The sun's image appears on a rear-projection screen of the kind often used in large-screen TVs. Many varieties of screen are available, each with different trade-offs in viewing angle, image brightness, sharpness and contrast. Hegerberg purchases a flexible Da-Tex rear-projection screen for $10 per square foot from Da-Lite Screen Company (800-622-3737). A 15-inch square will suffice. Secure the screen, polished side facing out, over the open end of the bucket. You can use a 48-inch plastic wire tie positioned just under the bucket's lip. The tie is the same type that can bind large bundles of wire, and Home Depot has them (part no. 728494104805). Pull the screen taut as you tighten the tie, so that the assembly resembles a drum. Alternatively, you can secure the screen with a large rubber band. Cut off the excess screen, leaving about a half-inch of fabric below the tie for future adjustments.
Finally, Hegerberg removes the bucket's handle and slips a large rubber band over it. After reattaching the handle, he connects the band to the finder scope to relieve some of the stress on the focusing assembly [see illustration above]. Depending on the size of your bucket and scope, you might also need to add a counterweight to the telescope tube.
To get a clear image of the sun, you'll need a good eyepiece and a filter that screws into it. Hegerberg recommends Pl¿ssl eyepieces because they deliver the sharpest and best color-corrected images, but Huygenian eyepieces contain no cemented elements and so may better survive long-term exposure to the sun's heat. You'll need focal lengths between 17 and 25 millimeters depending on the size of your telescope. If you happen to own a Schmidt-Cassegrain telescope, try a 20-millimeter eyepiece for a four-inch instrument and a 25-millimeter eyepiece for an eight-incher. Sirius Pl¿ssl eyepieces retail for about $50 from Orion. If your telescope's aperture is larger than four inches, you must attenuate the light using a piece of cardboard with a four-inch hole in it. Attach this cardboard to the front of your scope. Otherwise, your instrument could overheat.
For the filter, Hegerberg recommends #21 (orange), #11 (yellow green) and #12 (yellow), any of which Orion sells for about $15. But keep in mind that these filters were never intended for direct solar viewing. Just as you would never press your eye over the lens of a movie projector, so you should never look directly into the eyepiece--even with one of these filters. Doing so could permanently damage your vision. The projection screen on the Sun of a Gun diffuses the light so that it is safe to look at.
Because the finder scope can focus sunlight enough to cause burns, always cover it before using the Sun of a Gun. Of course, never look through the finder scope at the sun. To align the telescope with the sun, first adjust its position so that it casts the smallest possible shadow. Then use the focus to sharpen the image on the screen.
Armed with this powerful tool, you'll be ready to explore our home star on any clear day. You, too, may enjoy observing the life cycle of sunspots, recording the ratio of the umbra to penumbra area or mapping their size over time.
For more information about this and other projects, check out the Society for Amateur Scientists's Web page. You may also write the society at 4735 Clairemont Square, Suite 179, San Diego, CA 92117, or call 619-239-8807.
This article was originally published with the title Sun of a Gun.
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