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: http://www.atmob.org/library/member/BuildingADob.html
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I recently built was first telescope, a dream I had since I was a kid. OK, I didn't build *everything*; I bought the optics and some of the mechanical components. I thought it would be fun to share some of the information and ideas I had on building a Dob Newtonian scope with other ATMs (Amateur Telescope Makers), particularly the first-timers.
Please note: I am not an expert and this is not intended to be a set of "answers" or truths regarding building Dobs. Like any other project, there are a myriad of trade-offs involved: it is my hope to present some food-for-thought and questions to be asked in the course of designing and building a scope.
I had two primary sources of information, Richard Berry's excellent book "Build Your Own Telescope" without which I probably wouldn't have started. The other is tens, maybe hundreds, of "conversations" I had on the Internet with other ATMs. Their help and encouragement was invaluable. In fact, if you see a good idea here, it is probably yours or someone else's and not mine. I wish I could give individual credit, but there are way too many to list.
I only speak for myself and am not representing any commercial claims or interests.
The focus (pun?) of this document is the mechanical aspects of Dob building, not the optical side. My springboard was Richard Berry's book. My goal in building the scope was to achieve a scope with a great "feel", precise and solid.
In retrospect, I achieved a pretty decent "feel" but at somewhat the expensive of weight. The scope is a 10 inch Newt, f/6. I have not weighed it yet, but it is a bear to wrestle out of the basement. If I did it again, I would place low weight higher in the equation. That is what I mean by global choices. I kind of poo-pooed transportability in the beginning...my advice is to think hard about both the weight and the "awkwardness" of the design. Every extra screw and 1/8 inch thickness of plywood adds up. As I will discuss later, weight can be saved without compromising feel.
One global choice is the mirror diameter/focal number. This is a complicated area, heavy in the optics. It of course has a big impact on the design. I chose 10 inches at f/6 based on trying to achieve a general purpose design that would still fit (barely) in my station wagon. First time builders probably should ponder this choice some before leapping in.
One of those big choices is whether to use a truss tube design or a continuous tube design. The general thinking seems to be that really large Dobs (16 inches plus) are solidly into truss tube territory and that smaller Dobs (eight and below inches) are tube candidates. I went with a fiberboard tube design which is probably heavier than the truss tube design. It is my impression that truss tube designs can be lighter for comparable stiffness than fiberboard tube designs. They also have the advantage, if designed correctly of allowing easy adjustment of the focus placement without having to redrill lots of holes. They can be made to dismantle. Since I have no direct experience, I just raise the point for something to think about.
So pick a tube type, think low-fat and start building!
Another "tool" that makes things neater is a good set of brad point drills for drilling into the fiber board tube (and plywood as well). I found that lots of RPM, a light touch and a piece of wood to back the hole made for nice clean holes. Normal twist drills at the low speeds of battery operated drills sort of rupture the fiberboard.
More of a technique than a tool, I found that wax paper was just the ticket for marking around the circumference of the tube when locating holes for the secondary spider, etc.. Wax paper is just slippery enough to allow small lateral adjustments and it is transparent enough to easily see under/overlap when checking for alignment.
I was fortunate enough to have a drill press, though I think that is more convenience than necessity.
I designed and build a number of fixtures to aid in the construction of the scope. These included a "bed" consisting of a length of plywood with two rails in which to rest the main tube. Otherwise I found that the tube would scoot all other the work bench when I tried to work on it. Unless are able to palm large basketballs with one hand, this might be a good use of your time.
I also made a fixture to back up the inside of the tube when drilling holes in order to get a clean exit hole. It consisted of two chunks of pine, curved to the radius of the inside of the tube and spread apart with a length of threaded drill rod and two nuts. This gizmo could be located in the tube and the nuts tightened such that it firmly held the wood block against the place where the drill would exit.
Save any cutoff from the main tube to use to trying drilling technique and for measurements...it came in mighty handy for me.
I found it useful to make a small scale model of the mount out of foam core cardboard and hot melt glue to get an intuitive feel for the sources of flexure and to experiment with various trussing schemes.
I deal with a store/outlet which just does plywood. They are pretty common near the larger cities and typically have a better selection than the local lumber yard.
I did find (and use) some luaun (sp?) plywood with exterior glue for the ground board since I thought it was the most likely candidate for a good soaking. Both the birch and especially the luaun have a strong tendency to splinter on the edges so use sharp tools and break edges with sandpaper when appropriate.
In general, I don't think you can go too far wrong on the choice of plywood except if has big voids. These can drive you crazy when making any furniture-like creations.
As far as wood glue, I used one of those aliphatic resin glues (yellow color) that have some claim of acceptable outside use. As far as I know, none of this class can truly stand continuous immersion (can you?) but they are easy to obtain and use.
