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.