Does focal extenders need a ridiculously long back focus?

[brenchen]

Hi all, I'm a long time owner, first time user of the explorer scientific focal extenders (2x and 3x). Last night, as I prepare for the Jupiter opposition, I find I needed a 20-30cm backfocus to reach focus when I attach my 2x extender, and extension tubes ran out for the 3x.

For anyone that has used it, is this normal? If not, what could I be doing wrong? My imaging train consisted of: ASI1600MM > Filter wheel > Nose piece > 2x extender > loads of extension rings > focuser > more extension rings > GSO RC8.

[brenchen]

Answering my own question. I tried both extenders on my refractor, it was working as expected. But that was not the case for the RC. Perhaps there are something fundamentally the extender didn't like about reflectors.