Constructing the 8-inch Dob
Major Considerations
After major weight issues with my 12.5" dob, I decided to explore ways to make a lighter telescope. Additionally, I wanted to make a scope that could fit into an ordinary suitcase to be checked on an airplane. I am a big fan of Baltic birch. It is a very strong, nice-looking hardwood plywood. But, it is heavy. So I used some fairly obvious engineering approaches to make the boxes of this telescope lighter. I decided to make the telescope out of reinforced 1/4" thick Baltic birch.
What I mean by reinforced is shown in the pictures below. The picture on the left shows two pieces of 1/4" ply, with the rightmost having 'windows' cut out. By gluing them together, as in the right picture, you generate 1/2" ply where you need it. Mostly where something (e.g. truss tubes, gussets, mirror cell) was going to be attached. The 'X' was just for added strength.
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Another issue was that the telescope was going to be a 2-truss design with a focusing board attaching to the two trusses. Because the mirror was fairly slow (F/6.2) it meant that the truss tubes had to be fairly long. Too long to fit in a suitcase. So I designed a way for each truss to break into two pieces yet be joined quite strongly. To begin, two pieces of 1/4" ply were glued together with ~2-inch overlap on one end as seen in the leftmost picture. Then strips ~1.5-inches were cut and glued at 90 degree angles (center picture). One half of the truss has the short side 'in' and the other half has the short side 'out'. Remarkably, these fitted perfectly together to form 'L' shaped trusses more than 40-inches long that are screwed together with #6 bolts and nuts. You can see this in the right picture since the lower halves of the trusses are painted black and the upper halves are not. The joint mates perfectly, as if it was all one piece.
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How do you bolt the pieces of the mirror box and rocker box together?
Another issue for making this scope truly portable is that the mirror box and the rocker box would be too big to fit into my suitcase. Thus, they are designed so that they come completely apart and become just a stack of boards. Importantly, there needs to be some way to reconstruct the boxes into square, strong boxes at the other end of the flight. I had used furniture connectors before, but found that they only come in sizes to fit 1/4" bolts. Since the thickest my plywood was was 1/2", drilling a 1/4" hole in the side, parallel to the face of the plywood, would weaken the wood too much. So I made furniture bolts for #8 machine screws. What are furniture bolts? The picture on the left shows the two parts. One is just a bolt, in this case a #8 1 1/4" long. The other is what I made using 5/16" aluminum rod and drilling through and tapping with an 8-32 tap. I did this using a drill press and a table vice to hold the rod steady. I think it would be very difficult any other way. The rod was cut about 1/2" long and a little slot was cut with a hack saw at one end to help, with a screwdriver, to line up the hole.
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The threaded piece of aluminum rod was fit into a 5/16" inch hole drilled in the plywood about 5/8" from the edge as shown in the leftmost picture. The #8 bolt that comes at this rod at right angles was fitted into a 9/64" hole drilled into the edge of the plywood (parallel with the face of the plywood) with this second hole meeting up in the middle of the 5/16" hole. Then you drill a 9/64" hole in the board to be joined, about 1/4" from the edge. With this setup you can join two pieces of wood at right angles as shown in the leftmost picture below. Again, a drill press really helps accomplish this. You can see the slot in the end of the 5/16" aluminum rod. The #8 bolt is at right angles and holds one side strongly to the other. Both the mirror box and the rocker box are made this way with 8 furniture bolts total used for the mirror box. The mirror box has extra strength added by having triangular gussets half way down as shown in the center picture. These little triangles are bolted in using #6 bolts and threaded inserts in the side of the 3/4"-thick triangles. These triangular gussets not only add strength to the mirror box, but also provide a platform to velcro on a 1/4" board with an 8 1/2" hole cut in it that serves to keep the mirror protected, when a dust cover is placed on, as seen in the rightmost picture.
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The altitude bearings are made from medium density fiber board and painted with faux aluminum paint. They attach with bolts secured by threaded inserts placed in the bearings. The bearing surface is Ebony Star, glued on with Liquid Nails. The circles are cut in the fiber board (and the rocker box) using a plunge router as described.
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The rocker box is made with considerations towards weight as well. The leftmost picture is a fenestrated side to the rocker box. This is 1/2" ply. I then glued on a, non fenestrated 1/4" piece to give the final side. I think I could have just cut windows in 3/4" ply and it would have been just fine. The rightmost picture shows the front side of the rocker box. Its only function is to hold up the sides.
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The bottom of the original version of the rocker box was square and was just simply the bottom of the box as shown in the leftmost picture. This proved quite unstable as it allowed for a very small ground board (also shown). The St. Croix winds almost blew it over several times. So, I re-engineered the bottom of the rocker box which allowed for a bigger ground board as shown in the right side of the leftmost picture. This is much more stable. Notice that the ground boards are made of 1/4" thick plywood and are only 1/2" wide arms that hold the teflon pads for the rocker box to slide on and to keep the teflon pads above the feet of the ground board. The feet here are 3/4" thick plywood. There are no structural needs beyond this for the ground board. The rightmost picture shows the final rocker box assembled.
