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Building an 8" Open Tube Reflector
Tubes, Struts and Finishing
Tubing - cutting and selecting the tubing
The basic material used for the three tube sections was the cardboard Sonotube or Quick-Tube. It is the type that does not have a wax coating. Do not use the wax coated type - nothing will stick to it. Another type of tube to use would be a phenolic resin impregnated type sold by Protostar under the name Blacklight telescope tube. This type is stiffer than the cardboard tubes and will take a better external finish as well as being water resistant and not requiring a polyester or epoxy resin coating as does cardboard.
After marking off the places to cut, the cuts are kept square by rolling a piece of aluminum roof flashing around the tube so that it matches when completely encircled and secured with masking tape. When this is done a mark can be made completely around the tube and an accurate square cut made.
For cardboard and phenolic tubes the cut is started with a fine toothed saw and finished with an electric reciprocating hand-held saber saw. You can make the entire cut with a hand saw if you like.
Laying out parts on MDF or plywood
For this project I used medium density fiberboard to construct the interior tube rings and exterior central tube ends. This ws an experiment and it worked out fairly well but good quality birch plywood would have been lighter and maybe even better. I have used plywood in the past with very good results. When selecting plywood go to a real lumberyard and not to the big chain stores. Their plywood is usually of low grade with poor internal layers and voids. You don't need much so get the best. When laying out parts it is very important to measure with great care and keep things square. Keep the pencil sharp. Take your time, think a lot and don't rush or do this in a distracting environment. Label parts clearly on the plywood before cutting them out to avoid confusion. I mark the parts with dimensions at times to show long and short sides when the difference is close and confusion can arise. Mark parts to show proper orientation and how they match each other in assembly.
Draw parts with pencil and compass. Tools needed are an inexpensive compass, a carpenter's square, a 12 to 24 inch steel ruler. This drawing is of the external central strut support taken directly from the plan drawing. To the right is the same part cut out. Note that the that the holes for the bolts that hold the two upper external rings together (red arrows) are drilled for 1/4-20 bolts and are approximately 1/2" in from the cut off corners.
Cutting and gluing parts
Parts must be carefully cut with attention given to cutting along the outside of the line and not on the line. Practice with scrap if you are not a skilled table saw user until you can cut to about 1/32" accuracy. Observe all common safety precautions advised when using power tools of this type and keep children away from work area. I always unplug my table saw when I walk away from it.
Internal end rings are cut out with a band saw. In determining the ID and OD of the tubes one one be very careful. While Protostar's Blacklight tubes are very consistent from tube to tube at 9.90" this is not so for the cardboard Sonotubes. In order to make for efficient shipping, the tubes are of only of 'nominal' size, that is to say, they vary in diameter over a range of perhaps an inch or so. This allows them to be shipped nested within each other. The result is that you must carefully measure the tubes yourself and select one that is the correct size. Also, you must make the telescope out of a single tube so as to make certain all the sections are of equal diameter. Once you have measured the tube and know it's actual size every part of that tube will be the same. Squeeze the tube as round as you can until it measures pretty much the same across any diameter and then cut a test piece out of scrap wood to see if it fits snugly and is not too loose or too tight. To the right is an example of a well-fitting ring cut to exactly the correct size. It should be just a little tight.
If the ring is not tight it can be made tight by fitting in some index card material as shown at the left. If it's really loose, pieces of wood can be cut and fitted in as seen here. This filler material need only to be tack glued in. Later the Polyester resin used to seal the tube will glue everything solidly into place.
The pictures above show an external end piece. These mount at the ends of the central tube section and support the struts as well as give strength and shape to the tube. The saw cut seen at the left should be glued together with a little polyester resin and either taped together as shown to the right or held with a weight as shown in the center picture. With polyester resin use a piece of wax paper to keep things from sticking.
