Doug Tanaka's 12.5", f/6 string-truss Dob

It is based on the design used by Kriege/Berry in "The Dobsonian Telescope". I had never seen a Dobsonian telescope (in fact, didn't even know what one was!) prior to buying the book. My first real look through a telescope was through an ETX 90-EC I bought in the Fall of 1999. It was a few weeks later that I learned what a Messier object was, and not much later than that I was disappointed in what they looked like. So, the following Spring I was in a telescope store in Vancouver, BC, saw the Kriege/Berry book and decided to buy it. In May I ordered the mirror from Robert Royce and it arrived in July. At the same time (May) I saw the site for Dan Gray's string truss scope and decided to alter the Kriege/Berry design to accommodate strings.
Being familiar only with the Obsession-type design and Dan Gray's Design, I was in no position to experiment very far. First light was August 21, 2000, and aside from a telescope I looked through when I was a kid, the ETX and this Dob were the only telescopes I had ever looked through. It wasn't until I joined the Battle Point Astronomical Association, last November, that I could actually talk to someone who knew what collimation meant!
You can see that the mirror box is very shallow. My wife and I travel a lot in a van/motorhome, and one of the requirements of bringing the scope along was that it had to fit under the bed. That meant the mirror box had to be 10.25" tall to have .25" of clearance, and it has to be removed from the rocker box. The secondary cage fits inside and is flush with the top of the mirror box.
The frame of the secondary cage is made from a very lightweight wood called Paulonia. It is like basswood, but with more color and more grain pattern (similar to Oak). The Paulonia was used to make the two octagonal rings, the focuser and Telrad boards, and the two struts that hold the octagonal rings apart and carry two of the curved spider vanes (the third spider vane attaches to the focuser board). The wood is covered with fiberglass and epoxy for strength and to show the wood grain. The black panels are made from a material called carbon tissue. It is like dark gray Kleenex and weighs about 3 oz/yard. It was made into individual panels by brushing with epoxy and pressing between two pieces of plywood and waxed paper. After the panels were cured I cut them to size and epoxied them to the inside of the secondary cage. Oriented this way they are very stiff, adding sufficient rigidity to the cage with minimal weight. The inside of the cage is lined with black flocking paper. The three-vaned, curved spider is made from 3 inch wide strips of carbon ribbon sandwiched between two layers of carbon tissue. They are 60 degree arcs spaced at 120 degrees. The arcs were pressed in a form cut from a block of 4x4 cedar.
In one picture you can see the swing filter attached to the focuser. It holds a 2" nebula filter and with the filter removed doubles as the focuser baffle. Because the secondary cage had to fit inside the mirror box, it is not very tall, so I also have a baffle opposite the focuser (not shown), made from black ripstop nylon that attaches via elastic cord.
The string is made from four continuous loops (eight strands) of 450 Plus archery string. They are looped around small, stainless steel thimbles to reduce wear. Also to reduce wear, the string is threaded through a black, polyester jacket. The jacket is 3mm accessory cord with the inner nylon filaments removed. The loops are "served" at the thimbles, an archery technique that uses wraps of a special thread to keep things secured. This is preferable to using knots, which can slip, and also overly weaken the connection. The string was made up in a wood- framed jig so they all come out the exact same length.
The 3/4 inch aluminum tubes are covered with a black, hard-sponge material used as wrap for bicycle handlebars. They slip into plastic sleeves (plastic pipe) epoxied into the corners. 3/4 inch O.D. springs, 3 inches long, sit at the bottom of the sleeves to supply the compression. Earlier, I had different springs that supplied about 35 lbs. of force when fully compressed and there was a bit of movement at the secondary cage (visible with a laser collimator) when going from the zenith to the horizon. I thought that the problem was in the narrow (f/6) angle of the strings; future scopes might benefit from a wider string base. I found that replacing the original springs with stiffer springs that gave 50 lbs. of force when compressed solved the problem and the scope now holds collimation very nicely.
The scope is very stiff, perhaps because I'm using four poles instead of three. The club's 16 inch f4.5 string scope has three 80 lb. springs and mine has four 50 lb. springs. Another idea that we might use on our third scope (the club has a 26.5 inch mirror that we want to make a Springfield mount telescope out of, with the focuser at the balance point) are miniature DeStaco clamps used in woodworking. They are lever actuated and move a plunger that is adjustable. We're thinking we could loosely position the poles in sleeves and lever them up for compression. The mirror weighs about 420 lbs. and we know springs aren't going to cut it, but we still want fast setup. We'll probably use about ten loops of 450 Plus per string and go with an eight string, four pole design.
I tried screw type poles in an earlier incarnation of the string truss (there were several experiments), but I didn't like it because it took too long to screw them in and back out. I felt it took away from the fast setup offered by the strings.
In the picture that shows the swing filter, you can partially see the D-9 connector that delivers the12V DC power. It connects below the Telrad and the wires go into a black cloth tube that feeds into the truss tube. It reemerges at the bottom and feeds into the top of the mirror box. Two lines go to resistors on the back of the secondary mirror that produce about 1.7 watts of heat. Two more go to a jack below the focuser. A line plugs into this that provides resistor heat for an eyepiece. At one end of the Telrad you can see two plugs. One runs power from the two AA batteries that are housed in the mirror box (to reduce weight), and the other delivers 12V DC power to a 75 ohm power resistor that heats the Telrad. Small holes are drilled below the Telrad window, that work like a windshield defroster in a car. The finderscope is also wired with resistors.
The mirror box is made from 1/2 inch appleply. The trunnions are 1 inch thick and are held on by four threaded, plastic knobs. These thread into brass inserts in the side of the mirror box. For storage the knobs unscrew and the trunnions swing down flush with the top of the mirror box (to fit under the bed in the van). The mirror cell is 1/2 inch Plexiglas, as are the three triangles for the 9-point flotation system. A four inch fan blows onto the back of the mirror. In addition, two 1 inch fans are mounted in opposite corners of the mirror box and blow across the mirror face. They are offset from each other to create a swirling action with the air. The amount of air coming from each of these 1 inch fans is almost imperceptible, but the effect is quite amazing. Prior to putting them in, when I only had the 4 inch fan, I would often still see thermal currents hours after setup (possibly because the mirror could not quite keep up with falling temperatures). With the two small fans turned on, I get rock steady images in less than an hour, and they stay steady throughout the night. The fans are mounted on pieces of sorbothane cut from an old shoe insert, and there is no noticeable vibration.
You can see I transport the scope like a hand-truck. I often observe from the top of a 4-story condominium building and the upright handles make it possible to use the elevator. In a wheelbarrow position it would never fit. Black nylon straps with quick release buckles hold the eyepiece case and poles in place.
I currently balance the scope (because of the low profile of the mirror box) with a virtual counterweight system using bungee cord and also two small 12 VDC batteries (2.5 inches x 3.5 inches x 6 inches) that hang from the center of the trunnions. The battery holders are made of 1/8 inch Plexiglas with a piece of metal channel called FastTrac screwed to it. FastTrac is used to make adjustable jigs for woodworking. One part of the holder hangs from the plastic knob at the center of the trunnion. The batteries sit in a separate Plexiglas box attached to the FastTrac with a bolt and plastic knob. This way a battery can be easily raised or lowered to adjust for the balancing difference between a large and small eyepiece. The battery holders are a good solution for balance problems without adding purely dead weight - I needed power anyway and don't have to lug around a separate battery. I'm planning on changing trunnion size from the current 7 inch radius to a 10 inch radius. Raising the COG 3 inch will also allow me to mount the finderscope at a more convenient height.

Doug Tanaka

25 Aug 2001
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