Almost a year ago I decided to make a Dobsonian style reflector telescope. I finished it late in the spring and was able to enjoy it all summer, fall and even a little during the winter. I've observed Saturn, Jupitor, star clusters, nebula and spiral galaxies. It truly was a rewarding project and I still use it on a regular basis.

When I built my telescope I used a 1.25" rack and pinion style focuser. It has worked OK, but I knew there had to be something better out there. The motion on the rack and pinion focuser is not very smooth and it seemed to make large jumps in focus for small movements at the knob.

After some research I discovered that many amateur telescope makers use a Crayford style focuser due to its simple design and smooth motion. Many focusers also make use of a planetary reduction knob for fine focus adjustments. With these design goals in mind I set out to make my own.

Please be sure to read my summary at the end if you want to follow my Instructable to make your own focuser. I give a review on how well it performs and what changes I plan to make on my next revision.

Step 1: Features

There are a few features I want to point out that may not be obvious at first glance. They are what makes this focuser a step above my previous one. These features are on various focusers that I came across while researching how to make mine, but I don't think I found one with all of these features (except for the expensive ones).

  • Aiming set screws - There are 4 set screws on the mounting plate that can be used to adjust the alignment of the focuser. This is very useful for collimating the optics.
  • Eccentric bushing - This is used to apply pressure to the draw tube. By rotating the eccentric bushing the shaft that is attached to the focus knob is moved closer to or farther away from the draw tube.
  • Planetary reduction knob - This is helpful when trying to obtain a nice crisp image. It provides a 6:1 reduction for fine focus. 6 rotations of the focus knob results in 1 rotation of the shaft pressing against the draw tube.
To put it simply: metal. It's the only way you're going to get the rigidity you need out of this. Since weight isn't an issue, steel is your best bet. It'll be easier to weld and machine, and way more exact and stiff.
<p>I agree. I was also thinking that the next one I make will be out of metal. However, I was leaning toward aluminum. Weight is a concern because I need the telescope tube to balance at its side bearings. This focuser, made out of acrylic, is heavier than my old one and I had to move the tube back in the cradle to adjust the balance. There is not much more room left to move it before it hits the base. I could, however, use some counterweights if I needed to.</p>

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