Maybe a late reaction but maybe usefull for whoever reads this
The trick which helped me is: drive in the prism in the OAG as far as possible into the optical path - without vignetting the sensor. The deeper it sticks, the more it shadows the sensor, the higher out it sticks, the more light you loose and the more funny your starshapes become.
It really makes a big difference to slide the prism a mm down or not, especially in light. And hence in guiding stars limiting magnitude.
For a DSLR this means positioning the prism on the long side of the sensor. Doing so, and bringing it to the edge of the sensor without vignetting, will pick up stars inside the corrected field of view circle of the telescope (obviously assuming the DSLR FOV is well corrected). In that case your guide stars will be round!
You can test how deep you can drive the prism visually during daytime while looking through the telescope, through your DSLR or your CCD (on screen in the latter case).
When you have found that deepest prism spot, lock the prism location. Then slide the OAG camera along the prism bar to find the focus.
This can be done during daylight unless you're using a Lacerta e.g.
Clear Skies
Joostie
