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You can make any Barlow more powerful by putting an extension tube into it, and then the eyepiece into the tube. In fact, you can make a Barlow about 10% more powerful just by not putting the eyepiece all the way down into the Barlow tube (test this out on the moon).
With a 1.25" diagonal, you have to put a 2X Barlow into the diagonal to get 2X. But if you put the diagonal into the Barlow you get approximately 3X (or more if its a 2" Barlow). Erect image diagonals have a longer light path, so that they too would give more than 3X if placed into a 2X Barlow.
I have a short tube 2X Celestron Barlow and a long tube 1.8X Televue Barlow. I have at times swapped the Barlow lenses between the tubes, to get approximately 1.5X and 2.5X Barlows.
Regards,
Renato
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