Refractor Homemade Dobsonian-Style Mounts
In 1996 I purchased Richard Berry's book, Build Your Own Telescope, as I was looking to build Dobsonian-style mounts for my apochromatic refractors that would make it easier to set them up when I did not have time to set up a German equatorial mount. Since then I have owned apochromatic refractors from different manufacturers but have been able to continue to use the same mounts by modifying them as needed. For example, the tube diameter of a telescope from one manufacturer may be of different size than a tube diameter from another manufacturer even though the aperture of the telescopes may be similar.
The above photograph shows the homemade Dobsonian-style mounts that I made for two refractors that I use to own. On the left is an Astro-Physics 180mm f/9 EDT refractor, while on the right is a Astro-Physics 130mm f/8.35 EDF refractor. Since I took the above picture these two telescopes, as well as an AP 90mm f/5, were sold and I purchased a TMB 175mm f/8 refractor, TMB 130mm f/9.25 refractor, and TMB 105mm f/6.25 refractor to replace them. Here it a photograph of TMB 175mm f/8 refractor on the Dobsonian-style mount:
The TMB 175mm on the Dobsonian-style mount showing the side bearings, rocker box, and tripod. The cradle and tripod are approximately 66" tall. An Antares 12 x 80mm Finder is mounted on top of the telescope.
The mount for the 180mm and 175mm were based on the design for a 6" f/15 achromatic refractor in Richard Berry's book. The cradle box and tripod hub were made out of 3/4" plywood. The tripod legs were made out of pine board lumber and were attached to the tripod hub using 7" carriage bolts. The tripod hub and legs weigh about 30 lbs. The cradle box weighs about 10 lbs. and has a 12" diameter Formica bearing surface on the bottom which rides on three Teflon pads on top of the tripod hub.
Here is a side view of the rocker box, side bearings, and mounting rings, prior to painting on top of the tripod hub and tripod:
Here is a front view of the rocker box, side bearings, and mounting rings, prior to painting on top of the tripod hub and tripod:
When I built the Dobsonian-style mounts for the refractors I decided that rather then building a separate box to hold the tube assembly I simply rotated the mounting rings on their sides and used bolts to attached the side bearings to them. The side bearing for the 175mm is made from 8" wooden disks. I wrapped the outside edge of these disks in Formica and glued them to the disks.
The mount for the 130mm uses a cradle box and tripod hub similar to the one for the 175mm, although smaller and lighter as I used 1/2" plywood for it. It has an 8" diameter Formica bearing surface. However unlike the tripod legs used for the 180mm and 175mm the tripod legs for the 130mm are made of wooden dowels, 1-1/4" in diameter and 48" long. These wooden dowels are bolted together at the bottom, and have a small rope connecting the three legs so that the tripod cannot accidentally be kicked out from under the telescope. The tripod and cradle are approximately 50" tall, and weigh about 10 lbs.
Close-up of mount for 130mm refractor showing the side bearings attached to the mounting rings, the counterweights in the cradle box, and the legs made out of wooden dowels.
The side bearings for the 130mm are approximately 5-1/4" in diameter. I cut out the wooden disks, bolted PVC piping to the side of them, and glued Formica around the outside of the PVC piping. For maximum stability the side bearings should be at least the size of the objective, and preferably larger. This way when changing from heavy to light eyepieces the telescope maintains its balance and does not move.
For both mounts I used ideas gleaned from Telescope Making Magazine issue #44, Spring 1991. In it Dave Kreige of Obsession Telescopes suggests using Ebony Star Formica and Etched Virgin Teflon for smooth motions. The etched surface of the Teflon allows for the use of contact cement so there are no nail heads to scratch the Formica. Both the Virgin Etched Teflon and Ebony Star Formica can be found at Lowe's and Home Depot.
Also, he recommends waxing the Formica surface to help reduce the friction between the bearings (I use Turtle Wax which I find works very well). I have found his suggestions to be very helpful in reducing friction for smoother motions in these mounts. This has allowed me to use these telescopes at over 60x per inch of aperture when seeing permits. Most of the sketches on my web page hae been made using my telescopes on Dob mounts and alt-az mounts.
Article © 2000 - 2013, Eric Jamison, All rights reserved.