Homemade 12.5 Inch Dobsonian Telescope

For complete instructions on building a telescope such as this one, I strongly recommend buying a copy of The Dobsonian Telescope by David Kriege and Richard Berry. Their step by step instructions were invaluable to me as I built this scope.

Step One—The Tube

I had cut the 12 foot tube in half in order to get it home in the first place, and a 6 foot section was the right size to work with for my f/5 12.5 inch. The focal length of the mirror is 61 inches. (Technically, it should have been 62.5 for a 12.5 inch f/5, but when the mirror came it had a note on the back that the focal length was 61 inches.) I installed the spider first, about one inch down from the end of the square end of the tube. I then temporarily installed the secondary mirror so that I could measure the approximate distance to the center of the secondary in order to locate the holes for the focuser. The secondary holder is adjustable, so I could didn’t have to get the location perfect. I centered the focuser 8 inches from the top of the tube, using a 2.5 inch hole drilling saw with smaller holes for the bolts that would hold the focuser. I did run into a small problem with the focuser. Two of the holes for the bolts were blocked by knobs. I had to take apart the focuser in order to get my bolts in place.

To place the primary mirror, I installed the secondary and installed the primary in the mirror cell. I took the whole tube assembly outside. I marked the expected position of the secondary by measuring 61 inches down from the end of the tube. (The 8 inches from the center of the secondary to the field stop in the focuser is the same 8 inches from the center of the focuser to the top of the tube.) My wife helped me by moving the mirror into and out of the tube around the marked spot while I stood at the eyepiece trying to bring some distant trees into focus. We tried this in our back yard, but found that it was too difficult to aim both the tube and the moving mirror at distant trees in our suburban yard. We moved the operation to a nearby park and got a focused image of the most distant trees (300-400 yards away) quite easily. I marked the spot and moved things back inside to install the mirror.

Fast forward a few months to the next time I had the tube outside. I had nearly completed the base and put things together for the first time. Before finishing the tube with a wrap of veneer, I wanted to make sure that the primary was in the correct position. I set things up in daylight and collimated the scope. I pointed it at distant trees and had no problem bringing the image into focus with all of my eyepieces. I waited until dark and pointed the scope at the sky. I first tried my 32mm plossl eyepiece. Just as it was about to come into focus, the focuser bottomed out. I tried all of my other eyepieces and found that I could bring the image into focus in the other eyepieces, but it was very near the bottom of the focuser travel. #@$%^#@!!!!! I had installed the mirror about an inch too far away from the focuser. I was annoyed to have made the mistake, but very glad that I checked the position on stars before wrapping the tube in veneer. I redrilled the holes for the mirror cell and got ready to wrap it.

Before I wrapped the tube (and before I discovered the problem with the primary placement) I determined the balance point on the tube. The balance point determines the height of the rocker sides, so I had to get this far before I cut the wood for the base. I placed my telrad, optical finder, and an eyepiece on the focuser end of the tube and, using a broomstick as a balance point, found that the tube balanced 22 inches from the bottom. I added three inches for a margin of error and in case I need to rebalance the tube in the future, making a distance of 25 inches from the balance point of the tube to the rocker bottom.

I did not wrap the tube until I had completed the base. I wanted to wrap the tube in veneer because I had seen a homemade telescope on a website that used veneer and I liked the look. I also wanted to strengthen and protect the cardboard tube. My plan for wrapping the tube seemed good, but I am not too pleased with the final result. I drew a guide line straight down the side of the tube and cut the veneer to length with one extra inch on each side—68 inches long. The 48 inch width of the veneer was close enough to the diameter of the tube that I used it as is. I applied contact cement to the first inch of the veneer and the mating inch of the tube. After waiting for the contact cement to dry to the touch, and with my wife’s help, I carefully aligned the edge of the veneer with the guideline on the tube and pressed the first inch together. We then spread contact cement over the remaining veneer and tube surface, waited for the glue to dry, and carefully rolled the tube over the veneer. We were careful to press out any bubbles as we went, but when the job was done we found that there were multiple bubbles in the veneer. Contact cement is permanent, so the only solution would be to replace the tube! I decided to keep this tube for now and if I run into problems with it, I can replace it later.

Step 2—The Tube Box

I had designed the tube box so that there would be an extra ¼ inch around the tube. This extra space would allow the tube to be repositioned within the tube box to rotate the eyepiece or balance the tube. With an outer diameter of 14 3/16, I cut the Tube Box sides 15 5/8 and 14 5/8—slightly less than my extra ¼ inch extra on each side. Once the pieces were cut and I held them around the tube, I decided to cut them a bit smaller because the extra space was too much. The pieces were cut down to 15 3/8 and 14 3/8, allowing just under 1/8 inch on each side. The sides were attached with wood glue and clamps with a few small brads to hold everything in place. After the glue dried but before the clamps were removed, I added corner braces to all four corners of the tube using glue and brads to hold them in place.

Step 3—The Side Bearings and Rocker Sides

The side bearings are each made from 2 identical semicircles of 5/8 inch plywood glued together to make a thickness of 1 ¼ inches. To cut the circles, I used a router with a straight cutting bit and a circle cutting jig. This allowed me to cut perfect identical circles. It was important that the two circles be identical because the edges had to match perfectly when glued together and the two side bearings had to be identical in order for the altitude motion to work well. To cut the circles, I started with a depth of about 1/8 inch, then lowered the bit and cut a little more with each pass until I was through the board. I was certain not to move the jig at all before cutting the second circle.

