The focusser is really the only mechanical component of a refractor, and therefore most critical to the telescope's collimation. With those of entry-level kits, like this one, and of others I have, the draw-tube is not necessarily square to its run within the housing, nor is the draw-tube slop-free. At a mid-point, or when racked fully outward, at least, the draw-tube wiggles, wobbles.
I had given the original focusser the works. Strips of PTFE as bearings for the draw-tube's run...
Bronze bearings for the pinion-gear's shaft, even...
Alas, it was to be my "Feather Touch" or "MoonLite" -esque or -ish focusser, but in the end it didn't pan out, not to my complete satisfaction. The main problem was simply my having to cut down the focusser in the first place. It was inexcusable for the factory to have included a long draw-tube for a short achromat. Also, the draw-tube's run seemed defective, uneven.
I went ahead and used it as is, and for testing for the proper spacing of the doublet. I use HVAC-type aluminium tape for spacers, at 0.09mm in thickness...
I layer large pieces of the tape, one on top of the other, depending on the thickness required, and cut the spacers out from that; for example, this edge-blackened doublet from a 70mm f/12.9 achromat...
