The most dissatisfying feature of this focuser was excessive "focus shift". When turning the knobs to adjust the focus while observing an object, the field of view shifted pretty drastically. This focus shift seemed to get worse over time. Examination of the focuser led me to the conclusion that it was simply poorly designed. With the draw-tube supported on the bottom only by the rack riding on the pinion, and the lack of any effective bearing surfaces other than the pinion itself, turning the pinion to move the draw-tube caused the entire draw-tube to rock one way or the other.
One day I was reading articles on the internet about telescope focusers. I had decided that the only solution for me was to replace the focuser with a Crayford style focuser, but I also knew that many high-end telescopes use rack and pinion focusing mechanisms very succesfully - there is nothing inherently wrong with the concept, it is simply a matter of design quality. I ran across a photo essay on adjusting, lubricating and maintaining rack and pinion focusers. In the article were photos of a focuser that looked very much like mine. I read with interest the section on focus shift - the focuser in the article had a plastic bearing above the drawtube, held against the drawtube by pressure from two set screws. Adjusting those two set screws was the means of eliminating or minimizing focus shift on that focuser. If only my focuser had the same adjustment screws ...
Well, it does! After all the time I had spent adjusting the draw-tube tension, collimating the focuser and trying to figure out why there was so much focus shift, I had never seen the two set screws. In the photo above I have indicated their positions, on either side of the knurled plastic focus lock knob, with red arrows. Use an allen wrench to adjust the two set screws so that they apply equal pressure to each end of the plastic bearing, until the focus shift goes away. You may find that the focuser knobs are now harder to turn. If so, you can slightly loosen the four phillips head screws described in step 1.