After cutting the circles for the side bearings, I set up the jig to cut the curved edges of the rocker sides. The arc at the top of the sides needed to have a radius 1/8 inch larger than that of the side bearings to account for the thickness of the teflon pads that would fit between. To get this arc, I actually moved the radius of the jig in by 1/8 inch. This is because my router bit was ¼ inch and the arc would be cut by the outside edge of the bit (where the circle had been cut by the inside edge.) On the cutting diagram, you can see that the side bearings were cut 26 inches long. This was to allow the circle cutting jig to be attached to the board to cut the arcs from a center point of 25 inches above the ground. I used one of these scrap pieces to draw an angle of 70 degrees to the edge of the arc so that I could later place the teflon pads on the arcs.

I carefully cut exactly through the center of each side bearing circle to make my semicircles. To glue the side bearings and rocker sides to full thickness, I covered a side with wood glue, then carefully lined up the pieces and put two small nails into the pieces to hold them in place. Several clamps were used to hold the pieces together overnight. After the pieces were glued together, I used a belt sander to smooth out any rough edges, being careful to keep both rocker sides and both side bearings exactly the same. Finally, I glued a strip of Ebony Star laminante to the curves edge of each side bearing using contact cement, using a flush edge bit on a router to bring the edge of the laminate even with the edge of the wood after the cement had set.

Step 4—Attaching the Side Bearings and assembling the Rocker

Drawing lines between opposite corners of the tube box sides allowed me to locate the center point on each side. Lining up this center point with the center of the top of the side bearing, I brought the corner of the side bearing up to the edge of the tube box and attached it with screws. I did not glue the bearings to the tube box in case I needed to adjust the position later.

After a dry fit of the rocker to make sure that the tube box with side bearings would fit well, I glued the rocker sides to the rocker front, holding them in place with small nails and clamping overnight. The next day, I centered the rocker on the rocker bottom (which had been cut using the same method as the side bearing circles), drawing perpendicular lines through the center of the rocker bottom to use as a guide, and tracing the position on the rocker bottom. Before attaching the sides and front to the bottom with wood screws, I wanted to make sure those screws would not interfere with the Ebony Star ring that would be attached to the rocker bottom. Using a compass, I drew lines for the inside and outside of the Ebony Star ring on the top and bottom of the rocker bottom. I drilled holes for the screws that avoided the location of the ring, and attached the rocker bottom. I then attached the Ebony Star ring using contact cement. Finally, I attached Keepers to the sides of the rocker at the bottom of the arcs using screws and glue.

Step 5—The Pivot Bolt

The pivot bolt consists of a 1 1/8 inch brass spacer with an 11/16 inch outer diameter, a 3/8 inch bolt 2 inches long, a stop nut, and two large washers. Ideally, the spacer would be 1 ¼ inch long and have a smaller diameter, but I didn’t get this one right the first time. I started with a spacer that with a ½ inch outer diameter. I drilled the holes in the center of the ground board and rocker bottom using a 9/16 bit, thinking that I needed the extra 1/16 so that the spacer could move. It turns out the the extra 1/16 allowed far too much side to side movement between the rocker and ground board. I could not find a spacer to fit my 9/16 hole, so I bought the 11/16 spacer (1 1/8 long because that’s what the store had) and a bit of the same size. The hole did need to be enlarged slightly by moving the bit within the hole , but in the end, the fit seems right.

Step 6—Stain and Finish

At this point, the scope was pretty much finished. I attached keepers 1 ½ inches wide by 5 inches long, extending above the bottom of the arcs by about 1 ½ inches. Then it was time to stain and finish. I had considered using a clear finish alone, but decided to use a dark red finish on the base and a clear finish on the tube, just because I like the look. I sanded all parts of the telescope with 220 grit sandpaper followed by 400 grit, making a very smooth surface to apply the stain. Just to make sure I liked the stain, I applied some to a piece of scrap wood. I’m’ glad I did, because I didn’t like the first color that I had chosen. I got a different stain, tested it, and went back to work. After going over the surfaces with a tack cloth to pick up the saw dust, I applied the stain and went over it with cheese cloth rags for an even finish. After waiting for the stain to dry for a day, I applied polyurethane in two coats, going over the surface with 0000 steel wool between coats.

Step 7—Feet and Teflon Pads and Keepers

Using a compass I drew circles on the top and bottom of the ground board to mark the inside and outside of the teflon ring on the bottom of the rocker bottom. I attached three feet equally spaced on top of the ring, and on the opposite side attached teflon pads 1 X 1 ½ directly above the feet using very small brads and countersinking the brads below the surface of the teflon. I used one of the top scraps from the rocker sides to mark an angle of 70 degrees from the center of the arc to the curved cut in order to properly space 1 X 7/8 teflon pads on the arcs. I attached them with very small brads, countersinking the brads. I attached small felt pads to the inside of the keepers to protect the side bearings from scratches, but discovered that the side bearings would not fit because of the thickness of the pads. Using heavy grit sandpaper I brought back the surface of the keeper enough for the side bearings to fit inside the felt pads.

Step 8—Fitting the Tube in the Tube Box

To hold the tube in place within the tube box, I used a combination of pieces of felt inside the box and wooden shims between the tube and the box. This combination has proved to be effective for all positions except when the tube is pointed directly up. I have not yet figured out a better way to hold the tube.