The lower external end is a single piece but the upper external end consists of two pieces. These two are bolted together at the corners and form the separating point of the telescope. They must be identical or they will not fit correctly and look poorlt when assembled. The method I use is to cut two ends a little bit outside of the lines and then glue them together with rubber cement. This is an old model maker's trick. Rubber cement will hold well enough when spread over a large area but can be taken apart easily. Mark the piece with pencil or marker so as to make proper orientation easy. Later, before painting, make dimple locator holes with a small drill.
The external ends are now sanded smooth and even using a disk sander. I turn my table saw into a disk sander by getting a masonry cutting disk and putting an 80 grit sanding disk against that. One can even use ordinary sandpaper for finer surfaces. Just don't press hard or it will tear up. As always, take care to unplug these tools when not in use and keep children at a distance. At this time, prior to separation, the holes that hold the two halves together should be drilled to insure perfect alignment. Note hole mark at red arrow.
The pieces can now be separated with a flat knife or putty knife and the rubber cement rubbed off. Note the crows feet marks for properly matching and orienting the pieces. Note also that the holes that hold the two halves together had been drilled prior to separation to insure perfect alignment.
Other parts such as this ring can also be easily sanded and shaped to perfection with the sanding disk
The external ends are now tack glued to the center tube section. Use a flat surface you can put glue on. Wax paper underneath works very well to keep glue off of surfaces discourage sticking. Make sure not to forget the external rings that sit against the external ends. Once again, note the crows feet marks in the center picture. You can't over do this sort of thing.
Clamps can be very useful for holding things together.
The lower section where the mirror is housed is made from tubing with internal end rings capped with a disk that forms the back of the mirror cell. Once again, the disk sander can give a smooth attractive exterior surface to the cap and even make a nice chamfer on the edge. Matching this with a similar chamfer at the front creates a balanced appearance to the telescope.
After the end rings have been glued into the tube the cap is centered and taped into position and the six mounting holes drilled. At this time the tip-tilt plate can also be taped into position and drilled for collimating holes as shown at right.
Coating with resin
NOTE: Resin and acetone should only be used in an open and well ventilated area. Read the instructions on the resin and acetone containers and use good sense and take precautions in their use.
I use polyester resin. It dries fairly quickly. Ace Hardware can get it for you by the gallon or quart. They used to stck Evercoat resin but now they carry Bondo resin. They appear to be very similar products. Two quarts ought to be enough. Get some extra resin hardener, perhaps two of the larger 1.35 fluid ounce tubes. I use about three to four times as much hardener as suggested. You will use acetone to clean things up. Acetone is a flammable liquid and gives off potentially harmful fumes in a small poorly ventilated area. Do not use around an open flame and allow no smoking in the area you are working. Small and unsupervised children should not be in the area when you work with resins and acetone. You will possibly have these liquids in wax coated paper cups and similar containers and they can be mistaken for soft drinks. - Be careful.
Working with polyester boat resin is not quite the same as painting. Don't wipe your eyes with your resin coated hands - it burns. It will ruin any clothes you wear. This stuff acts as much like a glue as a protective coating, one of its main advantages and one which I have exploited in my design. The resultant structure is immensely strong as well as light for its strength. It also has a short working life and can be a bit nasty to clean up. All cleanup is done with acetone. I use about three or four times as much hardener as recommended. This speeds up drying but shortens the working life. Plan on using a batch for no more than 5 or 10 minutes. When it starts to gell up, stop at once and clean the brush in a container of acetone. It should dry enough to handle in about three hours in a warm and dry environment. On humid days and in colder temperatures it will take longer. All parts should be laid out on ordinary wax paper. It will not stick to wax paper but will stick to about anything else and it will have an extreme reaction with and destroy Styrofoam. Use cheap brushes of 1" to 1.5" width. Application should be on flat level surfaces. Paint one inside and one outside surface. Let dry and turn over and do another two surfaces. You'll try to be neat but things will get a bit messy. Just have plenty of acetone ready to clean up things.
Use boxes and cans to raise parts up above the floor level. Cover these with wax paper. Note picture to left. I have painted two surfaces, an inner and outer, and am waiting for them to dry before painting the end. More complex shapes must just be painted as best as you can. Watch out for runs and brush them out when you see them. Take your time and expect to take a few evenings to get the job done. Don't rush and keep your head clear.
