[Rod-AR127 (Rod)]

Hi everyone, beautiful sunny day here in Perth, should get some observing in tonight hopefully.

I've heard the term back focus and I'm not sure exactly what it is referring to.
I have a problem with my 420mm FL ED72 when l have higher powered EP in use, usually with a Barlow, in that l cannot get focus due to the EP being too far away in the light path. ie: focusser wound all the way in and still can't reach focus.
I'm using a GSO 2" SD, would a 1.25" make much difference? l don't have one to try at the moment.
Thanks.