Try this:
http://www.iceinspace.com.au/forum/s...d.php?t=105057.
With 5.6 micron pixels (eg DMK21-618), rule of thumb suggests f28 – your scope needs a little under 4x. you could use a 3x with an extension tube or pay twice as much to get the same result from a 4x Powermate.
If you go for one of the very good 3.75 micron pixel CMOS cameras - ASI120, QHY5L2 - you will need ~f19, so suggest a 2x Barlow and extension tube to get out to about 2.5x
if you decide to try the new Neximage5 camera with 2.2 micron pixels, you will need f11, so a 1.5x Barlow would be best.
This guy gets pretty impressive results from his 120mm f7.5 refractor using 4x and a DMK21-618 camera (5.6micron).
http://www.astrobin.com/users/FranckIM06/