The Sun Funnel
In preparation for the upcoming Venus transit on June 5th I was looking at cheap, but effective ways to view the sun with my telescope. I wasn’t very interested in buying a full aperture solar filter for my 10″ scope so I was thrilled when I found www.transitofvenus.nl. The accessory proposed on this site is the sun funnel, a rear projection, group viewing tool for the viewing the sun. The sun funnel is very easy to make and costs just about $20 to make.
The funnel consists of a large fuel funnel, rear projection screen, two hose clamps, and an eyepiece. I say it costs about $20 assuming that you own a telescope eyepiece already, if not you’ll have to pay a bit more. You don’t need an expensive eyepiece, just one that fits the focal length of the telescope and the length of the funnel to give you an optimal image size. More info on the eyepiece can be found on the website. The rear projection screen can easily be ordered online and costs about $17 for one square foot.
I put my sun funnel together this past Monday and attached it to my scope but alas, the sun had almost set and was behind the trees in my backyard. I had to wait until Friday evening after work to get another good viewing chance, but I was not disappointed! Once the image is focused correctly my 25mm eyepiece projected an image about 75% of the funnel’s diameter! The image is still rather bright, but is by all means safe to look at with the naked eye (although sunglasses might make it more comfortable).
EDIT: I would also highly recommend building an aperture stop of some sort to reduce the amount of light gathered by the scope. You can use any kind of material really, but all you do is cut out a circle about 2″ in diameter in whatever material you’re using (a cardboard box works well) offset from the center so that you don’t cover up the secondary mirror (for reflectors). This reduces the aperture of your scope from say 10″ to 2″ which is perfectly fine for viewing an object as bright as the sun.
Once set up the image of the projected sun is razor sharp! As the picture below shows, once the image is focused on my phone the detail on the disk is stunning! Several sunspots are visible dotting the fiery surface of the star. In the center left and clearly visible is the large sunspot 1459. Three other pairs of sunspots form an isosceles triangle around 1459 in the center. Not bad for my first attempt at viewing the sun. I’m really looking forward to viewing the transit through the sun funnel in less than two months!