Strange -
i have a stock as a rock GSO680, and this projection doohicky....
works absolutely fine for me....
all the way up to 6mm...
(my problem is that prime focus does not work...)
I have put one one of these:
http://www.bintel.com.au/Astrophotog...oductview.aspx
which brings the EP quite a bit closer to the secondary,
since you can remove the stock 2"=125" adapter,
and additionally the EP itself can be mounted further down the projection adaptor