Thanks for the comments!
Daniel, the theory behind this method is that in photography you can use a reversed lens, becoming a macro lens which has very short focus distance, and can give you 1:1 or more ratio.
This is the situation that i'm using to reach higher magnification.
As for direct prime focus, i will only get around 13x magnification (650mm / 50mm).
By putting a reversed lens between telescope and body, it will act like barlow (without any loss in light intensity, i guess).
I'm wondering if i could also put a real 2x barlow after the focuser to reach even more magnification, but i don't have one.
It also depends on the focal length of the lens. 28mm is the perfect combination. i've tried to put 50mm reversed lens but i have to provide longer extension tube to get in focus.
Please CMIIW, since i'm not technically capable in optics theory.
Here's another shot. using 28mm reversed.