The Light Bucket

I started getting interested in astronomy in my early teens the first time I saw the rings of Saturn through my cousin's 4" Newtonian telescope. Since then I've always assumed that telescopes were expensive, hard to use instruments that usually ended up in the back closet or in the garage because it was just too much trouble to set the stupid thing up.

A bunch of years went by (25 or so) and I heard of an organization called the San Francisco Sidewalk Astronomers. This group was founded by John Dobson, a strange little man that teaches telescope building at the California Academy of Sciences. This organization takes these huge (anything over 8" could be considered huge) telescopes out on city streets and lets people view whatever is visible at the time. This gets astronomy out to people who normally wouldn't have access. Pretty cool! Also, during John's time as a monk in Sacramento (a whole 'nother story) he devised a mounting system for large telescopes that basically anyone could build. This mount, appropriately called the Dobsonian, is probably the most widely used system today.

Anyway, I ended up taking the class from John and built this thing. I ground the glass myself to a focal length of 5.5 and, that my friend, is not a fun job. Grinding two pieces of glass together for hours on end is not a pleasant way to spend a month of weekends. I did end up having the final polishing done by machine. The advantage to this was getting the accuracy of the final product to within a tenth of a wave of light (call me if you want an explanation). The final figuring was done by the master himself, John. (Figuring is the process of creating the required parabolic curves on the surface of the glass.)

The tube was built from 6 foot strips of redwood using the same techniques as in canoe building, namely, I nailed and glued these strips to a removable form and when the glue dried, removed the nails, then the form. I then fiberglassed both sides and painted the inside of the tube a flat black and used marine varnish on the outside.

I then built the mount and installed all the accessories and hardware. It's hard to see in this picture, but I have a Telrad finder, a 8 x 50 illuminated reticle finder and a 2" rack and pinion focuser mounted at the top of the 'scope. For the numbers oriented folks reading this, a 12" (304.8mm) mirror with a 5.5" (139.7mm) focal length figures out to a focal length of 1676.4mm. With a 4.5 mm eyepiece (the highest power I've got right now) allows about 372.5x magnification.

There's not a whole lot of things, that on a clear night, you wouldn't be able to see with this sucker. Among the deep sky objects I've been able to view are the Orion and Ring Nebulas, the Andromeda and Whirlpool Galaxies and a ton of open clusters as well as all the planets. I was even able to get some good seeing of the comet impacts on Jupiter. The moon is especially cool. This telescope can resolve objects down to about a kilometer so the mountains and crater walls are spectacular.

I would like to add that this telescope received an honorable mention at the 1992 Riverside Telescope Makers Conference.

Please send me, Greg Sherwood (, an email if you have any questions or comments about telescope building or astronomy in general.