Hey Carly,
Since you're using a Dob (and hence don't have any tracking abilities), it's probably best to stick with short exposure photography (e.g. the Moon).
If your Pentax has a video mode, you could use that to take (very) short videos of the planets drifting through your view. You can then later stack the individual frames in something like AVIStack or Registax.
There is a nice guide here;
http://www.iceinspace.com.au/index.p...63,201,0,0,1,0
Best of luck and have fun!