Peter is right - I tried imaging GX's with an f/5.6 scope, and it was pretty disappointing. You could see them, but the detail was very sorely lacking. I also tried f/6.4 scope with not much improvement. an f/8 scope was starting to do it...
And now I go and get an f/5 refractor astrograph
GX imager never to be
I could TRY the extender I guess...