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A "Crayford" or "Crayford style" (I presume there must be some form of trademark on the name) focuser uses friction to raise or lower the focus tube(?). I understand that this allows finer control of the focus and less backlash also. The only other type that I know of is rack and pinion (teeth and gears) and is not very precise. I've just changed the one on my dob to a crayford.
Hope I've got this right and have helped some.
Travis
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