Introduction: Homemade 12.5 Inch Dobsonian Telescope

How to build a 12.5 inch closed tube Dobsonian telescope. I began building it in the spring of 2008, but the bulk of the work was done in July of 2008 with first light occurring on July 25. This Instructable describe the planning, design, and parts of the scope, as well as the process of building The scope. This is the first telescope that I have built

I found out pretty quickly that building my own telescope would only be a bargain if I made my own mirror and mechanical parts. This might have been tempting if I wanted to build a 6 inch scope—at that size, they say that first-timers do pretty well at grinding and figuring their own optics. But I wanted a larger mirror, and, at that size, grinding my own was not an option. I also wanted to buy the other mechanical parts—mirror cell, spider, secondary holder, and focuser—so that the performance of these pieces was not limited by my skills. When I looked at the costs, I seriously considered buying an Orion telescope instead of building my own. It would cost less and they have a very good reputation. But by this time I had thought about building my own scope for several weeks, and getting one off the shelf seemed pretty boring by comparison!

Once I decided to build a scope myself I had to decide on the aperture of the scope. 10 inch or 12 inch. For a while I decided that 10 inch would be the best size. Many sources point out that 10 inches is a kind of sweet spot in the balance between power and portability. A 10 inch scope would not be very much longer or heavier than my 6 inch, yet it would allow me to see much more. Yet I was also becoming infected with aperture fever. I had the thought that as long as I was going to build it I should build it as big as I could afford. The wood would cost the same, as would pretty much everything except the mirror, so why not spend a little more on the mirror and go BIG.

I was finishing this inner debate when I got a copy of the book “The Dobsonian Telescope” by Kriege and Berry as a Christmas gift. This book describes in great detail the steps to build a large aperture truss tube dob, focusing on scopes with a 12.5 to 40 inch mirror. It had been described by many as the most important recent volume for anyone planning to build a dob, and I have to agree. As I read the book I decided that I should build a 12.5 inch truss tube scope. As the authors describe the benefits of a truss design, it is hard to dispute their arguments. I even began taking careful notes about needed supplies and plans for each piece of the truss tube scope. As I continued to read, however, I was struck by the many complications of the design and, frankly, how many ways I could screw it up. The last chapter of the book describes a plan for an 8 inch sonotube dob with a few design elements from their truss design, and, as I read that chapter, the simplicity of its construction was very appealing. I decided that my best option was to scale up their 8 inch design for a 12.5 inch scope and sacrifice the portability and easier storage of a truss scope for something that I felt confident in building myself.

Step 1: The Parts and Budget

Price List

Primary Mirror (12.5” f/5): $850

Secondary (2.6”): $150

Spider/Secondary Holder: $60

Sonotube (14 inch diameter, 12 feet long): $70

Mirror Cell: $66

Focuser (GSO 2 speed low profile Crayford): $139

Ebony star strips: 2 @ $10

Ebony star ring: $59

Teflon strip: $11

Telrad base: $10

Baltic Birch ½” sheet (60X60): $42

Baltic Birch 5/8” sheet (60X60): $48

Circle cutting jig: $30

Birch Veneer (4X8 piece): $65

Hardware, stain, other supplies: $80

Total cost for supplies for this project: $1700

Once I had my design set, I was ready to buy the parts. Since my plan called for purchasing the mirrors and mechanical parts there were several decisions to be made and several vendors to contact. Actually getting the parts into my hands turned out to be more of a challenge than I would have expected, but in the end, I have a wonderful telescope that I enjoy very much.

Parts List (the full story of acquiring the parts)

The primary, secondary, spider and secondary holder—Discovery Optics

After researching several possible sources for a primary mirror, I decided to order an f/5 12.5 inch mirror from Discovery Optics. The company has a reputation for excellent optics, and it was neither the most expensive, nor the least. I phoned in the order in the last week of February and was told that it would take about 4 weeks to deliver the mirror. At their suggestion, I also ordered a 2.6 inch secondary mirror. I paid the 50% deposit and dreamed of DSOs. The next day, I called back to ask their opinion about the kind of spider and secondary holder to order. Although they don't usually sell these parts, because I had ordered the mirrors from them they offered to sell me the spider and secondary holder that they use in their scopes. They even offered to send the spider, secondary holder, and secondary mirror immediately so that I could begin working on the tube while I waited for the primary.

