You are hereAN ASYMMETRICAL MOUNT FOR A 17.5" NEWTONIAN
AN ASYMMETRICAL MOUNT FOR A 17.5" NEWTONIAN
by Steve Lieber
How can you build a large telescope that is both portable and sturdy? Most lightweight designs are not very rigid. A rigid instrument is often too heavy to be considered portable. Is it possible to have a portable telescope that can be easily transported and still be stiff?
I live in New York City. I would like to have a 17.5” instrument. I needed to design one that is light enough to take around in a car. This is not easy to do. I also had to consider the space it would take in a car as well as weight.
I do not have a garage or a basement. The design must allow me to make the mechanical parts outside in my backyard. This would involve using hand tools. It is difficult to carefully machine parts this way. I waned to design an instrument that could be built using a circular saw and a router, not a machine shop. This meant that the design had to avoid any tight tolerances. It had to “forgive” any uneven cuts or sloppiness on my part.
Telescope design has come a long way since John Dobson built his first 18” Newtonian. He had to transport that in a cargo van!
This telescope is designed in an unusual way. Many people try to make the instrument lightweight. My design would strive for minimum volume. Make the parts as small as possible. This way they can easily be placed in a car. I would not worry about the weight. They would all be light enough to carry, so the overall weight of the telescope would not an issue.
The basic Dobsonian design uses a mirror box. Trunnions are attached to either side of the box. The bearings are the outer surface of the trunnions. The strength of the mirror box is critical to this design. The box must be strong. It must keep the trunnions rigid. Any flexure here will cause problems. The mirror box also supports the truss poles. Here again the box must be rigid.
The trunnion’s center of curvature is aligned with the balance point of the telescope. This allows the scope to maintain its balance in different altitudes. In the past people would handle this by making a large mirror box. This raises the sides of the box high enough to reach the center of balance (and motion). The trunnions would be attached to the side of this large box. The trunnions would be rather small, often circular. The box would have to be very rigid (read “heavy”) in order to do all of this work.
Another solution is to make the trunnions large. Large trunnions may look unusual, but remember: they are flat. They do not take up much space when the telescope is taken apart. The mirror box can now be made very small. In fact it only needs to be large enough to hold the mirror. This resulting box is only 8” high. It also allows for a 9” high rocker box. Both will fit in the trunk of a car.
Steve Overholt developed this idea in his book “Lightweight Giants.” He made large trunnions and attached them to a small box. His original design had problems. The telescope would flex. The reason was that the trunnions were not well supported. They were attached only to the mirror box. The mirror box was too small to support these large trunnions, especially since the box was placed near the lower edge of the trunnions. The top portions were unsupported. They would flex. This design also placed a lot of force on the mirror box. The trunnions were bolted to the box. This concentrated a lot of strain at the place where the two would meet. This is also not a good idea. It resulted in more flexure at this area as well.
My instrument gets around these problems by clamping the trunnions to each other with tie rods. The mirror box is sandwiched in between. The box becomes a strut squeezed between the trunnions. Long threaded rods act as tie rods. The mirror lid is also used as a structural element. It is rigid. When folded open it braces the upper part of the trunnions. A tie rod is used here as well. The trunnions are clamped to the lid. This supports the entire length of the trunnions.
A strut / tie rod design is inherently very solid. It also does not require careful machining. The sidepieces are well supported even if the edges are uneven. An uneven surface will still clamp securely. It is not necessary to have perfectly straight edges. Such attention to detail would not add to the overall strength of the structure. The pieces can be made in my backyard with a circular saw.
The trunnions and mirror lid are made with a foam board core. Styrofoam is an easy material to use. Plywood and Formica are glued to it. This produces elements that are light and very rigid. The great thing here is that if you want an item to be stronger, just use thicker foam board. You do not have to use a heavier wood on the surface. Using a thick foam board with thin plywood produces a lightweight and very stiff unit.
The trunnions use 2” thick foam board with 3/8” plywood. I choose 3/8” rather than ¼” plywood because of the forces produced by the tie rods. That can be rough on the surface, even after using large washers. Also these trunnions are about four feet long. They get handled roughly when putting them in the car, so the surface needs to be a bit tough.
How to make the large curve on the trunnions? An arm was attached to a router. The arm was anchored to the top of the trunnion. This was accurate enough to generate the curve for the bearing. I then glued Ebony Star Formica to the surface.
The mirror lid uses 1” foam board and is covered with Formica. I used the leftover parts of the “Ebony Star” that was needed for the bearings. One great aspect about foam board is that it does not require any special glue. Ordinary carpenters glue works just fine. It is also fairly non-toxic and cleans up with soap and water.
