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Building an 8" Open Tube Reflector

Plan Drawings

The following contains drawings and plans of various parts of this telescope. These should be studied in combination with the construction pictures and verbal instructions before attempting construction. Some of the drawings are illustrative of design concepts while others are of actual parts to be fabricated. The entire set of pictures may be individually printed

Layout

The plan above shows the basic layout of the telescope. Three tubular sections separated by a Surrier type structure. At the place marked Disassembly Separation Point the telescope comes apart and is only one half of its full length. The telescope has a rotating saddle for convenient equatorial mounting and use.

The tube sections

The 8" f/8 telescope uses three 10" diameter round tube sections: a central section, 14" long; and upper section, 12" long; and a lower section, 8" long. For the prototype I used Sonotube Quick Tube, the type without the wax impregnated cardboard. Even this material, however, requires scrubbing with acetone to clean off any release material that may be in the surface prior to coating with polyester resin. Various other materials can be used. There are companies that will sell you cardboard or kraft tubing similar to Sonotube. Protostar offer something very nice with their Black Light tubes made of phenolic resin impregnated kraft tubing. This is really something quite different in that it's a very hard tube and water resistant. The exterior is very smooth except for the 3/64" space between the tube windings. You will have to fill these with either wood filler or body putty. (See article on 8" f/6 Newtonian - TBD)

The exterior end pieces and internal end rings

In developing this telescope considerable attention was given to the overall aesthetic appearance, particularly with regard to the cross-sectional shape of the central strut support structure. This is commonly referred to as the central box and is almost invariably a square section with the strut ends attached as near to the corners as possible. In a purely utilitarian sense this is a very good design and results in a light and rigid structure, and in the case of the Palomar telescope a beautifully appearing instrument. However, the design, when applied to a smaller Newtonian to be equatorially mounted can present problems, particularly when the telescope needs to rotate so as to present comfortable eyepiece positions. Rotating a Newtonian has been a perennially nagging problem for designers. The simplest solution is to rotate the entire tube. But if this is applied to an open tube telescope one winds up with a round central section that essentially separates the central square so that there are two squares, one at each end of the central tube. Suddenly, the square sections look too square, too big, the corners too pointed, and not very attractive when compared to the circular cross-section of the two end tubes. I realized that something needed to be done to mollify this extreme squareness. After a lot of thought and scribbling on index cards I came up with the idea of effectively slightly bending the sides of the box and then nipping off the corners. Also, the saddle tends to fill up the area around the central tube section and further balances things out. The result was something that functioned like a box but looked more round and did not clash with the round end tubes. Also, by eliminating box construction anywhere in the telescope and exclusively using tubes, construction becomes very simple when compared to my 10" f/6 telescope with a separate rotating head and box construction throughout - very much simpler, in fact.

The single biggest problem in building this telescope is drawing this curious bent box end (external end piece) for cutting. It requires a bit of thought and explanation, but nothing that can't be overcome with a little effort. Above is the basic drawing showing the external bent square end structure (external end piece), in red lines; the internal ring, in blue lines; and an external spacer ring, red and green lines. Looking at the drawing you can see how this design breaks up the heavy square geometry as exemplified by the dotted square box marked "1/2 OD + 1/2". Another subtlety is that the struts can now lie flat against the outer walls of the upper and lower tubes and still be in line with the external end pieces. This is all accomplished by having the sides bend in toward the corners as they do. If a standard square piece were used it would require the struts to be placed 1/2" out from the sides of the tubes. I would then have to use a 1/2" thick external ring on the upper and lower tubes and this would not look nearly as good. External rings break up the sleek exterior appearance. The method for drawing the external end piece is given below.

 

Begin by carefully finding the exact outer diameter of the tube you are using. This is best done by first making a correctly fitting internal ring. This ring will be 3/4" thick and extend 1/2" into the tube. Once you make a correct internal ring you can accurately measure the external diameter. This now becomes the OD and the diameter of the hole for the external end piece.

Draw a square on the wood you will make the end piece out of that is the OD + 1". This will allow for 1/2" of material thickness around the tube exterior. Draw a vertical line and a horizontal line bisecting the square. The intersection of these lines will form the center of the piece. From this center take a compass and draw a circle having as its radius 1/2 of the tube OD. The circle you draw should be exactly the OD and leave 1/2" between the top, bottom and sides of the square as shown in the drawing above. Now draw another square whose sides are tangent with the circle you just drew. You will now have two squares with sides 1/2" different from each other as seen by the two dotted line squares above. Take your compass and now draw a circle that is tangent with the outer square. You will now have two circles as seen above. The inner one will be the tube OD and the outer one 1/2" larger in radius and will be tangent with the outer square.

On the inner square, mark the right leg 4" out from the center as shown as point B. Take a ruler and hold it so that it comes against point B and is tangent to a point near the circle near the center as shown above by the line A - B. The tangent point will not be exactly at the vertical center line but perhaps about 1/2" to the right. You want to create a straight line that sweeps into the curved line.

 

Repeat this procedure but now with the vertical line as shown above. 

 

Now take a ruler and connect the lines at the corner as shown. You have now created one corner.

 

Repeat this procedure until all four quadrants are completed as shown above. Once one is made you will find the other one much easier. My procedure was to draw out two, #1 (the exterior ring at the bottom of the center tube) and #2 (the exterior ring at the top of the central tube) and then trace out the third, #3 (the exterior ring at the bottom of the upper section). The telescope separates into two pieces and the joining place is the combination of #2 and #3 exterior rings. These must be exactly the same size and the attachment holes must match exactly or things will not look right. To accomplish this #3 was cut a little oversize and rubber cemented to #2 with #2 on top as shown in the construction photographs. Using the lines from #2 as a guide both pieces can be sanded to shape together and the attachment holes drilled. This is a simple methodology that insures exact alignment.

Mirror cell

The mirror cell consists of a tip-tilt plate and bottom cover or end piece and are made from 3/4" medium density fiberboard or good quality plywood. The tip-tilt plate is made to fit through the bottom interior tube ring with about 1/8" to spare. The sides of the tip-tilt plate triangle can be left straight or curved in somewhat as shown in the construction drawings. The collimation holes are placed as close to the end of the corners as is reasonable so as to achieve maximum leverage and stability in mirror support and collimation

  

The end piece can be made in a variety of ways, two are shown above. Black circles are open cut outs for ventilation. The design on the left was used for the prototype 8" f/8 but the design on the right might, in fact be better since it allows for access and removal of the mirror mounting bolt without having to remove the tip-tilt plate, a decided advantage in hindsight. When drilling the collimation bolt holes, have the two pieces taped together with masking tape and mark the orientation of the pieces for later reference. In this way the holes will align perfectly.

The rotating saddle and clamping mechanism

The saddle was made from two thicknesses of good quality plywood 3/4" glued together. The design is essentially a split circular clamp held together with 1/4-20 threaded rod through the line A as shown. Wing nuts or threaded knobs are used to hold the two pieces together and control the amount of pressure or tension on the rotating central mounting tube. The two ring assemblies are connected by a bottom piece made of the same thickness and material as the rings. The whole assembly is glued together with polyester resin. I used no screws.

Struts

The drawing above shows the basic dimensions and method of attaching the struts to the tubes and external end rings. The construction drawings fully explain the detailed method of attachment using threaded insert nuts.