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Focuser Drawtube Length

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Posted by Dave Sellors on April 17, 2003 16:09:58 UTC

Hello:

I'm building a 10" dob, with a custom made crayford focuser. My question is about the optimal length of the focuser drawtube.

My concerns are that too short a drawtube invites stray light into the eyepiece (poor image contrast ?) and too long a drawtube could block incoming light when the focuser is close to being totally racked in (vignetting?). If I'm understanding these issues properly, how should I choose how long to cut the tubing ?

Dave

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