Retrofitting a 13" Dobsonian By Dave Aucoin

It was once said "If you build it, they will come". My slant on this saying is, if I build it, they will come…to the starparties.

I had the opportunity to purchase an old Coulter 13.1" telescope, from a friend who obtained the scope from another friend who in turn purchased the scope from me many years before. So finally it was coming back to me. I had retrofitted the scope, not the older box and tube, but the newer tube design, to break apart to fit in my then Chevy Chevette. It could detach about 8 inches from the focuser and be re-attached with 4 snap latches. Furthermore, the mirror could be removed in its cell into a hatbox type configuration. All this was nice, but I grew tired of lugging it out, after about 3 years of hardcore observing. I sold the scope and soon realized that I missed it. I had another opportunity to purchase a 13.1", so I grabbed it. This was the real old tube and box design, so to be able to fit it into my car I converted it into a surrier truss design. I hardly ever dismantled it and was able to fit it in my car anyways.

So now having the opportunity to get the scope again, I bought it from my friend. Now I had to figure out if I could fit it in my car, now having a Hyundai Scoupe and having little cargo space. Not being a hatchback, I had few options on getting it into the car, with my other equipment and still have my girlfriend accompany me observing and to go to starparties. So I had these options in mind:

· Portable enough to fit in my car.

· Set-up and break down in 5 – 7 minutes.

· Light enough to carry by one person.

· Telescope lines to be smooth and simple.

With these ideas in mind, I scoured the net for ideas to fuel my imagination. I came upon one site, of an amateur who lives on the west coast. He was building an 18" f/4.55. He explained about how he made it, but didn’t include many dimensions or drawings, just a few pictures and small text. I liked the design, so I printed out the web page and set to work. After looking at the pictures and gleaning what dimensions he did mention, I made a set of drawings based on what I knew. His design, although original, was by no means "his". I thought that the best way to flatter him was by imitation.

I sent out an e-mail asking for some answers to how he built the scope, but I didn’t get any replies. So, not being daunted, I set to work. These next few pages will outline how I did it. I had at my disposal was a table saw, band saw, planer, scroll saw, drill, 2 sanders, misc tools, and the use of a drill press and belt sander at the local astronomy clubhouse.

The Layout

I purchased a sheet of ¾" Baltic birch furniture grade plywood. Technically it is called .7 inch thickness. I obtained it at a local Home Depot (plug). Getting it home was another problem. It was a 4 X 8 sheet and my girlfriend’s car could not take the uncut sheet, so we had it cut to 2 equal 4 X 4 sheets. It still didn’t fit, so I shaved off 10" from both pieces. When I made the drawings, I had a specific layout in mind. I thought I had just blown the project by not having the right sizes to make the pieces I needed. Well, after getting the wood home, I started to measure out the pieces I needed by taking the dimensions I knew and plotted them out on the plywood. I managed to get to sides of the mirror box and front and rear mirror box pieces penciled in. Now the design is a radical one. The mirror box and the altitude bearings are one piece, as the picture will show.

Mirror Box

The bearing/mirror box is an 18" radius, as was the mentioned in the web page that the radius he had was 18". So I drew the design out, using a 36" circle and cut it out using a friend’s router, getting 2 identical pieces. I cut it out with a jigsaw and I sanded the edges and the surface. Next I used my girlfriends fathers table saw to cut out the front and rear panels. Then I cut out the first of 2 designs for a spreader bar. Just in case someone bumped into the extended bearings that protrude beyond the mirror box, I wanted to insert the spreader bar to prevent that and reduce flexure. I had also routed out the circle design for the rocker box and ground board, which I will talk about shortly.

The mirror box had an inside dimension of 16.5" square. The height of the inside is 11.25". The outside dimension was 18" square. I wanted to be able to replace the 13" mirror with a 15" mirror at a later date, so I kept the inside dimension. I had an opportunity to place the mirror bottom board in another configuration to be able to put the truss tubes on the inside, but I chose to leave it as it was and resort to placing the truss tubes on the outside, since the inside dimension would change, therefore eliminating the future placement of the larger mirror. When I started to assemble the mirror box, I wanted it to be of a solid design and rigid structure. I wanted to glue and screw the pieces together. I didn’t want to use all screws, so I opted for dowels glued into the pre-drilled holes I made. I contacted another web site amateur and he told me of a way to attach the pieces before I actually started to glue to maintain squareness. By nailing in four nails per side, I could set the glue and screws and then glue the dowels into place and still maintain a square edge. The holes the nails used were converted into the 4 holes I wanted to use screws for. I used drywall screws, about 1 ½" long. They were also in a unique tan color that would blend into the surrounding wood, but I countersunk the screws and placed short dowels into the holes to cover them up.

