The big things with prime-focus CCD is to get i) good polar alignment and ii) good focus. Even with stacking short exposures you'll find that the quality of the image will improve if polar alignment and tracking is good. This is because the image isn't 'smudged' across the exposure. Ideally you want the image not to move on the chip for the duration of your exposure. Judging by the look of the core in your pic I think 5 secs is a good place for you to get started. If you can setup your mount, with tracking on, so that you get no visible drift measured over 5 seconds then you're in business. To get Polar alignment use the Drift Method. For focus use a
Hartmann mask. Hope this helps in some way.