For focusing, you need to make yourself a hartman mask. Slew to a star and focus on the star. When the 3 points overlap into 1 with nice diffraction spikes, then your focus is good.
Slew back to Saturn and start your imaging.
To get rid of your dust motes, use a can of compressed air and blow them off. I've had to do it a few times on my ToUcam, and it's worth with the dob because the image moves across the whole chip.. If I was tracking, I could position the planet on an area of the chip with no dust motes.
You should think about getting an IR filter that screws into your 1.25" adapter, aparently it will result in better images, and it will also protect your CCD from the dust invasion!
Great image, you're really improving.