After the 2 sides were attached to the front and rear panels and the mirror box bottom, I glued 2 screws and one dowel in to the rear and front panels for added strength. I went a tad over board when I placed the dowels into the mirror box sides. Each side has about 35 dowels and each dowel is 1 ½" long. But the box is strong enough to handle any flexure and linear torque. So I set this aside and worked on the rocker box next. I sanded the mirror box to get the dowels down to the surface. I cleaned and then tack clothed the surface and proceeded to stain with Minwax Natural #209 stain to maintain the beauty of the wood. After one coat of stain, I prepared for the Helmsman Spar polyurethane in a semi-gloss texture. I coated it 3 times, sanding after the second coat. On the bottom side of the mirror box, I placed a fan assembly. It has an exterior power supply and a toggle switch. The batteries can be replaced at any time by undoing the cover covering the batteries. This picture shows the fan on the back of the mirror box, without the battery pack. The fan parts cost about $22. There are 4 major holes underneath the fan, 1 1/2 " in diameter. There are also 21 - 5/16" holes surrounding the fan for ventilation. The inside of the mirror box is coated with gray primer, then a coat of Stone Creations black spray paint. It has a very rough texture. I wanted a non-light scattering surface and this seemed like a great way to do it. Then I sprayed flat black over this in 2 coats. It looks real good.

This picture also shows the knobs that adjust the mirror cell. The cell I purchased from University Optics. For a 13.1" primary, it cost $50. The wing nuts that came with it were out of character with what I wanted, so I got some white knobs from Home Depot, drilled the center out and threaded them by placing them on the bolts and rough cutting them into the thread. The light baffle is made from ¼" oak luan. It has a 14" circle cut out and I can put in or take out the mirror cell without unscrewing the screws that hold it in place. It is stained and polyurethaned like all the rest, and flat black spray-painted on the underside. The mirror cover is 16" in diameter and will fit over the baffle and attach with Velcro. It is stained on top and has flat black on the underside. On the edges of the altitude bearings, I glued strips of Ebony Star formica with contact cement. At the ends of the strips, I drilled and screwed small screws with grommets to both keep the strips from peeling off and to stop the bearings from running off the Teflon.

Rocker Box

This part of the scope cannot be called a rocker box because it is not a box. I coined the term "Zero Height Rocker Box (ZHRB)". It has practically no height to it whatsoever. The picture will illustrate what it looks like.

It is essentially a ring, glued to a disk. Both diameters are 16". I did this to maintain strength but reduce weight. It was cut out with a router and the top ring and bottom ring share the square sides on which you can discern the Teflon pads which are attached to these square "pads" Now, after I had cut out and glued and screwed the 2 rings together and doweled, yes DOWELED them together, I clamped them together and let them dry overnight. I took the ZHRB (see above), and cut 45 degree angled slots into the squared pads. When the Teflon in placed onto these pads, the altitude bearings will slide over these. I placed the completed mirror box over these pads and POOF they didn’t work. The front of the mirror box would not rock over the ZHRB because the mirror box was digging into the ZHRB. I was annoyed to say the least. Then my girlfriend mentioned that I should build up the pads with blocks so that the mirror box would slide properly over the Teflon and not catch on the ZHRB. I cut out, from spare plywood, 1" high by 2 " long by 1 " thick pads, screwed the Teflon onto the pads using the drywall screws and glued them in place. After I did this, the bearings actually worked, but the previous cuts on the pads were canted either in or out. I again was annoyed, but after getting some rasps and files, the next evening I set out to square these off. When I got the results I wanted, I settled for an almost, within .010 of an inch, squared pads on which virgin Teflon sat waiting to bear the weight of the mirror box.