It took a lot longer than I expected to receive my mirror and other parts from Discovery. A week after they said they would send the secondary and other parts, I called to ask if things had been sent. I got no response to phone calls or emails after another week, and decided to wait until the primary would be finished in another two weeks. 5 weeks after I had ordered the mirror, I called to ask about its status. After the second call, I received a response that it would be ready at the end of the following week. I told them to take their time and get it right. After another 3 weeks, I was concerned that I had not received the mirror. My concern turned to alarm when my wife told me that Discovery had charged the balance due on all items to my account a few weeks earlier. I called to express my concern and was told that it had all been packaged a few weeks ago and that it should have been sent. It turned out that they had an incorrect zip code and the package was still waiting to be sent. It took yet another call a week later for the items to actually be mailed. When they arrived, they were beautifully and carefully packaged with the secondary already installed in its holder. It was May 12th.

The Sonotube

From everything I had read, I expected that getting a sonotube would be as easy as running to any hardware store and picking it up. It turns out that sonotubes are a stock item in 8 inch, 10 inch, 12 inch, and sometimes even 16 inch diameters. For my project I needed a 14 inch diameter, and I had a tough time finding one. Home centers and hardware stores did not carry large sizes. Two different construction supply companies told me that sonotubes were not available with a 14 inch diameter, even by special order. Concluding that I would not be able to buy a tube off the shelf, I found a couple of companies that took special orders on cardboard tubes. Two such companies would not give me a price because the order of a single tube was too small. The one company that did provide a price quoted over $500—more than I was willing to pay for cardboard! Frustrated by these difficulties, I stopped by one more hardware store to ask for advice. They suggested a construction supply place on the other side of the city that I had not called yet. I gave them a call and they said they had one on the shelf—14 inches in diameter and 12 feet long! I told them I was on my way. I brought a saw with me so that I cut it in half to fit in my minivan.

The mirror cell: University Optics (eventually...)

One of the companies I had considered as a supplier for the primary mirror was Anttler's Optical. They have many items for ATMers and I had frequently visited their website when planning my scope. I had talked with their owner about mirrors and mirror cells once, and I was very impressed by his knowledge of telescope building. I decided to order a mirror cell from them because the owner had talked about a new cell he was excited about. When I ordered the cell, I paid through paypal, which immediately deducts money from my checking account. I didn't receive any confirmation email after a few days, so I tried to send them an email. The email bounced back to me. I tried to call, but the voice mail had a recording stating that the customer was unable to receive messages. I suspected that the company was either in trouble or no longer in business. After a few weeks of no communication from them, no ability to phone or email them, and no mirror cell arriving. I file a dispute with Paypal to begin the process of getting my money back. It took about 3 weeks but, since Anttler's did not even reply to the Paypal dispute, I received a Paypal refund. The day I received the refund, I ordered a mirror cell from University Optics. I called in the order and explained that I would be using a mirror two inches thick. They said they would make the brackets extra long to accommodate the mirror and it would take about a week. The next day my wife called me at work to tell me a box had arrived with telescope stuff. Realizing that it could not be the University Optics cell already, I thought, “ couldn't be...” but, in fact, it was. The cell from Anttler's, for which I had received a refund, had arrived. I knew that I would be returning the mirror cell to them, but just for fun I decided to see how it would fit in the tube. (I didn't have the primary mirror yet). As it turned out, the mirror cell was too large to fit into my 14 inch sonotube. I laughed, packed up the cell with a long letter describing the series of events that led to the return, and put it in the mail. A week later the cell arrived (on schedule) from University Optics.

The Focuser and other items: (Scope Stuff)

After looking at many focuser options with a big range of prices, I decided to order a GSO 2 speed Crayford focuser from ScopeStuff. I had considered a Moonlite or Featherlite focuser, but I knew from my experience with my 6 inch scope that I wanted a 2 speed focuser, and the price tag of their 2 speed focusers scared me away. I ordered the focuser, received an immediate email confirming the order, and it arrived two days later. ScopeStuff is awesome!! A few months later I ordered some other things from Scopestuff, including strips of Ebony Star laminate for the side bearings, a ring of Ebony Star for the rocker bottom, a strip of teflon to cut pieces for the ground board and side bearings, and a new base for my Telrad finder.

