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So it begins. My plan is to build an 8" Newtonian reflector based on a simplified design first built by John Dobson, AKA a"Dobsonian" or more simply, a "Dob". I want to make as much of the scope as possible. I decided to start with building a focuser. This is probably the most critical component in terms of precise fitting and movement. After careful study, I decided on a Crayford type focuser. These use a friction wheel to move the focusing tube instead of the more common rack and pinion. Advantages are a much smoother "stepless" mechanism that is also more suited to the kind of fabrication I am capable of. This would probably be much more suited to making on a machinist's lathe but I don't have one of those. As work progressed, I found myself thinking about how things were made during the pre industrial revolution period when each part had to be carefully hand fitted to achieve the desired result. The hardest part was getting the focusing tube to fit into the base properly. This has to be an easy slip fit but with no wobble. With repeated test fitting and sanding with 220, I was finally able to get a pretty sweet action.
The base has a PFTE bushing and a key to allow the tube to have as little resistance to movement as possible. The key also serves as a focus lock via the thumb screw visible on the side of the focuser body. Tensioning screws were added to the adjusting rod tube. These are necessary because you need to be able to compensate for the varying weights of different eyepieces. The focuser will accept a standard 2" eyepiece and, via the adaptor, the more common 1 1/4" eyepiece.
Of course, it would have been much easier to simply buy a focuser. They are many good quality units available for very reasonable prices. But then, it would just be something I bought and bolted onto the tube and that was not in keeping with my intent which is to actually make the thing and not just assemble a bunch of parts.
The other thing I discovered was that there are "dual range" focusers which are much preferable to single range units. Dual range focusers have fine and course adjustment dials and incorporate a planetary, typically with a ten to one reduction, using ball bearings instead of gears. While trying to find a plan to build the planetary, I came across an adjuster that is designed to retrofit an existing manufactured focuser. It looked like it would be pretty simple to adapt this and it was very reasonably priced so I ordered one and set about working it into my "design". Another critical element was getting the shaft positioned exactly right to bring the rubber friction roller into contact with the focusing tube. Too tight and the whole thing would bind up. Too loose and it would not work at all. Again, this would have been much easier had there been a machine shop available. As it was, I had to "sneak up on it" until the fit was correct.
So here we have it. A fully functional dual range Crayford type focuser made mostly out of wood. This is a truly "form follows function" object. Few embellishments were added outside of what was needed to make it work. We are often discussing the various ways that this kind of work provides satisfaction. The thrill I felt when I put all of these parts together and realized that it was actually going to work, well there are few things in life more satisfying!
The mounting plate still has to be fine fitted to the main scope tube and there will be a few other minor adjustments necessary but these will not be done until the scope is basically completed and the focuser is mounted. A bit down the road at this point.
Materials: Cocobolo, Honduras rosewood burl, African blackwood, brass, stainless steel, Teflon.
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