Ground Board

The original design was a spoked rocker box on a tri-spoked ground board. I chose a solid ground board with three wooden rectangle legs screwed, glued and doweled onto the underside of the ground board, from the center out to the edge.. Each leg has a ¼ " hole drilled into it with a corresponding 1" hole on the topside of the ground board. The 1" hole was for a t-nut to be placed into the hole for which a leveling screw was to be placed. I epoxied the t-nut into the hole and capped the hole with a 1" hole cap found at Home Depot. In the ZHRB I drilled a 5/8" hole for the pivot bolt that attaches to the ground board. I got the pivot bolt and Teflon pads from AstroSystems, Inc (AS) in CO (plug). The Teflon with the pivot bolt was pre-drilled to accept the bolt to pass thru it. The Teflon pads were pre-drilled and counter sunk to accept any type of wood screw. I placed the Teflon pads on the ground board in 120 degree points about 80% out from the center to insure good contact with the bottom side of the ZHRB onto which I glued with contact cement Ebony Star Formica obtained from AS. You can, however, get Ebony Star at Home Depot: a 2-foot X 4 foot sheet for about $10. Very cheap compared to what most telescope making supply stores want. After all was said and done, I spray painted the ZHRB and the ground board flat black and over coated then them with Helmsman Spar polyurethane from Home Depot. 2 coats, sanding, another coat of paint and then final coat of Spar was all it took.

Upper Cage Assembly

The upper cage assembly was purchased from AstroSystems. I got a great deal from Alan and Randy out there. I have called many times since getting the parts and was rewarded with the offer to come and work for them. It is tentative at this point, but I plan on moving to Co within the next 8 months, so I already have a job ready and waiting. The upper rings, bars and focuser board cost me $40. I was given the upper clamp blocks for another $40, saving $30 from the original price of $70 ( I was told they were the last ones so they gave me a deal), along with the Teflon pads for the ZHRB and ground board, 2 sided tape for the Kydex light shroud and the Truss tubes (6061-T1 aluminum at .049 thickness, 1" outside dia.), Phase 4 focuser that cost me $150, spider and diagonal holder and a collimating tool. The spider and diagonal holder will accommodate a 2.6" ma secondary mirror. The original Coulter secondary was a 3.1", too big of an obstruction, about 26 –28%. The smaller size will reduce that to about 20%.

I assembled the cage in one evening. The focuser was a little tricky, trying to get it as close to the inside of the ring yet leave enough space so the Kydex didn’t touch it. I had to cut the support blocks for the focuser in a semi-circle, using the inside ring as a template to get the right radius. I then attached the upper clamp blocks, setting them so that the center of the tubes, when placed in the clamps, would be just outside the center of the middle of the lower ring. The thick aluminum that came with the clamp blocks was unworkable, so I cut out from thin aluminum sheet using tin snips, the pads in which will take the brunt of the ends of the tubes when placed in the clamps and will not destroy the wood ring underneath. See the picture below.

After the construction, I stained the wood with 2 coats, as opposed to the mirror box with just one. The upper cage was made from appleply plywood. It has about 9 – 13 layers and is very light. The Baltic Birch is 5 layers and quite dense and slightly more heavy than the appleply. I applied 2 coats of polyurethane, sanded after the second and sprayed a third coat. After the overcoat was dry, I applied contact cement to the inside of upper ring and lower ring and the cage supports. When I set the 2-sided tape to the rings, the contact cement adhered to the tape and made a tighter seal. The Kydex was placed on the inside of the rings, adhering to the tape creating a light shroud. The Kydex is glossy on the outside and rough and flat on the inside. It can be obtained in 4 X 8 sheets for around $32 from any plastics company, most notably Cadillac Plastics in Woburn, MA.

Lower Clamp Blocks

This part was tricky. I didn’t have any plans or drawings for these parts. There were no affordable alternatives, as most dealers didn’t sell these separately. I asked AstroSystems for help and they said to drill holes in the tubes and then attach them to the outside of the mirror box. I didn’t want to drill holes in the tubes, let alone in the mirror box, but I conceded and did so. Then I had an idea to make compression blocks. See the attached drawing below.

As the drawing shows, it is a simple compression block using ¼ - 20 socket head screws, with plastic knurled knobs attached to the screws with epoxy. The block is 3" wide, 2.5" high and 1.5" thick. It is made from gluing 2 pieces of plywood together. I drilled a 1" diameter hole using the drill press for accuracy down thru the block. Then off to one side I drilled a ¼ - 20 hole in which the knob would tighten the block and produce the necessary tension to hold the tubes in place. I had to make 8 of them, but I made a few extras just in case. I am glad I did. On the other side of the ¼ - 20 hole is a 1" counter sunk hole in which I placed a t-nut and epoxied it to the block. The slot in the block was made using the table saw set at the height at which it intersected the 1" hole for the tube. Is that clear? The slot is about 1/8" to 3/16" wide, based on the width of the saw blade. I attached the block to the mirror box in 8 places. I used 2" wood screws for one spot and 1 ½" in another; the side that is closer to the tube uses the longer screw, but where the slot is cut in the block, I used the shorter screw which does not attach to the block until it hits the lower part of the slotted block. Whew! Rather than the 4 holes in the drawing, I opted for just 2, just in case I had to re-position the block, based on the angle that the tube was set at to meet up with the upper clamp block.

