Barlow Lens Query

[Rautospoon]

For use in a refractor what are the "pros" and "cons" for using a Barlow "between the tube and the diagonal" versus "between the diagonal and the EP" - does it make any difference ?

Thank you.

[sixela]

The magnification factor is larger in the former case, so you can do both (especially with overly cheap shorties, though, you might see more aberrations if you put the barlow in front of the diagonal).

[brianb]

does it make any difference ?

Yes.

The eyepiece should be placed directly in the barlow, which means that if a diagonal is used, the barlow should be placed in the diagonal. Long ones will probably foul the mirror.

Moving the focal plane back as far as placing the diagonal in the barlow would, makes the magnification very different to the value the optics were designed for, and the results will very likely be poor in quality ... as well as getting more magnification than you think. Especially if you're using the barlow with a short focus scope (and there's no way you need one with a long focus scope).