You might like to be aware of a peculiarity of the 2.5x Powermate. With all barlows and most powermates the image scale increases as you increase the distance between it and the sensor. With the 2.5" the image scale actually decreases slightly with distance. There are graphs showing this on the Televue pages.
http://www.televue.com/engine/TV3b_p...p#.Ux4qbueSyrM
Having said that, I've used the 2.5" happily on my 11" SCT.