David,
""
the drawback is that with the powermate being so far removed from the eyepieces the magnification is huge, I estimate X4 although I admit that I don't know how to work it out mathematically"
Hmmm, this is not the case with the Powermates (with conventional barlows yes ) the magnification is almost constant with any spacing - it will be very close to x2.
See:
http://www.televue.com/engine/TV3b_p...id=53&Tab=_app