On a whim, I looked in the phone book and found a local "paper tube" factory! Yes, they would sell one to someone off the street. For about $42 I got a 12 foot long tube which I cut in half. I sold one half to a friend and wound up with a great tube for $21! It was almost perfectly concentric, no wax and almost a full 0.2 inches of wall thickness. I have no idea if this kind of factory is common. I will say that the quality of the tube kind of set my whole frame of mind... it upped the ante in regards to doing careful work.
So pick your tube carefully and good luck. As a footnote, I knew one guy who used the varying diameters to great advantage. He build interlocking tube sections by nesting pieces of the tubes together.
I spent some time thinking about how to trim the tube to the right length. I have read that a good sharp utility tube works great. My experiments with my tube were not too successful. I wound up building a fixture for my table saw (insert safety lecture here). It consisted of a giant miter fence about 12 inches tall and about two feet long. I lowered the saw blade below the table surface and adjusted the fence such that the tube was directly over the blade. To locate the tube axially, I made a "collar" with my trusty router which clamped around the tube; the collar referenced against the side edge of the table of the saw. With the help of assistant (father-in-law), we rotated the tube while slowly raising the saw blade. Little by little the tube was cut with an almost perfect edge.
I built the end plywood reinforcing rings with the router but embellished them slightly by leaving a "lip" of plywood such that when mounted on the tube, the outer edge of the scope is the plywood edge, not the raw fiberboard edge. Not necessary, but at this point in the project the mirror was on order and I had time to fool around.
Here is what happened to me. I talked to lots of people and decided to go for 0.75 inches of in-travel. My mirror arrived marked as 59.0 inch focus. I measured everything four times, cut the holes and found that I had lost about 0.3 inches of in-travel! I figured I might be off by maybe 0.1 inches. I now suspect that the mirror was not what it claimed in this regard. My advice is to not take any claim about the mirror for granted...measure it! How, I am not sure, but it can be done. Better yet, figure out a way to temporarily mount the mirror in a way that you can adjust it. Find the focus of choice and then commit to boring holes.
Also, I found that I could get eyepiece info by directly calling the manufacturers. And don't forget barlows and cameras. And make sure you understand in-travel vs. out-travel...these relative terms can drive you crazy.
The first step is to make sure there is enough tube extending beyond the focuser to shield the eyepiece/focuser. A rule-of-thumb is about one primary mirror diameter's worth, though it is possible to do some ray tracing on paper and come up with your own rule. I have about ten inches of extra tube.
As far as baffling materials, various fabrics coated with ultra flat paint are claimed to be best. Fabrics like velvet or corduroy are mentioned. It is acknowledged however that they can be a bear to apply to the inside of the tube. Backing up a bit, fundamentally what you are trying to do is to 1) absorb the light i.e. convert it to heat or 2) reflect it or direct it to a place where it cannot enter the optical path.
The area near the focuser and on up and out the tube are the most critical. Once you get deeper into the tube, the angles are steeper and the light paths more indirect.
Some people make thin rings out of thin plywood or hardboard (another use for a router) and glue them in the tube at various places. Light hitting these baffles bounces back up the tube and generally out of trouble. The rings have the added benefit of greatly stiffening the tube, possibly of great importance near the secondary "spider" since most spider designs tend to collapse the tube. One concern about this approach is that they can encroach on the useful aperture of the scope and limit the angle of acceptance for "good" light.
One simpler way to make baffles is to use open cell foam weatherstripping. I have experimented with this stuff and like it. It can be obtained in a variety of widths and thicknesses. I have found that the stuff I bought is resistant to the solvents in the spray paint (including the adhesive on the foam). It sticks well to the fiber board and the paint sticks well. Test yours before making a gooey mess of your scope.
Other roughening agents which people talk about include sawdust or kitty litter glued to the inside of the tube and then painted. You may want to be careful about using anything which, if it breaks loose from the substrate, can dust up the mirror with "grit". I looked into using "flocking" which is used to give a velvet like covering to small jewelry boxes and the like. It can be obtained in black with special black adhesive. It should be applied with a special hand pump gun to get the right alignment and dispersion of the fibers. The gun is somewhat expensive.
Recently, I read that the Hubble Space Telescope uses something called "Martin" black which is claimed to be the blackest coating on earth! (Don't embarass yourself by asking for it at your local hardware store).
Another idea in the redirection camp is to use glossy black paint deeper in the tube such that light bounces off and away from the mirror. It seems pausible.
One of the better suggestions was to do what seems appropriate and not too difficult and then to look through the focuser without the secondary and shine a bright flashlight into the front of the scope and look for light. Address trouble spots accordingly.
One thought for an embellishment on the rocker box. Several people have claimed that it is very convenient to be able to rotate the position of the focuser as a function of the altitude angle or height of the observer. The rocker box is a convenient place to locate this feature.
To save some weight, I cut two big circular holes in the top and bottom sides of the box.
My purchased mount has a three "puller" bolts and six "pusher" springs. I am thinking about modifying it by adding three "pusher" bolts to lock in the collimation, though I don't think it is a big deal.
