A Simple Solar Filter for Observing the Sun


A wise man once told me, “You can view the sun through an unfiltered telescope twice … once with each eye!!!”

In order to safely view the sun and attenuate enough light to make CCD images, I constructed a solar filter out of a piece of black postar board, some Scotch tape, masking tape, Elmer’s glue, and the key element … a small piece of Baader astrosolar Safety Film, (available at Starizona @ $29.95).

An 8-inch Schmidt-Cassegrain is far too big of a light bucket for the sun, even when filtered. So, the filter must block most of the apeture, and then filter what little bit you leave open. First, I constructed a postar board cover for the telescope. I cut two 1-inch wide strips from the postar board. I wrapped one strip around the aperture of the telescope and taped the ends together. The fit was as snug as I could make it. Then, I applied a small amount of glue to the second strip and wrapped it around the first, holding the second strip in place a few minutes while the glue dried. The double thickness of postar board gave the edge of the cardboard cap some rigidity.

From the remaining postar board, I cut a circle with a diameter equal to the telescope aperture plus a little. To get a perfect circle of the proper diameter, I simply traced the plastic Celestron cover. Then, using masking tape, I taped the end circle onto the postar board rings (still on the telescope to keep their shape). I now have a cardboard cover for my telescope.

I removed the cover, scribed out a one-inch off-axis circle, and then carefully cut out the circle with a razor blade knife. I could have then applied the Safety Film filter material directly to the back of the cap behind the hole, but as I thought I might want to use the filter cap without the filter as a moon attenuator, I mounted the Safety Film between two pieces of postar board with similar one-inch holes cut in them. Then I positioned the Safety Film filter card over the off-axis hole in the cover, taped it in place and … you have a solar filter with a one-inch off-axis aperture.

I first tested the filter by holding it up to my eye and carefully and gradually looked toward the sun. I was ready to abort this test at the slightest discomfort, but I was able to comfortably view the sun through the filter. Then, I placed it over the apeture of the telescope and pointed at the sun. To my delight, the filter passed the perfect amount of light for solar viewing. Sunspots came into sharp focus with good contrast. And to my even greater delight, I found I could also acquire good CCD images with a 0.02 second exposure.

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