The mirror box is ¾” plywood. I modified an 18-point Novak cell to accept a nylon sling. (“Mirror Cell Design For Large Dobsonians” by Lew Haberly, Amateur Astronomy #6, also A.A. #1, page 39.) Underneath, three pieces of angle aluminum further strengthened the box. (A.A. #1, page 28. Yes Tom, I noticed that detail.) The angle pieces of aluminum are bolted to the mirror cell, the bottom of the mirror box and the sides of the mirror box. The entire box functions as a beam. All stress forces are distributed throughout the entire box. Forces are not concentrated. It makes the entire box very solid.
The outer mirror lid is made of 1” thick foam board. Formica is the skin. When folded open the lid is also clamped between the trunnions. It becomes a strut. When folded shut it can protect the mirror when transporting. I made a test section of this design 8 years ago. I glued Formica to foam board. The Formica covers four sides. Two sides are open. I left this outside in the elements for 8 years, summer and winter. It has not deteriorated at all.
There is also an inner mirror lid. A plain sheet of Formica is used here. This inner lid can cover the mirror while I am assembling the telescope. The mirror can also be covered while keeping the telescope assembled.
Aluminum angle pieces are used with the threaded rods. They are shorter than the struts and do not serve any structural purpose. They only keep the pieces aligned when assembling. They also keep softer parts away from any damage by the threaded rods. The rods could abrade them otherwise.
The diagonal cage is built entirely out of aluminum. It is made of ½” box aluminum. It has a 1/16” wall thickness. This is very light and very strong. I cut the pieces and had a machinist weld them together. This was the only part of the construction done away from home. The resulting cube is very strong. You can stand on it. It also weighs very little. It weighs less than the plywood rings that are popular with many telescope makers. Black fabric covers it.
The diagonal cage is not as small as you may see on some telescopes. I wanted a large one in order to protect the delicate spider vanes. This is the only part of the design that does not try for minimum volume.
The poles are 1” aluminum, 0.035” thick, seamless. They are attached to the mirror box using 1” Lucite. The plastic was previously used as a bank teller’s window. I simply drilled and tapped holes for setscrews. Threaded inserts keep the Lucite from being damaged over time.
Look again at the mirror box. I need to be able to open the lid. Where do the aluminum poles attach to the box? The answer was to make the box asymmetrical. Two of the clamps are inside the mirror box. Two are outside. There is nothing wrong with an asymmetrical telescope. Does the diagonal cage sit directly over the mirror box? No. Does that affect collimation? Not at all. I collimate in the usual manner: first by squaring the focuser onto the diagonal, then adjust the diagonal to be at a 45-degree angle to the primary. Then adjust the primary. The final telescope may look odd, but it works like any other Newtonian.
The mirror is a homemade 17.5” f/5.5. Most mirrors of this size are f/4.5 or shorter. That produces a deep curve. I knew that I was too inexperienced with optics to make such a fast mirror. The answer was to try for a shallower curve and a longer focal length. I made the mirror in my living room, using a 55-gallon drum as a workbench. The mirror blank came from Coulter Optical.
Two full-sized tools were used, one for the back and one for the front. I ground the back flat with one tool. Then I placed the mirror on the tool. Wet newspaper held them together. The tools were made of reinforced concrete. A non-reinforced tool failed and flexed. An epoxy coating sealed the concrete surface. Some of the techniques for making the 1¾” thick mirror came from old “Telescope Making” articles.
Every possible mistake that could be imagined happened to this mirror. I dropped it. I scratched it. I read the Foucault test incorrectly, polished the wrong zones, turned down the edge, etc. By the way, it is a fine mirror. It just took a long time.
The diagonal has a minor axis of 3.0”. This makes for a very small 17% central obstruction. It yields a fully illuminated one-degree field of view. You cannot ask for more.
And how did it turn out? The telescope performs well. It is a little stiff, but that is deliberate. There are often strong sea breezes where I live. When the telescope’s bearings were too smooth it would blow over. The large focal ratio caused it to act as a sail. I adjusted the bearing so that they do not glide so easily.
It quickly dampens down. If I tap the frame the vibrations stop in a couple of seconds.
It did have one problem. The tubes would flex. The telescope would bend when it was used at lower elevations. The solution was to add two small clamps, one on each trunnion. They attach to two of the tubes, about 18” above the base. This holds everything solid. This was the only major shortcoming of this construction.
I can now take a large telescope in a car. In fact I have taken two passengers in my car with me, including a tent. The diagonal cage goes behind the driver. The mirror, mirror box and base get stored in the trunk. All other pieces go on roof racks. (Remember the weather test – I do not have to worry about rain.)
Several people helped me, especially with the mirror figuring; Barry Levin, Prof. Phil Pinches and Stew Rorer provided the most assistance.
The end result is a solid, very portable telescope. It is a useful design. It is not the final say in telescope making, but it is a unique construction that works well: an asymmetrical Dobsonian with oversized trunnions. It is also a design that is not available anywhere commercially. That is a fine reason for going through all of the trouble of making an instrument.