Constructing a 12.5" Newtonian
Commercial units are often of metal and can cost from $35 to over $300. There are 2 basic types, with variations on them: 1) rack and pinion and 2) screw thread. I have used an inexpensive rack and pinion on my 6" telescope and found it difficult to get exact focus. When is was f/10 it was adequate, but at f/5 it was difficult. Focus is much more critical on fast telescopes, like mine.
For my 12.5" telescope, I decided I wanted something easier to use and more accurate so I chose a threaded design. Threads work like a lever so that a large rotation produces a small amount of travel along the axis of the thread (in other words, nearer or away from the main telescope tube). This is exactly what we want so that we can get the focus perfect (the eyepiece at the right distance)
I decided to use plastic since it's light, easily obtainable, and easily modified without a machine shop. I found plastic pipe commonly found at a hardware store would work quite well. My design cost less than $3. Here's what the nearly complete eyepiece holder looks like:
The eyepiece is inserted on the left side (the gray piece). The black piece on the right goes onto the telescope tube. My eyepieces are nearly parfocal, so I only need my focuser to move at most 1/2". This design allows 2-3 times that, so it's adoptable to many other scopes. Shims could also be used to extend eyepiece travel if needed.
The completed eyepiece holder assembled on my 12.5" telescope. As can be seen from the photo, this is a low-profile focuser. Part of the black base sticks into the tube about an inch and blocks a bit of light, but the effects of this on the image are minimal since the main tube is 14" diameter and it only blocks a small area of off-axis light.
And here's how it was made, from just two pieces of plumbing.:
That's all there was to it!
Next, it needs to be collimated.