[Wavytone]

Using a small aperture off axis the scope effectively becomes an unobstructed Herschelian of the same focal length, and reduced aperture.

There's no real gain by doing this unless you really want a light loss, eg to observe the moon which is otherwise painfully bright in a 12".

Robh - you're right re the reduced exit pupil appearing off-centre in the eyepiece, it does, having done this with a 60 mm aperture on my C8. Not that it was noticeable as in my case it becomes 60mm f/30.