[bojan]

Erick, you can go as far as it is practical. I think I used ~250mm extension, and obviously the magnification you will be getting will depend on the barlow focal length.
Of course, this will result in barlow going in (closer to the primary) a bit more (but not much... Actually in the extreme case, when the barlow external focal point coincides with the focal point of the mirror, the effective focal length of this system will be infinite. This is also a case of Galilean telescope optical system.. he used the negative lens as an eyepiece.