12" f/5.3 Ultralight Dobsonian (Mark II)

After completing the 12.5" dobsonian using "C" channels, I immediately began thinking about designing and building the next one. I am very pleased with the first scope. It's an outstanding performer. Unsure of how the "C" channels would really work, and it being my first dobsonian, I overbuilt it just to make sure. I wanted the next scope to be much lighter.

 

This scope is the result. The entire telescope weighs about 37 lbs., including optics. The optical tube assembly, with optics, weighs 23 lbs. This is approximately 1/3 the weight of the first scope. It uses only three "C" channels. Their combined weight is 5.7 lbs., about the same as the usual eight trusses. The channels are 1 1/2" width X 3/4" height X 1/8" thick. All of the wood is 1/2" Finnish Birch. This is a great material to work with. Its nine plys are glued together during fabrication with a water-resistance glue. It is very strong and dimensionally stable.

The upper "cage" assembly borrows from designs from Mel Bartels and from Bruce Sayre. The upper "cage" is a single ring cut from 1/2" Finnish Birch plywood. The "C" channels attach to three blocks that are glued and screwed to the bottom of the ring. The ends of each channel are slotted so that the ring can be slipped onto the ends of the channel. The 1/4"-20 bolts slide into these slots and are then screwed tight. When disassembling, the bolts are just loosened and then the ring is slipped off. No loose parts. Note that the channels attach at a spacing of 120 degrees around the perimeter of the ring. With the 6" f/6 dobsonian, I could get away with using only two channels spaced along the sides of the tube. That construction permitted the adjustable bearings to be attached directly to the sides. By moving the location of the side channels from their position on the 12.5" dobsonian , the channels are equidistant from each other and allow the side channels to be attached. Although the center of gravity of the scope is offset a bit from the attachment of the bearings, the torque is minimal. It gives the illusion that the scope is floating. The spider is a conventional model made by Novak. It attaches to the ring via four aluminum angle pieces.

From this angle, you can see how the focuser is attached. It's angled at 45 degrees, 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. The 8X50 right angle finder is positioned so I can star hop comfortably while sitting down.

Close up of focuser and focuser shelf. I started with a standard 1 1/4" helical focuser available from many sources (e.g. Orion, University Optics). I milled the base flat and drilled and tapped four holes in the base. The focuser is then attached to a focuser shelf made of 1/8" thick aluminum angle. The shelf is attached to the ring via two 1/4"-20 bolts that screw into tee nuts permanently inserted into the ring. You can see the tee nuts in one of the above photos. The shelf itself can be quickly removed from the ring by loosening the two 1/4"-20 screws. The shelf has two holes that are "keyholed" like most of the attachments on the scope. By loosening the screws the shelf can be moved to where the screw heads are aligned with larger holes, permitting the shelf to be removed while leaving the bolts attached.

This is a close up of the diagonal 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. 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 then held in place by silicone caulk. In later versions of this mirror cell, I mill out the middle of the aluminum block almost to the base in order to reduce weight. The diagonal is a 2.14" with an enhanced coating.

The mirror box without optics or mirror cell weighs about 5 lbs. 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 lightweight tube. The tube is then epoxied into the routed grooves in each of the top and bottom plywood pieces. There are also three wooden pieces joining the top and bottom. They each have two threaded tee nuts inserted into them. The wooden pieces serve to align the channels and the tee-nuts hold captive screws that are used to attach the channels. The channels are "keyholed" to permit easy attachment and removal without removing the screws.

The mirror box is the single heaviest component of the scope when it is disassembled for transport. The mirror box, with cell and primary, weighs 14 lbs! From this angle you can see the three alignment screws for the mirror cell. By attaching the mirror cell directly to the bottom permits the weight to be as low as possible. The distance from the top of the mirror to the bottom of the mirror box is 2-1/2".

I 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. 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 firmly attached. Adhesives are very good in shear.

What really makes this scope lightweight is the primary. It is a 3/4" thick Pyrex mirror. It weighs only 7 lbs! Unfortunately I don't know its figure. But foucault testing by the people who run the Chabot College mirror making class (thanks all) indicates it to be a reasonably figured mirror.

The bearing surfaces are the conventional ebony star Formica 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. The two knobs you see 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 the "C" channels. Each of the side "C" channels has slots cut into them. Carriage bolts insert into these slots and pass through the bearings to the tightening knobs. 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 have just completed this scope and have had little time with it. However, I have star tested the scope and it performs quite well. I can easily resolve the double-double in Lyra. Views of Saturn and Jupiter are quite good. Stars aren't quite the pinpoints I'm used to. I don't know if this is due to the primary or the mirror mount.

