Prime focus = no eyepiece - the telescope acts as the camera lens.
If you want more magnification than this, use eyepiece projection, where you will need one around 12 ... 25mm and a barrel set up for this, with the eyepiece inside, see figure 4 here
http://televue.com/engine/TV3b_page....E#.U_CtuGIaySM
The magnification = A/B. As a first approximation B is slightly longer than the eyepiece focal length.
You can also use digiscoping, figure 5, but this means an awfully large number of surfaces in the optical path and you can expect a lot of scattered light degrading the image.
For the planets like Saturn yes you will need more magnification, possibly 5x, and also set the mount to track reasonably well. You main enemy will be atmospheric turbulence, for one good shot you'll probably take 100 bad ones.
