

photos of the Sun is using a filter attached to a telephoto lens (ideally with a focal length of 250mm or above). You can use any ring or clear filter (e.g. a UV filter) that attaches to the filter threads on the front of the lens (in the photos I used a macro reversing ring).

Cut out a circular section of the solar filter paper, and carefully attach to the edges of the lens ring or filter with instant adhesive. Just a note: polarizing filters are probably not ideal for this, but experiment! You might get an effect you actually like.

Step 4: Test It Out!

Once you’re sure the filter is properly fixed and operational, you can start observing! Most sources advise using the shadow of the telescope or camera to initially align with the Sun. If you are using a camera, using Live View is generally preferred to the eyepiece, as in the event of a failure, the focused sunlight will only reach the camera sensor, not your eyes. The exact settings to getting a clear, properly exposed picture will depend on several factors, although if you are using a telephoto lens instead of a telescope and adapter, metering will still work. One thing to bear in mind; keeping Live View on for an extended period of time can cause the sensor and camera to heat up, or possibly become damaged - it's recommended to switch off this setting intermittently to allow the camera to cool down.Sol

Have fun, experiment, and stay safe!