12.5" f/5 Ultralight Dobsonian (Mark III)

After completing the 12" f/5.3 ultralight, I immediately began thinking about designing and building the next one. The 12" f/5.3 ultralight was a study in finding how far I could push the design. I went a bit too far. See the 12" f/5.3 ultralight webpage for a discussion of the issues.

I wanted to accomplish two goals with this scope. I wanted it to follow the same design principles as the earlier scopes, i.e. I wanted to continue to use three struts, but have them be just stiff enough without adding unnecessary weight. And I wanted to make its assembly tool-less. To accomplish these goals, I decided to use cylindrical struts.

12This scope is the result.

The entire telescope weighs about 50 lb., including heavier optics, focuser, and finder. The optical tube assembly, with optics, weighs 30 lb. The large 80mm finder adds another three pounds. It uses three 1 Ĺ" diameter cylindrical aluminum tubes. Their combined weight is 5 lb. All of the wood is 1/2" Finnish Birch.

12The upper "cage" assembly is essentially the same as the 12" f/5.3 ultralight (with appropriate credit to Mel Bartels and Bruce Sayre, by whom I was inspired). The upper "cage" is a single ring cut from 1/2" Finnish Birch plywood. The aluminum tubes fit inside clamps that are glued and screwed to the bottom of the ring. Each block, used here and everywhere else on the scope, has been formed by first gluing two Ĺ" pieces of plywood together. Each has a thinned curved section shaped into it that it provides the spring force in the clamp. Then each is shaped for its intended location on the scope. Each has a 1 Ĺ" hole drilled through it that admits the tube. To hold the tube in place, each block has been made into a clamp by cutting a slot along one side. A ľ"-24 bolt and wing nut is used to apply force to clamp the tubes in place. I used 24 tpi threads to provide greater mechanical advantage since wing nuts are used.
12From this angle, you can see how the focuser is attached. I relaxed my weight constraint in order to use a quality 2" focuser. This is a JMI NGF-1 with the optional black anodized finish and flat base. The focuser is then attached to a focuser shelf. Since the focuser is larger and heavier, I used a stiffer piece of aluminum machined out of a large channel with nominally ľ" thick walls. I cut off one side of the channel, drilled a 2-1/4" diameter hole in the center, and then drilled and tapped holes to attach the focuser to the shelf and attach the shelf to the ring. The shelf is attached to the ring via two 1/4"-20 bolts that screw into drilled and tapped holes in the shelf.

The focuser is positioned 45 degrees from vertical, which permits comfortable viewing no matter where the scope is pointed. Also, with this focal length and focuser position, anyone over about 5' 8" can view anywhere in the sky while standing on the ground.

The baffle on the side opposite the focuser is cut from a sheet of Kydex plastic. One improvement on this scope is that I put a permanent set into the Kydex before I cut it. I wrapped a sheet of Kydex together with some laminate (think of a jellyroll) to form a tube that was the diameter of the inside of the ring. I held this tube together with twine and placed into a convection over at 200 Fahrenheit for about 20 minutes. I took the rolled tube out and allowed to cool. The temperature is critical. Higher temperatures make the Kydex too soft. Lower temperatures donít allow the material to take a uniform set. This baffle now behaves as if it were cut from a rigid telescope tube. It shows no tendency to sag or deflect into the light path. Two small strips of Velcro that permit easy installation and removal hold it on.

The 80mm right angle finder is positioned so I can star hop comfortably while sitting down.

12This is a close up of the diagonal and its holder. To make the holder, I started with a 1-1/2" aluminum rod stock. I cut it at a 45 degree angle and the milled the sides of this aluminum block a bit so I would have good flat surfaces to hold onto in the vice. The center of the block is also milled out to almost the base in order to reduce weight. I then drilled and tapped three holes spaced around the bottom to accept the three alignment screws. In the center I milled a spherical hole to accept the brass acorn nut. A threaded post passes through a circular 1/8" plate and threads onto the brass acorn nut. A locking nut is installed on the other side of the plate. Then the three alignment screws pass through three holes in the circular plate and screw into the bottom of the aluminum block. This construction is very similar to most diagonal mirror cells you see. The only difference is that the diagonal is held in place by silicone caulk. The diagonal is a 2.6" with an enhanced coating. I painted the edge with flat black paint in order to reduce reflections of this surface.
12The mirror box without optics or mirror cell weighs about 5 lb. and is very strong. The lightweight construction consists of the top and bottom pieces, cut out with a router from 1/2" Finnish Birch plywood. The top piece has a 13" diameter cutout and serves as a baffle for the primary. The lower piece has a 4" diameter hole. This construction baffles light coming from the bottom, going up past the mirror to the diagonal, yet allows air to circulate. On the inside surface of the two wood pieces I routed a groove with a diameter of 14". This groove accepts the fiberglass tube that I made out of Ebony Star Formica. I made a form that held the Formica in the right diameter cylinder. The Formica is curved into a cylinder, and the ends are overlapped and epoxied in place. After the epoxy cures, the inside of the cylinder is covered with a layer of fiberglass cloth and epoxied. It forms a nice stiff, lightweight tube. The tube is then epoxied into the routed grooves in each of the top and bottom plywood pieces. There are two clamps for each tube that align and hold the tubes in place.