The rigidity of the scope is adequate for visual use. There is no visual sag or displacement when I move the telescope around. The motions are great - buttery smooth on both axes. It maintains collimation very well. I can torque the front end several degrees, but you never do that in practice. It certainly isn't rigid enough for photography.

The only problem I have to solve is vibration. If you tap the tube, the vibrations take 4-5 seconds to damp out. The amplitude is OK at low magnifications, but up at 150X or more; it is very annoying and interferes with focusing. I need to find a solution.

Above, I mentioned two areas of concern: stars not being the pinpoints I expected, and the long vibration damping time.

The first thing I did was to remove the 12" f/5.3 3/4" thick mirror and replace it with a 12.5" f/5 1" thick mirror. If I had any doubt about whether the silicone caulk would safely hold the mirror, those doubts were laid to rest. I had to use a razor blade to remove the support pads. There is no way it could ever fall off. With the 12.5" thin mirror I had excellent star images when vibrations settled down. This indicated that the figure on the 12" wasn't too good or the six-point cell wasn’t adequate.

So I turned the six-point cell into the 18-point flotation cell shown to the left. In this image, the mirror clips are not yet installed. PLOP indicates that an 18-point cell is overkill. Also, there is no silicone caulk to influence the performance. The mirror floats on 18 small support points. When I substituted the 12" f/5.3 back into the telescope using this cell, the star images were a bit better. However, any residual abberations, I believe, are due to the mirror.

I then addressed the long vibration damping time. I believe there are two major sources of the vibrations: the C-channels and the rocker box. In my goal to make an ultralight, I pushed the limits of making everything as light as I could, knowing that I would be trading off stiffness. I went a tad too far. In my first 12.5" dobsonian, the Mark I, with the Parks full-thickness mirror, there was no problem with vibration. Its design is different, but it used 1" X 2" channels. This ultralight uses 3/4" X 1 1/2" channels. In addition, the rocker box plywood is only 1/2" thick vs. 3/4" on the original. The ultralight rocker box also has larger cutouts. There is noticeable play in the rocker box. In fact, it is probably the greatest source of vibration.

The arrangement of the channels provides great stiffness in the primary axes (up-down and sideways) where the forces are the greatest. However, the arrangement allows some twist (where there is very little force). However, this twisting vibrational mode appears to be excited when the scope is moved. One way to resist vibration is to increase stiffness. One way to increase stiffness is to increase the size of the channels (actually, you want to increase the moment of inertia). The 1" X 2" channels are about 2 1/2 times as stiff as the 3/4" X 1-1/2". However, another possibility is to use rectangular tubes. Rectangular tubes provide the greatest stiffness of any geometry given the same mass and diameter. Cylinders are almost as good. In fact, compare this design to my latest scope, the 12.5" f/5 ultralight. Overall, it adds about 10 lbs, but it is a lot stiffer. It uses three cylindrical struts, but otherwise, it looks very much like the 12 " f/5.3 ultralight.

However, since the problem is vibration, not stiffness, I solved it by using "constrained-layer damping" which is the most effective way to damp vibrations. To be most effective the second constraining member should be approximately the same stiffness as the first member and cover at least half its length. I used a second 3/4" X 1 1/2" strut placed back-to-back along the lower half of each strut, with a thin layer of damping adhesive between them. The completed scope is shown. The difference is remarkable. Without the upper cage assembly attached, the struts used to vibrate like a tuning fork (of course the scope isn't used this way, it was just a good test). Once I made the modifications, they damp out within a second. This added a couple of pounds in strategic areas that improved the performance immensely. Lightweight was one of my primary goals for the ultralight. Pushing the limits helped me understand what was important. In the ultralight's rocker box, I could see where it was flexing. So I redesigned the rocker box. I haven’t made one with the improved design for this scope yet, but you can see the design on the 12.5" f/5 ultralight. When I rebuild the rocker box for this scope, I’ll confirm its effect on this scope as well. I added a couple of other features that improve convenience. The first was to add the three small feet on bottom of the mirror box that can be seen in the previous image. 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 it 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.

The other feature is that I made a mirror box cover that also holds the upper ring securely in place during transport. I started with a 1/4" Finnish Birch plywood disk slightly larger than the mirror box opening. I drilled three holes through the plywood, spaced equidistant near its outer edge. I notced dowel rods on both ends and glued them into these three holes. The notches and dowels are aligned so that the lower end keeps the cover centered over the mirror box opening, and the top ends keep the upper ring centered and secure suspended above the mirror box.

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.

When I design and build, I sketch the ideas and design as I go. Consequently, 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.

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