Coulter Makeover Part 1 - An Easy to Make Six Point Mirror Cell

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I bought a used 13.1" Coulter Odyssey II. These scopes are relatively inexpensive, and make the most out of inexpensive materials. In my mind, this makes them ideal. You get a scope for very little money, and use it while you are rebuilding it. Yes, you can pay a lot and get a scope you don't have to rebuild (like an Obsession), or you can pay a medium amount and have a scope that you can tweak, and live with the rest of it (like a Meade/Orion/Celestron/etc.), but I want everything just the way I want it, so I figured the cheaper the better, I'm going to replace everything anyway, why pay for stuff I'm not going to use.

One of the little marvels of economy is the mirror cell, shown here. The back of the scope is a circle of oriented strand board (OSB, the stuff they use for house sheathing) 16" in diameter, painted black, with rubber feet on it. On top of that is another circle of OSB that fits inside the tube, to hold it in place (3 drywall screws through the tube hold it in. On top of that is the mirror cell proper, which consists of a circle of particle board. 3 push screws and 3 pull screws serve to tilt the particle board circle, which has 3 rubber pads stapled to it. The mirror sits on the 3 pads, and is held in place laterally by duct tape. The duct tape is secured to the mirror with a large hose clamp, and to the particle board by staples. I assume the staples and clamp are so that the mirror doesn't fall out if the duct tape glue gets soft from heat.

Despite the way it sounds from my description above, this cell is actually nicer than the ones I've seen on other scopes, in that the external parts are made of OSB, not particle board. OSB is an external grade material, and stands up to dew well. Particle board warps and eventually comes apart if it gets wet.

The problems with this cell are that it is difficult for the mirror to cool down, because air can't circulate around it. In addition, 3 pads don't adequately support such a large thin mirror (the mirror is 1" thick, 13.1" in diameter). Also, I've never quite gotten the hang of push-pull bolts. I prefer a spring-loaded mechanism with only 3 bolts to turn.

The scope saw first light at Jersey StarQuest '99. I had taken the mirror out and cleaned it, so it needed collimation badly. Armed with a borrowed laser collimator, I discovered that the best I could get was a bit off. It was ``good enough'', but the dot was clearly outside the circle marked on the primary. Also, I discovered the focuser was wobbly. When the collimator was touched, the dot would bounce around wildly. I used the scope as it was, and decided the first thing on the agenda was to get a new mirror cell for it.

My first thought was to make a cell, but I found no references on-line about making anything more than a 3 point cell (here are another and another). The only written reference to making a more-than-3-point-cell I found was The Dobsonian Telescope by Kriege and Berry. In the course of looking, I found that University Optics sold a 13.1" cell that had ``9 support pads''. I asked around, and discovered that what that means is that the cell has 9 rubber pads that support the mirror, but it is still a 3 point cell. Meanwhile, Richard Schwartz shamed me into building my own. I downloaded a copy of Plop and away I went.

Plop showed me that a 6 point cell was adequate, which was good, because fewer points is easier. In particular, I only had to cut lengths of bar to make ``teeter totters'' instead of finding a source for aluminum sheet and cutting triangles. I'm going to have to do that eventually, as I get on to bigger scopes, so if anyone out there has a good, inexpensive source for 1/4" or thicker aluminum sheet, please drop me a line.

Rather than make a ``tailgate'', I decided to make a ``T'' shaped cell, just like I did on my 6" scope. One of the nice parts about the ``T'' is that the collimation bolts can be spaced much farther out than the mirror supports, so each turn of the screw tilts the mirror less, and fine tuning the collimation is easier. I ran down the Home Depot and picked up some 1" square aluminum tube, some 1/4" x 1" aluminum bar, and some 2" x 1/8" aluminum angle. Next, I stopped at the Sears Hardware store and picked up all the nuts and bolts (these stores have a great selection of little doodads, perfect for the ATM).

Plop suggested 3 pads 3.92" from the center, and 3 more pads 3.96" from the center and 60 degrees between each pad. I changed the numbers to 6 pads all at 4" (much simpler to measure) and ran a check. Looked good. I like 60 degree angles, because a right triangle with a 60 degree angle has 30-60-90 degrees, and the ratios of the sides are 1-2-sqrt(3). Likewise, 6 points is easy because the sides of a hexagon are the same as the distance from the center to each vertex. Makes all the math easy.

I first figured out where the 3 supports would be, relative to the center. Thanks to the neat numbers, the supports are all 3" apart. I next figured out how long the top and sides of the ``T'' had to be, and I was ready to start ``cutting wood.''

