There are three main options for dealing with your problem (insufficient back focus):
- Create more back focus by moving the primary/secondary mirrors closer to each other. Easy to do if you have a extendable truss tube Dob, otherwise it's rather drastic with solid tube reflectors.
- Add a telecompressor to magnify the image, e.g. a 2x TeleVue PowerMate or 2x barlow. This will make your focal length longer, but also gives you more back focus.
- Replace the focuser with a smaller, lower profile focuser if you're very close but not quite there.
Second option is generally the easiest. If you search on the forums, there are many discussions about this. Good luck!
