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Old 17-07-2019, 05:20 PM
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skogpingvin (Bill)
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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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Old 17-07-2019, 05:50 PM
Wavytone
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Yup thats a rack & pinion.

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.
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Old 18-07-2019, 12:31 PM
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skogpingvin (Bill)
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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.

Cheers!
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Old 18-07-2019, 04:00 PM
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https://en.wikipedia.org/wiki/Crayford_focuser
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Old 18-07-2019, 08:08 PM
Ukastronomer (Jeremy)
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I never liked the Skywatcher (and others) ones that were just a pressure roller setup they always slipped

Invented in the UK
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