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Hi Trevor,
I agree with Peter's explanation.
In other words, the focal length of the eyepiece (eg. 10mm or 20mm) is actually the distance of its focal point towards the primary mirror. ie. the focusser needs to be moved in/out so that this coincides with the focal plane of the primary.
The light cone emanating from the eyepiece is fairly parallel (and how far it remains crisp for viewing/imaging is "eye relief", typically about 10 to 20mm). That's my current understanding anyway.
Cheers,
Jeff
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