Found this on the astronomy.com forums
Quote:
A rack and pinion focuser typically has a plastic (sometimes Teflon) "bearing" which is just a flat strip against which the focuser tube slides. By turning the focus knob, you turn a small pinion gear which engages a long, straight "rack" gear, thus moving the focuser tube back and forth. Friction along the tube provides the "feel" to such focusers and is not usually very consistent -- particularly under heavier loads -- and the fine-touch to your focusing depends entirely on the way the gears are cut and how sensitive your fingertips are. In the wintertime, when you wear gloves or mittens, an R&P focuser can feel "rough" and make it hard to achieve critical focus.
A Crayford focuser usually has multiple (three or four) bearing surfaces, often provided by little rollers which "clamp" the focuser tube between them. This gives a more consistent "feel" to the focusing action and, in turn, allows you a finer control over critical focusing. Since there are multiple bearing surfaces, the Crayford design generally performs better in the cold.
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Although looking at my "crayford style" focuser it seems to still have a rack....
I might have to look into this further....
