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A sun funnel is a solar projector designed by Rick Fienberg and Chuck Beater and was featured in the
Sept/Oct 2011 issue of ASTRONOMY TECHNOLOGY TODAY.
More information can be found here.
This refractor is made from a used photo copier lens, commonly called a "copy scope". It's approximately 60mm in diameter and projects the suns image onto the screen thru a 9mm eyepiece. The funnel can be found at any auto parts store and the screen is the high contrast stuff used in rear projection TV's. I don't know if the screen material is still available, but I've been told that some people have used white or gray vinyl shower curtain liners with good results.
Low cost solar scope version 2
This scope is a 70mm f/10 refractor I found at a thrift store. The longer focal length of this scope allows the use of a longer focal length eyepiece (17.5mm) which makes focusing easier.
August 2017 Solar Eclipse from Tarpon Springs Florida
The drive uses a simple worm wheel made from a stack of three wooden disks. The middle disk is 1/2 inch smaller in diameter then the other two. I filled the gap with quick setting epoxy and used a threaded rod coated with silicone to impress the "gear teeth" into the epoxy before it set. The wheel is driven by a 1/4-20 rod attached to a 1/3 RPM motor.
The best thing about sun funnels is that small groups can view the sun together. If the scope has a finder, you should remove or cover it for the safety of your observers.
Another option is a Sunspotter. This little device projects an image of the sun on one of it's inner surfaces using a lens, an eyepiece, and three mirrors. No fancy mounts, just put it on a table or other flat surface.
I made this one using a lens from an inexpensive refractor that was given to me. The first step was to measure the focal length of the lens.
I mounted the lens in it's cell and a focuser with an eyepiece to a board and slide the focuser back and forth till a distant object (the Moon) came into focus. The distance measured from the lens to the midpoint of the eyepiece helped determine the size and shape of the scope. I should have measured this before I took the scope apart and discarded the tube, but I didn't think of it. The light path will pass close to the eyepiece twice so it is a good idea to draw or plot the light path to make sure that the eyepiece doesn't interfere with it. I used SketchUp Make to model it and found that an equilateral triangle shape would work.
The mirrors should be “first or front surface” mirrors to prevent “ghost reflections”. The eyepiece needs to be made of glass and metal. Plastic eyepieces will quickly melt or burn when exposed to the concentrated rays of sunlight. I used cheep plastic eyepieces to determine what focal length I wanted and scorched two of them in the process. Once I determined the focal length I wanted, I ordered an inexpensive plossl eyepiece that works well.
On the subject of heat, the area immediately above and below the eyepiece can get very hot so if kids are going to be using it you may want to block or shield those areas. Also using a longer focal length lens will increase the overall size of the scope but you don't want it so big that someone can stick their head in it. If you hear someone say”hey hold my beer, I want to see what makes this thing work” try to stop them.
Partial solar eclipse from Dade City Florida on October 14, 2023
Front surface mirrors can be found here
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