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Timing without barlow/Timing with barlow=magnification factor.
This one can be used:
Threaded on the eyepiece
Threaded onto a 1.25" adapter
In its own tube in front of the eyepiece
Threaded to the front of a star diagonal
In its own tube in front of a star diagonal.
So in some scopes, it can be used to yield a number of different focal lengths for the eyepiece.
Throw in some barrel extenders (about 10 different lengths of these available, from 14mm to over 50mm), and you could almost create a whole collection of eyepieces with only one or two eyepieces.
Unfortunately, not all distances of the lens from the eyepiece are free of vignetting, or spherical aberration.
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