I spent alot of time making special mounting "grommets" for the primary mount which nominally bolts through the side of the tube. I didn't want the tube to get all frayed from pulling the mirror in and out. The grommets were triangular piece of brass sheet metal (about 3 inches on a side) with a 0.2 inch thick Life Saver (tm) like slice of brass pipe silver soldered to the center. The brass pipe fits into a hole in the fiberboard tube and lines it. The triangle is mounted to the tube (on the inside) with three truss head screws. This was project unto itself.
I found the secondary collimation adjustment to be very tedious and unsatifying. The above helped alot. Good luck.
Basically, it seemed to me that Ebony Star-ES (tm) was all wrong. It is sort of a fine pebbly like surface. The fine "pebbles" are smooth but the overall impression is that the material is sort of rough. So I tried a variety of other materials. None of the other plastic laminates had anywhere near the right combination of low friction and low stiction. Backing up a bit, what you want is some friction and no stiction. Zero friction is bad because the scope will drift around with the slightest breeze. Excellent roller bearings will give you almost zero stiction but also zero friction.
I thought maybe glass on PTFE would be better. It seemed to have a higher coefficient of friction. Why is ES so great? One person I talked to said that PTFE has a decreased coefficient of friction under heavier loads. Thus the multiple points on the ES place very high local loading on the PTFE. Sounds plausible. Bottom line it was by far the best combination in my experimentation.
One down side to ES is that if you leave the scope resting on the pads, say overnight, the pebbles make an impression in the PTFE (cold flow phenomenon) which causes a "click" when the scope is rotated over the resting location. If you unload the pads, the PTFE will return to its original state and the "click" seems to disappear.
OK, what shape bearing pad? I don't know the science here but I chose circular pads with a bevel around the edge so that the transition from the ES to the pad would be more gentle.
How to mount the pads to the plywood? The simple way of course is to use small brads and drive the heads below the surface of the pad. I used small countersink head screws to capture the pads around the beveled edges, probably not worth the effort. There are sources for "bondable" PTFE which somehow etched on one surface to make it glueable with epoxy. It seemed to only be available in 0.06 inch thickness but I have not researched this exhaustively. The non-bondable stuff is available in a convenient 0.125 inch thickness.
I also did something which may NOT be a good idea. I placed small disks of inner tube rubber under the pads to give a smoother feel and to promote less stiction (the pad can give some under load). It was pointed out that this decreases the overall stiffness of the scope mounting system which is a no-no for any scope. I may remove the pads.
As far as to where to mount the pads, the further out from the center of rotation, the greater the torque needed to rotate the scope. Of course, the farther out, the more stable the mount. Tradeoff time again.
The last fine point here is there is another degree of freedom in this bearing system. By loading the center point, say with a spring and a PTFE washer, you can off-load a portion of the weight on the three outer bearing pads, possibly to advantage. I did not do it and it is not needed on my scope. On really big scopes, some people use roller bearings and add a "brake" to supply the friction.
Bottom line is I would NOT suggest using just any `ol plastic laminate, particularly for the azimuth bearings. Take a small piece of PTFE sheet and walk around rubbing it on a variety of surfaces to get some direct experience; people may think you are strange, but what the heh.
For the center pivot, I used something called an axle bolt which is just a large shoulder bolt in a hole lined with a bronze bushing. These are fairly common items in the better equipped hardware stores; they are in those little boxes, all stacked up with the flip up lids (I love looking through them!).
As far as to where to place the altitude bearings i.e. at what angle, the higher up the greater the bearing loading and the more friction. The lower down, the lower the friction but at the cost of decreased stability. I opted for about 45 degree placement. Most people say try various positions and use the one that feels best.
As far as the diameter of the rings, I used 10 inchers. The larger the diameter, the greater the torque required, but also the more stable. I think about 8 inch diameter is pretty common for scopes of this size.
I wanted the two trunion bearings to be well aligned. Rather than making one big hole in the middle of a piece of wood and sawing it in half to generate the two trunion sides, I cut the two pieces of wood a little over size and temporarily nailed them together. I drilled a small pilot hole through the center of the pair. I pulled the pieces apart: now I had a pilot hole in each piece which perfectly aligns with its companion. The pilot hole served as the center for my router circle jig.
I was concerned about the stability of the bearing board which serves as the substrate for the laminate. As every good woodworker is taught, treat both sides of a piece of wood the same if you want minimal warpage. So I laminated both sides of this piece of plywood. One side with ES, one side with a nice color standard matte finish laminate.
Of course any feet should be placed directly in a radial line from the bearing pads. This is to minimize flexure.
Last but not least, you can get these nifty corner reinforcers like you see on a steamer trunk in a variety of sizes to cover the covers of the rocker box...solid brass, no less.
I have just begun to think about handles to aid in carrying the beast and maybe some sort of "wheels".
Everything is "raw" right now. I am going to paint the scope but of course first need to seal it. I will look into using pigmented shellac which dries fast and I understand that shellac is very impervious to moisture. This is really my next area for research.
Last updated: 13-Nov-95