A simple question regarding a Barlow

[slothead]

Please excuse the newbie question, but if I should insert a 2X Barlow in my light path (assume I'm going to use the same eyepiece I was already using - say a 10mm for simplicity's sake) will I need to be extending the focus (backing the eyepiece out) or compressing it (moving it in) from where that 10mm eyepiece was previously focused?

[ronin]

Usually it seems to extend it.

However as you put the eyepiece in the barlow you will likely find that the bit of the barlow tube after the actual lens takes up some of the additional distance. So then it depends on the relative length of this bit.

[RikM]

Yes, you generally need to move the focuser out a bit with a Barlow.

[Herbig]

Really good barlow are par focal meaning you shouldn't need to adjust focus, in practice and for most of us that can't afford them then yes you will need to adjust focus, how much and which way depends on the barlow you are using

[slothead]

Thanks guys.  I would have gone out and tested it myself, but we are so clouded over right now it was not worth the effort of getting it all set...  And I knew you guys would come through for me!