The mirror box is the second heaviest component of the scope when it is disassembled for transport. The mirror box, with cell and primary, weighs 18 lb.

From this angle you can see the three alignment screws for the mirror cell. By attaching the mirror cell directly to the bottom of the mirror box permits the center of gravity to be as low as possible. The distance from the top of the mirror to the bottom of the mirror box is approximately 2-1/2".

Also note the three small feet on the bottom of the mirror box. These feet keep the mirror box from resting on the adjustment screws before I assemble the scope. Also, they keep the adjustment screws from digging into the rocker box when the mirror box is placed within it for transport. I drilled holes into the bottom of the mirror box about half way through. I then glued dowel rods into the holes and cut them the same length on my table saw after the glue dried. I use inexpensive rubber feet available from most hardware stores. The three supports are longer than the adjustment screws, but short enough in their positions so they don't interfere with telescope motion.

12I used the program "Plop" made available by David Lewis (Thanks David) to investigate various mount designs. Surprisingly, a six-point mirror cell, where the support points are arranged at a single radius, always provides better support than the conventional nine-point mirror cell, where three support points are at an inner radius, and six support points are at an outer radius! Since a six-point mirror cell is easier to make than a nine, and because I was curious to try out the prediction, I made a six point mirror cell. The cell is machined from a plate of 1/2" aluminum stock. The beams are machined from 3/8" X 1/2" aluminum bar stock. The mounting hardware is the same as taught by Bruce Sayre. The beams pivot on stainless steel shoulder bolts. The support pads are stainless steel tee (or weld) nuts. 
12 The mirror is then attached to the support pads using silicone caulk. Even though the total adhesive area is approximately 7% of the mirror area, it is very firmly attached. Adhesives are very good in shear.

What really makes this scope lightweight is the primary. It is a 1" thick Pyrex mirror. It weighs approximately 12 lb. Itís a very nice mirror made by Enterprise Optics and has a QSP enhanced coating. Here you can see the mirror attached to its cell before it is placed into the mirror box. A stainless steel spring is placed over each of the three support bolts before the assembly is placed into the mirror box.

12The bearing surfaces are the conventional Ebony Star laminate and Teflon. In this image you can see a close up of the altitude bearings. Note that I routed the bearing to have a 1/4" lip that protrudes past the bearing surface. The outside surface of this lip, one on each bearing, rides against the inside surface of the Teflon pad, and keeps the scope centered in the rocker box.

Two clamps, each with its own knob, hold each of the bearings in place and permit the scope to be easily balanced. This is one of the features I like most about using three parallel struts. When I want to alter the balance point, I simply loosen the knobs, nudge the bearings to a new position, and then re-tighten the knobs.

I also made these bearings larger than on the previous scope (the Mark II). They are 18" diameter. I designed it this way so the bearings would overlap the mirror box. This prevents a possible rotational vibration mode from occurring.

12This next image shows a Kydex baffle in place over the top of the mirror box. This provides a tighter baffle for the primary mirror and minimizes reflections from the top of the mirror box, improving contrast.
12This is what the scope looks like when it is disassembled for transport. Short lengths of 1-1/2" aluminum tube substitute for the long lengths used when the scope is assembled. Most pieces fit nicely into the rocker box. This is also a good view of the rocker box. This is much stiffer than my earlier rocker boxes. It has five improvements that increase stiffness. The first is that the sides are shorter because I used larger bearings to support the tube assembly. Second, and probably most important, are the gussets extending along the back sides. In order for the gussets to provide support, the base had to be widened. The wider base also adds stability. Third, the lip around the back stiffens the bottom board. Fourth, the bottom board is 1" thick (two 1/2" pieces glued together) within the walls of the box. This resists twist.
12Finally, I use an eyepiece shelf within the rocker box, which stiffens it and provides ready, safe access of eyepieces during observing sessions. The eyepiece shelf can be seen best in the image at left. It is made from 1/4" Finnish Birch plywood and is glued into slots routed into the front and sides of the rocker box. It has space for two 2", and four 1-1/4" eyepieces.

I completed this scope just in time for the Riverside Telescope Makers Conference 2000. I am pleased to say that I won a Merit Award for this scope.

The rigidity and motions of the scope are great. There is no visual sag or displacement when I move the telescope around. The motions are smooth. The scope stays put when released. It maintains collimation very well. Vibrations are small and damp out quickly.

Can you make a scope for me? Do you have any plans you can send?

I received numerous requests from friends, people I meet at star parties, and people who stumble across my website, to make them a scope or to send them plans so they can use or copy my designs. Since I received so many requests, I built a limited number of the 12.5" f/5 ultralight dobsonian (the Mark III) telescopes. I am no longer building these. However, If you think you would like to buy one, please contact Custom Telescopes by Plettstone. I am not planning to build any of my other designs at this time.

I'm sorry, at this time I don't have any plans for my scopes. I am happy to answer the questions of any amateur who is interested in making a scope for his or her own personal use.


Selected Links

Mike Rawling's version of the Mark III 12.5" ultralight, with some of his own innovations. Excellent description of construction details.

Michael Rasmussen's version of the Mark III 12.5" ultralight, with some of his own innovations.


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