Jig made from a stick for resining exterior of tube.
Sanding and smoothing
All resined parts must be carefully sanded by hand. How well you do this will largely determine the appearance of the telescope. Flat surfaces may be sanded with paper over wood blocks. Start with # 80 and finish with # 320.
In the picture to the left the red material on the end is filler putty. This can be picked up at automotive stores and is good for filling in minor voids. The the end you can create a very smooth surface suitable for painting or covering with contact paper as I did.
Machine screw inserts
The struts are connected to the corners of the external end supports on the central tube and the ends of the exterior of the end tubes as shown in the picture to the right. Exact dimensions for hole positioning is given in the plans section. Inasmuch as wood is soft attaching the struts with wood screws was not considered advisable. Over time these would loosen and eventually strip. Rather, I chose to use wood inserts that are threaded to take a machine bolt of the desired size, in this case a 10-32. The inserts need to be threaded into a hole pre-drilled with a .275 drill. The end of the insert takes an Allen head wrench as shown below.
The inserts are positioned in the upper tubes and central tube as shown below. Turn them in until the insert is just slightly below the outer surface. This will allow the strut to bear firmly against the wall of the tube and the exterior end support. The \inserts were put in after resining to avoid having the inserts fill up with resin. The inserts can be locked into place with any thin cyanoacrylate glue (super glue) after positioning in telescope.
The struts are made from 1/2" x 1/2" aluminum channel stock that I got from Home Depot. You can also get it from McMaster - Carr or similar suppliers. Typically, Surrier truss type telescopes use round tubes. For large Dobsonians this has worked well and suppliers make end fittings so the tubes can be attached to the rocker boxes and upper end tube, but for smaller telescopes there are no end clamps available that work at all well and the ones I found were expensive. Thus, I was motivated to use channel stock. This allowed me to trim away two sides at the end of the strut to form a flat tongue that would bear against the tube and allow for secure mounting. This has worked out very well, and an added benefit is that the struts can be made to face inward and avoid protruding from the sides of the telescope, thereby giving a trim and sleek appearance. The pictures below show how the struts can be cut on a band saw or even by hand with a hack saw and filed smooth.
This shows how the strut end is measured and marked with a pencil.
The strut is cut as shown. After cutting the corners are nipped off the ends as seen at the right. Holes should drilled at the ends of the struts to take a 10 -32 screw. These holes should be 3/8" from the end. It is very important to place the holes accurately. I suggest that they all be elongated somewhat so that the struts will be loose when assembled and can be adjusted as described below.
Above can be seen the completed but unpainted parts ready for assembly.
The telescope can now be preliminarily assembled as shown above. There is a simple methodology to assembling this type of telescope so that the tubes are properly parallel to each other. This is discussed below. Note telescope separated for storage or traveling.
Covering and Painting
Prior to painting. all parts must be sanded smooth starting with #100 and finishing with #320 gray emery paper. Any defects will only be amplified by the paint. To a much lesser degree it will imprint through contact paper. Try and not sand through the polyester resin and into the wood.
At this point I will discuss the use of contact paper as a covering for the telescope tubes. This was another experiment. It has the potential to produce and very attractive exterior but it can be peeled off and can be subject to tearing if abused. However, people have apparently used this stuff to cover telescope tubes with success. It covers a multitude of surface defects and takes paint very well, particularly Rustoleum Hammertone Paint. Rustoleum makes this paint in of two types, one for metal one specifically made for plastic. I used this paint for everything on the telescope. Because of the curious 'hammered' appearing texture small surface defects are more readily hidden.
The contact paper I used was a leather texture type I got at Home Depot. The leather texture pattern is very small and tight and creates an attractive mottled effect. Roll the paper on evenly and cut the paper off straight an clean not like you see in picture to the left. Ragged patterns like this will show through the next adjoining layer.
Where possible use a single sheet and roll on smoothly. Excess is cut off with a new, sharp razor blade.
