Make a Homemade Solar Filter for a Backyard Telescope for Under $50
Have butter tub and solar mylar, see sunspots!
If you've got any kind of backyard telescope, you can make a homemade solar filter and view sunspots, solar eclipses, or other solar events!
The hard part, of course, is finding the right mylar for solar viewing. I picked up mine at Scope City in Costa Mesa, CA. Their online store sells Baadar Astrosolar Safety Film in a 7x11 size that's not too expensive.
You won't be able to buy solar filter film online at the last second, so plan ahead. Or Google for a telescope store in your area, if you live near a city.
Example Photos with My Solar Filter
(note: no camera mount, and I'm just using a point-and-shoot camera)
I was using an old TeleVue Pronto refractor, but I didn't have a proper camera, so I just held a digital point-and-shoot over the eyepiece. With a proper camera mount and camera, you'd get better detail than this.
How I Made My Homemade Solar Filter - It will depend on the size of your telescope
- Sheet of solar viewing mylar
- plastic tub (see step two for how to find the right size for your scope)
- sharp scissors
- (optional) posterboard or newsprint to use as work surface so you don't get glue on the table
- Like I said, the hardest part is finding solar viewing mylar or the equivalent. I recommend the Baadar Astrosolar Safety Film listed on Scope City's website, since this review of mylar vs. baadar solar film shows that some kinds of mylar sold online are inferior and will yield poor images.
The mylar must NOT have any pinholes, which could let in enough sunlight to damage your eyes or equipment when magnified through the telescope's lens. Hold the mylar in front of a bright light to make sure.
- The other hard part is to find a cup-shaped plastic container that will fit over the end of your telescope. My 2.8" (70mm) aperture Televue Pronto was capped snugly by a 16oz butter tub:
Trace the end of your telescope on a stiff piece of paper, then take that to the grocery store and test it against butter tubs, dip tubs, salsa tubs, and round plastic storage tubs until you find one that matches.
Try to avoid tubs whose bottom outer edges are rounded (see diagram). It's okay if the bottom has an inset area that's recessed, as long as there's at least a third of an inch of flat "rim" on the bottom.
- Bring your plastic cap home and test it to make sure it fits snugly over the end of the telescope. Mine isn't a perfect fit, but it will stay on without needing to be held or taped there. Once you're satisfied that you've got a good cap, it's time to cut a circular hole in the bottom, leaving about a third of an an inch of a rim of plastic. The hole doesn't have to be perfectly circular, but the flat rim is important, because that's where the mylar will be glued.
It took me two tries to figure this out:
I used a metal nail file to smooth the edges of the hole so they wouldn't cut the mylar (see interior view -- it frayed the plastic edges slightly, so I'm not sure whether this is a good idea).
- Place your mylar filter or equivalent on posterboard or a mailing envelope on the kitchen table. The instructions I got from the telescope store recommended putting kleenex under the mylar to cushion it and taping the mylar to the table to keep the sheet flat, but I just laid it loose.
Practice setting down the cutout end of the plastic container or tub on the mylar, with at least an inch of mylar extending beyond the edge of the tub. You want to make sure that you can set the container down flat without the mylar wrinkling (although it will still work with minor ripples or imperfections).
- Run a line of superglue all the way around the BOTTOM RIM of your tub. Now, gently and carefully, set this surface face-down on the mylar. Once the glue touches, you can't adjust it, so get the mylar as flat as possible first!
You will need to make sure it's a complete seal all the way around -- no gaps to let in the sunlight. Let the tub sit there until the glue is dry.
- Once the glue has thoroughly dried, use sharp scissors to cut off the excess mylar extending outside the tub. Leave a "fringe" all the way around; again, we want the end of that cap completely covered.
- Tape down the "fringe" of mylar with duct tape on the sides of the tub protect the edges of the mylar sheet from getting snagged and peeled away.
- (Optional) Use the remaining mylar to make solar viewing filters you can look through -- very helpful when using a spotter scope! Cut a hole in cardstock and tape a piece of your filter material over it: the tape should seal all the edges flat against the card stock.
Spotter scope filter: I cut a round hole in card stock and covered it with a leftover piece of solar viewing mylar. The curved edge of the card let me drop it behind my sunglasses to cover one eye, closing the other.
For my neighbors, I took another piece of card stock and cut a rectangular hole in it wide enough to look through with both eyes, and covered that with another piece of mylar to make a homemade version of the solar eclipse viewing filters like this one available on Amazon:
Important: NEVER look through any kind of binoculars or telescope at the sun unless it is capped on the end that lets in the light. If you only use a solar filter on the eyepiece or to cover your eyes, the magnification of the main lens will transmit so much sunlight that it will overpower the protection provided by the filter. You must filter the sunlight BEFORE it enters the first lens!
Example Sunspot Photo With This Filter - Using a Point-and-Shoot Digital Camera
Click photo above to enlarge for sunspot details. Remember, I was just holding a digital camera against the telescope eyepiece; you'll do better if you have a camera mount.
© 2012 Ellen Brundige