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.
Nothing to fret about, the modern ones are great if they have the teeth on an angle like that (and a matching helical pinion) - it means it will be smooth. The key aspect of the Crayford - which I think almost every manufacturer has adopted - is the use of 4 tiny precision bearings to support the draw tube rather than a sliding friction bearing.
The rack & pinion IMHO has a big advantage in its ability to hold heavy loads without slipping whereas a crayford roller drive needs some sot of adjustable drag or a locking screw if you're using a heavy eyepiece or camera.
Thanks for that Nick, what I think you're saying is that it's the four bearings around the drawtube that make it a "Crayford" focuser, and that the existence of a rack and pinion mechanism don't disqualify it from the Crayford name.
