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
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.
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, “No...it 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 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.