Now the lower block, attached to the mirror box has 2 angles that have to be used for the tube to reach the upper clamp block without too much bending. One angle is the tube canted inwards towards the wall of the mirror box heading up towards a point at the upper cage block. Imagine a straight line drawn from the point at which the mirror box ends and then intercepts the upper cage. Now imagine the tube running from the lower clamp block up to the upper clamp block. The next angle is the angle at which the tube is canted towards the center of the mirror box, but ends at an almost point where the 2 tubes on each side of the mirror box meet to align with the upper clamp blocks. This compound angle is difficult to set until you are ready to attach the truss tubes and align them with the upper cage assembly. Then you have to determine the focus of your eyepieces and probably have to re-align the blocks again to set the proper distance from the primary mirror and the focuser. As with the mirror box, I sanded them, coated once with the stain, polyurethaned 2 times, sanded after the 2^nd coat and applied a 3^rd coat after that. This next picture shows the blocks in place on one side of the mirror box.

Truss Tubes

The truss tubes I purchased from AstroSystems. They are 8 tubes, 48" long at which point I cut them down to 43". I calculated that I needed about 39" to 41" of tube length for the ideal focus. The tubes arrived very dull finished. I used Scotch Brite to lighten up the tubes and give them a high gleaming shine. Inserting them into the compression blocks is a snap. And they easily detach from the upper clamp blocks. I plan on encasing them in seamless tube foam, leaving about 3" of bare tube showing at the top and bottom for that Obsession look!

Breakdown

After the assembly of the scope, it takes about 7 minutes to fully break apart the main sections of the scope. Upper cage, truss tubes and mirror box/bearings plus the ZHRB and ground board. These next 2 pictures show the fully assembled scope and then a breakdown of the major parts.

Believe it or not, this package fits snuggly into the back cargo space of my car. It weighs about 60 lbs. totally assembled, and the mirror box is about 35 lbs.

Hardware:

All screws, bolts, nuts, knobs, paint, stain, polyurethane, clamps, glue and contact cement came from Home Depot, Waltham and Norwood, MA.

Upper cage assembly, Truss tubes, Ebony Star and Teflon provided by AstroSystems, Inc, LaSalle, CO.

Kydex from Cadillac Plastics, Woburn, MA.

Fan assembly from U-Do-It-Electronics, Needham, MA.

Patience, ideas, frayed nerves, creativity and imagination provided by me.

Final Thoughts

Well, I certainly learned a lot. I was very anxious to get this built, so I might have rushed things along a bit faster than originally anticipated. Had I another scope to use while I was building this, I would have taken more time during construction. I plan on making a 10" version of this design, just for the hell of it. I will certainly upgrade to a 15" mirror within 6 months.

My girlfriend is a seamstress as a hobby. She will make for me booties for the altitude bearing protrusions so they don’t chip and smash into my rear window. She will also make me a light shroud and an upper cage bag to store the cage when not using it. I will try to have her make me a cover to drape over the mirror box during down time, one that fits the outline of the mirror box.

If you have any questions for me, feel free to e-mail me at Graywwwolf@mediaone.net If I don’t get to your e-mail as quickly as you need an answer, bear with me, there maybe others before you, or I may be busy myself with my other hobbies and my job. At some point in time, I may have a package put together with drawings, dimensions, a few photos and supplier info for a small price to cover expenses of putting it together.

Bottom Line

The final cost of this project was about $800 + another $600 for the scope. The scope was a bargain, because if I bought the mirror separately, it would have cost about the same as the complete scope. I still have to buy some eyepieces so I figure another $500. I plan on using just 2 eyepieces, a 9mm Nagler, a 32 mm UO 2" Konig, and a Televue 2X Barlow. I will get great eye relief and get 2X magnification and get 4 focal lengths for the price of 2. All in all the cost was worth it.

When the 15" is ready, I will have to buy 8 new tubes, the primary, a new secondary and a new diagonal holder. I may have to reset the lower clamp blocks, because the angles might be different.

I hope you enjoyed this article. I spent most of a Friday night/Saturday morning writing it. I am now falling asleeeeeeeep…………….