I cut 2 squares of 3/4" birch plywood, and routed them into rings. One is 16" outer diameter, and 11" inner. This replaces the bottom of the mirror cell. The second is 15 1/8" outer diameter, 13 1/2" inner. It merely serves as a guide to center the cell on the back of the tube. I cut 3 sections out of the smaller ring, to allow clearance for the ``T''. I then cut 4 sections out, which were replaced by 2" x 1 1/2" x 3/4" blocks of maple, which are how the cell is attached to the tube. The maples blocks are screwed (with 2" drywall screws) and glued to the cell, and have holes in them to accept 1/4"-20 furniture screws. The furniture screws have a tapered end, and a large flat head. I used the same kind of screws for the collimation bolts. The holes in the maple were drilled slightly tight (drill size ``A''), and the screws were turned in the holes to thread them. After the wooden parts of the cell were done, they were fitted in the tube and a small drill was used from the inside to make holes in the tube to mark where the screws should go. I then drilled slightly oversized holes in the tube from the outside.

Now it was time to ``cut metal.'' The idea of this cell is that the mirror is siliconed to the ``T'', which is all aluminum and stainless steel (well, except for the ``Tee'' nuts, which I couldn't find locally in stainless). When you want to clean the mirror, you don't take it out of its cell. Instead, you remove the collimation bolts, and the metal parts, with mirror attached, all come off together.

I used the already cut edge of the tubing as the top end of the base of the ``T'', as I figured it's closer to being square than I can do with a hacksaw and file. After cutting the pieces of tubing to length with a hacksaw, I cleaned the cut edges up with a file. I next cut two 1" lengths of angle, to reinforce the joints where the base and top of the ``T'' meet. I clamped the pieces together, 2 at a time, and drilled holes all the way through with a #19 drill bit, to accept #8-32 stainless steel screws with lock nuts on them (in some of the photos, regular nuts replaced the lock nuts. That's because I didn't want to loosen things by repeatedly putting the lock nuts on and off. In the final assembly, the regular nuts were replaced by lock nuts).

Next, I cut out the 1/4" x 1" x 5" aluminum bars for the teeter totters. The supports are 4 inches apart, but since the silicone pads are 1" in size, there's an extra 1/2" on each end to make room for them. The cuts were cleaned up with a file, and a hole drilled in the center of each one (#19 drill again). The holes had to be countersunk a little to make the screw heads a little lower, so the 1/8" silicone pads would let the mirror clear the screw heads. Next time, I'll use flat head screws, but I didn't think about it until late in the project. The Kriege book said to use pan head screws, but I saw an Astrosystems mirror cell that used flat head screws.

Next, I went back to the ``T'' and measured and drilled holes for the 3 supports. I also drilled holes for the 3 collimation bolts to come up through the bottom of the plywood circle and into the tubing, and for the ``Tee'' nuts to fit down through the top of the tubing. This turned out to be a mistake, as the range of travel with the ``Tee'' nut there was restricted. I had to re-mount the ``Tee'' nuts on the bottom of the tubing facing the other way to make things work.

I had picked up 2 different diameter springs at Sears. As it turned out, neither one was quite as stiff as I wanted, so I used both springs on each bolt (fortunately they nested). Because 1 kind of spring was 3/8" longer than the other, I cut out rings of 1/4" plywood to act as spacers. All the spring/plywood places have washers between the wood and spring, to keep the spring from compressing the wood, which might throw off the collimation.

The third problem was how to keep the ``Tee'' nuts from rotating when I turned the collimation bolts. I was going to just epoxy them to the aluminum tubing, but at the last minute I got fancy, and drill 3 small holes through which I passed aluminum nails, which are epoxied in. No particular reason for doing it this way, it just seemed like an interesting thing to do.

Once I worked out the issues of placement of the ``Tee'' nuts and got the spring tension right, the whole thing went together without any trouble. I took the metal parts only and laid them on a table. I balanced 6 pieces of #10 threaded rod on the ``teeter totters'' to act as spacers (anything about 1/8" thick will do, but the rod was handy). I practiced putting the mirror on several times so I knew I could place it accurately without moving the supports. I cleaned the back of the mirror and front of the supports with acetone. I then squeezed out blobs of silicone (I used Silicone Plus because it was handy - I was told Silicone II is the best stuff to use) and sat the mirror down on the blobs. They weren't nearly as big as I wanted. I was afraid to put on too much and have it ooze all over, so I guess I put on too little. I let it cure for 36 hours, then went back, removed the spacers, and ``caulked'' the space between the mirror and supports. I think I'll do it this way on purpose next time; it worked great.

I added 3 nice looking wooden drawer pulls to the back of the cell, to act as feet. I thought about using the rubber ones from the old cell, but decided I liked the look of the wood, even if it gets scratched up right away. The wooden parts were all assembled with screws and glue. I took the screws out of the thin inner ring after the glue dried, as nobody sees this part, and it's not structural, and the darn scope weighs enough already without adding extra screws to it. Then the entire ``wooden part'' was covered with 4 coats of satin spar urethane.

Some final assembly, and the scope can now be easily collimated. Of course, now that the back end is lighter, the scope's front heavy, so I have to move the bearings. Hmmmm. I smell another project coming on.