The Dobsonian Sun Telescope

dave north looks
Dave North, former President of SJAA looks  
through Ray Cash's Sun Scope

suntube.jpg (12338 bytes)
Here's a closeup of the OTA.
Notice the removeable section
with focuser and diagonal.
Use an oatmeal tube container for
ID diameter alignment.

Key Materials:

  • Partially aluminized parallel-plate glass (5% transmission). A "one way" mirror also works fine; the stuff, in other words, commonly used in casinos and department stores.
    Be sure to mount the aluminized surface inside (toward the primary and eyepiece).

  • An unaluminized primary.   Keep it small, 4.5" or 6" works just fine. Keep it "slow" F/8 or F/10.

  • Sonotube. With a Sun Scope, we do not have to worry about "tube currents" and have our tube 2" larger in diameter than our primary--if you can fit your primary inside the tube, it will work fine.

  • Sky and Telescope, August '89 p.207ff.  Sidewalk Astronomer Tom Mathews' article.In 2005 while at RTMC, John Dobson introduced me to him, Tom was kind enough to give me a copy that he found and bought for me at the swap meet that next day!  Thanks Tom.

Left is an example of another incredible contribution to amateur astronomy by John Dobson: an extremely safe Sun telescope. 

One immediately notices the mirror tilted at 45-degrees at the front of the tube... This is actually a piece of one-way mirror, which serves two purposes: One, to deflect up to 95% of the incoming sunlight, thereby acting as a filter, and, two, to act like a normal  diagonal mirror: to bounce the light gathered by the unaluminized primary out the side of the tube to be magnified by a suitable eyepiece--but not before this light passes through a welders' glass to filter out any harmful infrared or ultraviolet radiation.

Designed into this failsafe method to view the Sun, is this: If the front mirror is broken, or becomes accidentally dislodged, the viewer will be protected since no direct sunlight will reach the eyepiece; the eyepiece will, in fact, be pointed safely at the ground!

Construction Tips

Note the cut of telescope tube in half about 12" from the focuser--this is necessary since it was glued (with 100% Silicone glue) the one-way mirror directly on the cardboard sonotube; it is necessary, that is, so you can mount, or de-mount, the focuser or welders' glass which is glued to the bottom of it. (Line--glue-- the inside of one half of the tube with "sleeve material,"--cardboard tube in which you have cut a section out, or other appropriately thin material).The focuser used here is a "2-inch to 1-1/4-inch adapter"--a $20 item available from telescope outfits like Lumicon, Scope City or Orion; or you can use the standard Dobson style of focuser and eyepiece.  The one shown is low and sturdy; and provides a flat bottom in which to glue the welders' glass. You will need to experiment with the grade of welders' glass you will need, by the way. Ray Cash's 6-incher requires a "grade seven". He  also recommends the "gold anodized" version of welders' glass over the traditional green glass--to yield a more natural, yellowish, transmission of light.

You must cut the front of the tube at exactly 45-degrees, if you glue the front mirror directly on the cardboard as he did. Tape enough 8-1/2" X 11" paper together end to end to go around your tube. Cut both ends at 45-degrees, making a trapezoidal shape. Bring the two ends together so that one edge of the paper is snug against the tube (some trimming will be required). Trace a line; cut to it.

With all that weight up front, you will have to make a tall rocker box, or use weight at the rear of your tube (Ray did both).

What will you see? Sunspots, faculae, granulation!

Full credit for this article goes to Ray Cash, 

Edits and additions Kenneth Frank

Copyright © 2005