In painting this telescope I chose something new, Rustoleum Hammertone paint. This paint has a kind of "hammered" or textured finish and covers a multirude of surface defects. Rustoleum also seems to give you a good deal of paint in the can. I chose two colors for the telescope. The tubes were painted black (which is really a very dark gray) and the struts, end cap and saddle, the next shade of gray down. It worked well, I think, and give the telescope a little variety of shade, if not color.
Apply paint moderately and evenly keeping the spray can in constant motion. Start spraying pointing off to one side and quickly move over the surface to be painted to avoid having the sprayer spit little blobs on the surface. Put on one coat followed by another in a few minutes. Be careful not to put on excessively thick coats. Putting on additional coats may result in crinkling if additional coats are too thick and the underlying paint no thoroughly dry. Flat black is applied to the inside with a brush to avoid over-spraying. The problem with brush-on flat black is that the black is not really 'flat', just matte. If you are willing to be careful and mask areas off you can spray in a good flat black. Only the front tube section requires a truly flat texture and this can be achieved with flocked paper better than any paint.
Drying outside on a warm day. Note use of a jig made from a stick for painting interior and exterior of tube and holding other parts as well.
The flocked anti reflection paper I got from Protostar. It has a sticky back side and can be directly applied over a painted surface. It is really almost a velour or velvet type of surface and will greatly suppress grazing incidence reflections as well creating a dead surface to look into. This is where many Newtonians fail. If the eyepiece looks out on to a surface that will reflect stray light you will have reduced contrast when observing dim objects. Also, you will note that this telescope has the upper tube section extend a full eight inches beyond the eyepiece. This is also important in reducing stray light. Newtonians often have their eyepieces very near the end of the tube and the observer looks over the end of the tube and into stray light emanating from horizon areas. Long front ends used to be a characteristic of English Newtonian telescopes from the twenties through the forties. It was a good idea.
If you are painting directly on the Sonotube, sand after the first coat with very fine 400 emery paper to take down the inevitable fuzzies that rise up after paint is applied
Struts were painted with Rustoleum Hammertone paint and hung to dry on nails. I painted one half first, hung it to dry and then painted the other half. The struts required two coats. This type of paint creates a gas a part of the hammertone finish process and results in tiny pinholes that need a second coat to cover. I cleaned the struts with acetone prior to painting to remove any possible surface oil.
Final assembly and placing of struts
Begin attaching the struts by arranging the bottom and center boxes on small pieces of plywood of equal thickness and making sure they are square and in line and separated by the correct distance. Distances between tubes and the correct position of the center box for correct balance is determined by using the spreadsheet program provided on this web page. Once the distance between the boxes is established the distance between the holes is measured and marked off on the strut tubes, leaving an excess length of 3/4" beyond the holes. 1.5" of the end of the strut is squashed in a vise and the holes drilled. I used 1/4-20 round head bolts 3/4" in length and one washer and a hex nut on the inside. One must take care to make the distances of the holes the same on all struts. The holes should be oversized a bit so as the not be tight or bind and allow for slight differences in hole position and to assist in final alignment. If you are careful, the boxes will come out straight and true. Do one side and the turn over 90 degrees and do another, keeping careful check on alignment of the two boxes as you proceed.
Set the tube sections on the plywood pieces making sure that the exterior tube ends are clear of the floor and the tubes are parallel to the floor. Start as shown in the picture above left by setting up two struts on the center and lower tube sections. Do not tighten the screws. Leave them a little bit loose. Rotate the tube 90 degrees and set up two more struts. Repeat until all four sides are set up with the screws loose. Press the tubes into the plywood boards with your hand and make sure they lie flat against the boards and tubes are parallel to the floor. Start tightening the screws on the struts making sure everything remains parallel to the floor. Repeat and continue the process with the central and upper tube sections.
When completed sight down the tube from from to rear and make certain that everything looks perfectly concentric.
When painted and assembled one can now add the final upper end hardware; spider and secondary, eyepiece focuser, finder, etcetera. The focuser seen here is a Moonlight dual speed.