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Question about a focuser
Hi folks, Bill here
I've got what is probably a dumb question, but which I've never really considered before.
I understand that there's a difference between a rack and pinion focuser and a Crayford, but I was always under the impression that a Crayford, because it uses a round roller plus some geometry to do the job, does not need a rack strapped to the drawtube.
My Sharpstar 107PH has an obvious brass rack along the drawtube that indicates to me that it's a rack and pinion focuser. Actually the grooves on it are a crosshatch pattern so it might be a double rack and pinion - not too sure. However, I've been working with an Esprit 100, and this also has a brass rack on it. I've attached a photo, although I assume most of you know what an Esprit looks like.
I'm puzzled - doesn't a Crayford simply have a flat facet, or can it too have a rack? A number of other focusers I'm familiar with just have this flat smooth facet, but not this one.
Can someone explain this to me?
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