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Building an 8" Open Tube Reflector
Basic construction principles
The fundamental design concept of this telescope is based upon a form of composite construction that I developed many years ago. It involves the use of polyester resin in combination with wood or other porous and fibrous materials to form a relatively light-weight as well as strong and water resistant structure. The fact that this construction technique makes the structure water resistant is also extremely important since swelling and consequent warping of the structure during an observing session may throw the instrument out of accurate collimation. A coat of paint is simply not good enough for this purpose. Critical to an understanding of this composite structure is the notion that the polyester resin acts not only as a protective coating but also as an adhesive. The initial gluing of the parts, using Elmer's glue or similar, so as to assist in forming the assembled shape, is only a preliminary act and in no way contributes to true structural integrity of the finished product. The gluing should only be seen has a tacking together of the parts in a minimal way. In fact, applying excessive amounts of glue will only weaken the final structure inasmuch as the polyester resin is intended to seep in between the joints. The eventual structure is incredibly strong, and, in fact, is strongest at the joints. Merely gluing the pieces together in the conventional manner results in the joints being the weakest part of the structure, just the reverse of what is intended. This type of construction allows the use of wood materials that are thinner than what would normally be considered necessary or prudent for a structure of this type where not only the ability to remain intact is important but even small amounts of flexure can be detrimental to good performance. I emphasize this point only inasmuch as so many people have suggested that other conventional gluing methods might be employed as a substitute. People have also suggested the use of fiberglass cloth as a form of additional strengthening. This is completely unnecessary and will only create additional weight.
In designing this telescope I have attempted to reduce weight as much as possible without compromising rigidity. If I have any criticism of telescopes that I have seen it is mostly a tendency to over-build in certain areas. And this also extends to professionally built telescopes for amateurs. Frequently, the wood is too thick and the struts too heavy. The Surrier design is a brilliantly simple and effective structure that uses the fundamental principles of the truss in a multidimensional embodiment without retaining any superfluous elements; quite literally, a stroke of genius. When compared to open framework or skeletal tube designs of the 19th and early 20th centuries its elegant simplicity makes them look almost absurd by comparison. With this in mind, most telescope builders use truss tubes that are far too large diametrically and produce a structure that is needlessly heavy.
Making this telescope will be a fairly straightforward construction operation to those individuals who have been involved in a variety of woodworking projects from bookcases to bird houses to model boats. Needless to say, anyone who is experienced in cabinet or furniture making will find it extremely simple. Accuracy and cutting the parts is critical. I suggest that one be able to cut parts to an accuracy of approximately 1/32 inch to ensure a reasonably accurate end product.
As for the time required to make this telescope, working fairly steadily on weekends and odd evenings should result in a finished telescope in two or three months. Working very consistently I made mine in a bit over two weeks.
Materials to be used
Primary construction materials include medium density fiberboard, a good quality birch plywood and a section of concrete form builders tube, commonly known as Sonotube. The use of MDF was an experiment and I'm not certain I would repeat it. The end product seems to work well but is a bit heavier than what it would be had I used plywood. The problem is getting good quality plywood. The tendency at present is to find plywood having good looking exterior face sheets with the interior material soft and punky. I suggest going to a real professional lumberyard and staying away from the chain stores.
The builders tube used for the telescope is to have a nominal outer diameter of 10 inches. When purchasing this one must be careful to try and get a fairly accurate measurement of the specific tube that you are purchasing. The sizes printed on the tube are only 'nominal' and can not be assumed to accurately represent the actual dimension. This is done to facilitate the shipping of these tubes inasmuch as they are telescoped into each other so as to make for more efficient transportation. Take a ruler to the store with you and find the exact size you need. Try to find a tube that is almost exactly truly round. Many are somewhat egg shaped.
Laying out and cutting parts
All laying out of parts should be done in a quiet and undistracting environment. Mistakes in these initial stages can be frustrating later on. Print out the parts to be cut from the plans given here and study them carefully before drawing out the pieces on the MDF or plywood. Develop a good and clear understanding of the project you are undertaking. Time spent here will pay dividends as the project progresses. The old carpenter's dictum of 'measure twice, cut once' is very sound advice.
The cutting of circular parts is best done on a band saw. If you do not have, or have access to, a band saw a saber saw will do as long as you can hold it very accurately.
I have taken great pains to create a design and construction procedure that requires very few power tools. About the only power tool that I feel is truly necessary is a table saw, but that might even be dispensed with. A table saw, however, is extremely convenient in that it allows for accurate cutting and can also function as a disk sander. A drill press is also a decided convenience, though this can be dispensed with if one is good at holding a drill accurately. One can also purchase inexpensive drill guides that will allow for accurate alignment of holes. Other then these, only hand tools are required.
Installing a standard mirror
Although this article describes fitting a conical mirror, one can also install a standard full-thickness mirror in this telescope. Elsewhere on my web page I have given suggestions on how to make a simple cell for the standard full-thickness mirror. It is fundamentally the same as the cell used for the conical mirror except that there are three supporting pads for the mirror and retaining struts in four places around the side of the mirror. This article may be found here on this web page
Along with these written instructions one should also study carefully the construction photographs provided here. These are heavily annotated and give what I hope will be useful information regarding assembly techniques. In particular, they give information as to the use and application of the polyester resin, painting tips, assembly techniques and things to avoid. I have made many mistakes over the years making telescopes, and even a few making this one, and my hope is that I can make you the beneficiary of my experience. I look forward to hearing from many of you who have successfully constructed this telescope or used these pages to help design and build one of their own design.