Other Supplies

Other supplies that I bought for this project include wood, various bits of hardware, stain and finishing supplies, and a circle cutting jig for a router. The only thing on the list that was a bit difficult to find was baltic birch plywood, which had to be special ordered from the lumber yard. I decided to use baltic birch because it is recommended by Kriege and Berry as well as many online sources. I also figured that the extra expense was worth it, considering the amount of money I was investing in the project overall. The lumber yard also gave me a lower price than I expected. Rather than use a sheet of plastic Kydex to finish the tube, as recommended in K/B, I ordered a sheet of birch veneer. I was concerned that it would be difficult to find, but I was able to order it from a local woodworking shop.

Step 2: Tools Used for This Project

Table Saw

Router (with circle cutting jig)

Joiner (helpful, but not necessary)

Belt Sander

Finishing Sander

Power Drill

Hand Saw

I did not own all of the tools that I needed for this project, particularly a table saw and router, so I did a lot of work while visiting my Dad. I suppose that one could complete this project without these two tools, but it would be difficult to cut the pieces accurately enough for a good fit.

Step 3: The Design

In scaling up the K/B 8 inch design, I made a few changes that, I hope, will improve the design for 12.5 inch aperture. They use a simple plywood mirror cell, stating that for an 8 inch mirror this cell will be adequate. I decided to order an aluminum mirror cell from University Optics instead so that the mirror could be supported at 9 different points. I also doubled the thickness of the rocker sides (not the front wall) to provide better support for the heavier tube. Thicker walls also required thicker side bearings so that the outside of the bearings would line up with the outside of the rocker sides.

Below are cutting diagrams and diagrams of the rocker and tube box with side bearings. The drawings are only approximately to scale. The rocker sides are each made of two identical pieces of ½ inch plywood glued together, creating two sides one inch thick. Likewise, the side bearings are made of two identical semicircles of 5/8 inch plywood glued together, creating side bearings that are 1 ¼ inches thick.

The dimensions of the Tube Box Sides are based on the outer diameter of my tube—14 3/16. This allows for a small clearance between the outside of the tube and the inside of the tube box (14 3/8). I had initially allowed for more of a gap, but realized once I cut the pieces and held the tube inside them that my gap was too large. The length of the Rocker Front also had to be changed so that the outside edge of the Side Bearings matched the outside edge of the Rocker Sides precisely.

The cutting diagram in (figure 1) shows how I cut the required pieces from two 60 X 60 pieces of Baltic Birch plywood, one ½ inch thick, and one 5/8.

The blue pieces of the rocker box and ground board are arranged as in the diagrams in (figure 2).

On the bottom of the Bottom board is a ring of Ebony Star laminate that glides over the three teflon pads located directly above the feet on the ground board. The curved sides of the Side Bearings are also covered with strips of Ebony Star that glide on the teflon pads on the curved edges of the Rocker Sides (figure 3).

In order for the Rocker Box to pivot on the Ground Board, a center pin is placed in a hole at the exact center of the Ground Board and Rocker Bottom. The specific hardware used for this is described on the Building page.

Step 4: Building the Scope

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.


July 25, 2008

RocheStar Fest

At some point in the building process, my goal became to finish the scope by RocheStar Fest, an annual event of the Rochester Astronomy Club. I was still doing some construction on the morning of the event, but I succeeded in making first light on that clear, dark night in late July. As I set of the scope, several club members came over to check things out and it was nice to hear the positive comments on the appearance of the scope. When the skies were dark enough for observing I found that the scope moves easily into position and holds that position well. That evening I spent most of my time rediscovering the many DSOs around Sagittarius, especially the Lagoon Nebula and the Trifid Nebula. I will have a lot of fun building my homemade 12.5 inch Dobsonian Telescope!!!

Celestron Space Challenge

Participated in the
Celestron Space Challenge