In this article, I will outline some engineering principles that apply, and show how to use them to optimize the mount design. I have built two mounts this way; one for a 6 inch f/8 that weighs about 15 lbs, and one for a full thickness 12 inch that weighs about 9 lbs. Both mounts have been in service for several years, and perform as well as their heavy-weight cousins.

The key to light weight construction is to understand the difference between strength and stiffness. Strength is how much load the material can take before it breaks. It can be found by multiplying the tensile strength of the material by the area under load. Stiffness refers to how much it will bend under much lower loads. To predict how much bending will take place is more complex, and involves calculating the moment of inertia. This basically means that the thicker shapes are stiffer that thinner ones, and that the outside layers of a material take more of the load than the inside layers. If you take a straight bar of wood or some other material, and try to bend it, the one side will be stretched _ in tension, and the other in compression. The center of the piece, called the neutral axis, will take no load at all.

So what good is this? Well, since the center does not contribute to the stiffness, it can be built lighter than the outside areas, provided that there is still enough left to hold it all together. In the first mount I built using this principle, I used two layers of 1/8" plywood, separated by strips of ¾" pine. Because it was largely hollow, the mount is much lighter than solid ¾" plywood. However, because it was thicker overall, it has the same stiffness as solid wood. As for strength, a one foot cross section of ¾" plywood can support a dead load of about 28,000 pounds before failure, so losing 75% of its strength doesn't really hurt.

There are two drawbacks to this method. One is that you must use more care in construction. The strength of the finished system depends on good glue joints, which in turn means accurate cuts, and clean mating surfaces. Special allowances must be made for the bearing blocks. For the azimuth bearing, I made sure that there were cross braces under the bearings. The altitude bearing required gluing in extra blocks inside the frame.

The second problem is wear. Solid plywood can take a lot of abuse without affecting the performance, but hitting the side of a 1/8" plywood can do serious damage quickly. Again, this can be controlled somewhat by careful design. There is another way, however, and it results in even lighter, stronger construction. We can reduce weight from the center of the system by using materials other than wood. My favorite is that marvel of materials science _ Styrofoam. The second mount I made used the same basic principle, but I used ¼" plywood for the bracing instead of ¾" pine. Then I carefully cut Styrofoam to fill all the gaps, and glued the skins in place. This method calls for even more care in construction: The bolt used to center the azimuth bearing has its own wooden block, which in turn is supported by the ribs. Still, the Styrofoam fits tight to all of it.

There are a few things that can be done to ensure a successful project. The altitude bearing blocks will tend to damage the frame unless they are properly supported. One way is to drill a hole in the Styrofoam, insert a piece of doweling, and put the bearing block on the end of the dowel. You can expect that the load will be spread at a 45» angle from the dowel, so use this as a guide. Six inches of ¾" dowel should be sufficient.

The Styrofoam should be cut on a tablesaw to ensure accurate, square cuts. Be careful when you cut it this way. It doesn't handle as well as wood, and if you let the blade drift, it will chew up your sheet pretty quickly. Practice without power on a couple of times, then cut quickly, and don't stop partway through the cut. Use lots of glue, to make sure all joints seal. Use a brush or roller to spread the glue. Use only water based finishes, as most solvents will dissolve Styrofoam. Test your finish on scraps first.

With this method, there is a lot of added stiffness and strength, and it is much less susceptible to damage. Overall, the mount I made weighs in under 9 pounds, and supports a full thickness 12" f/6 Newtonian, and has operated successfully for several years. Compared with its solid wood relatives, the mount is less than ¼ the weight. Since it's less of a chore to move, it spends more time under the stars _ where it belongs.

Doug Angle is a member of the RASC Kingston Centre.