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A focusing board holds the focuser, the spider, and the secondary mirror. The focusing board was also built with reinforced 1/4" ply as seen in the leftmost picture. I bought a VERY LIGHTWEIGHT (2.2 oz) focuser HC-1 from KineOptics. This is a 1 1/4" helical crayford focuser. It works quite well. I only have one 2" eyepiece, which I can't use in this scope. Thus, my widefield is limited to my 30mm Ultima (42x & fov of 1.2 degrees & exit pupil of 5mm). I tried to buy a spider, but failed (Protostar refused to make a custom design), so I designed and built one myself. It is made from 1"-wide, 0.032"-thick brass sheet (Small Parts B000FN119G) and 5/8" square brass bar (also Small Parts) as seen in the leftmost picture. The square bar was drilled with a 1/4" hole to accommodate the secondary holder, which was from Parks Optical. The brass could have been welded, if I had such talents. Instead, I used PC-7 epoxy and screws. The square brass was tapped for #4-40 threads and these, along with the PC-7 hold the spider together as seen in the center picture. The problem with this style of spider is that there are not lateral or forward/backward adjustments. You get it right, or else. Well, almost. Actually, in one direction, front/back, you can move the primary mirror cell to align the mirrors. The focuser and focusing board, attached to the two trusses can be seen in the rightmost picture. My 30mm Celestron Ultima is in the focuser. This is a great low-power eyepiece. Note the lead weight, covered in black nylon, hooked to the front of the scope. I seem to be conservative in setting my balance point and my scopes are always bottom heavy. With my 7mm Pentax, however, no counter weight is needed. Note also that the scope is covered with black ripstop nylon to keep down stray light and dew.
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The mirror cell is pretty much the design suggested by Richard Berry in his book "Build your own telescope" where he describes and illustrates a simple mirror cell for a 6" dob. I had used this design on my 6" previously and it works well. When I travel, I take out the mirror cell and put it in its own box as carry-on luggage, along with the secondary, it its own box. (pictures of these later).
To get a sense of how this all goes together, see the left picture below. Note that the two L-shaped trusses bolt into the mirror box using threaded inserts. These trusses hold the focusing board. Note also that I attached 1/2" wide, 1/2" thick, ~40"-long plywood 'trusses' at the other corners of the mirror box. They are also two piece, made and joined like the L trusses. The purpose of these 'trusses' is to allow the wrapping of the nylon shroud to keep out light and dew. These extra 'trusses', and the 'box' of the telescope tube, are strengthened by cross members made from 1/4" thick foam core with a veneer of birch (much lighter than baltic birch plywood). One cross-member holds the 6x30 erect image finder (Orion). There is also an attachment plate, up near the focusing board to hold a Rigel Quickfinder reflex sight. Note that the skinny trusses are longer than the L-shaped trusses. This was accidental, but worked out well. The nylon shroud opposite the focuser and eyepiece is several inches above the focusing board and this helps block stray light from entering directly into the eyepiece. I would design this in the next time. From this point, the altitude bearings need to be bolted on, the Rigel Quickfinder attached, and the nylon shroud wrapped around. The tube is then complete and ready to be put on the rocker box and used. On my trip to St. Croix, it took me 45 minutes to go from pieces of wood in the suitcase to an assembled, ready to use telescope. To be able to easily carry the scope, I added a handle to the mirror box as seen in the right picture. It is placed so the tube balances nicely in your hand and you can bring it out to the backyard, put it in your car, take it to the beach, or whatever.
So the telescope works great. The optics seem very good and the movements are smooth and easy. I have a nice range (42x to 180x) of magnifications to choose from and, when the seeing is good (ala St. Croix in the Caribbean), 180x on Saturn is really nice. However, one thing I discovered after using the scope is that the eyepiece height is low. Not surprising since the focal length of the mirror is ~50". I built a great tripod for the scope that was beautiful and should be in the MOMA (see left picture). But, with the telescope mounted on top, it was frighteningly shaky. Vibrations in the scope took forever to dampen. So, I dumped that idea. Then, rather than raising the telescope, I decided to lower myself. So I built an observing chair, based on a concept that I had used before, and the seat adjusts from the ground level to 21", high enough to look through the eyepiece at zenith as shown in the two right pictures. This chair also disassembles into pieces that fit into my suitcase.
This telescope is so portable, before or after it is built, that I find I use it a lot at home. I leave it built in my basement. When I want to go out to the backyard to look at a planet or the moon, I just pick it up and in two minutes I am observing. I think this proves the old adage that the telescope you use the most is the one that is the easiest to deal with. Having said that, I would not give up my portable 12.5" for traveling. There is nothing like that much glass when you are under one of the darkest skies in the world (e.g. Australian outback or the Atacama desert).
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