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Okay, here's what I know....
When you focus with an eyepiece in the focuser, you adjust the focuser up and down to acheive focus for your eye.
Your telescope has a fixed focal length, say for example, 1250mm (if it is an f5). So the distance of the light path from the surface of the primary mirror - to the secondary mirror - up the focuser - to the focal point at eyepiece is 1250mm...
Your focuser will have a fair bit of travel on both sides of focus, but...
because you want to attach a DSLR camera on the focuser in place of an eyepiece, some focusers are not able to give enough "travel" in their mechanism to allow the image to come into focus on the CCD chip of the DSLR. Sometimes all it needs is an extra 15-20 mm, at on end of the focuser travel.
There are 2 main ways to get around this:
1. Replace the focuser with a "low profile" focuser. This type of focuser has a reduced height on top of the OTA, allowing the attached DSLR to come into focus.
2. Move the primary mirror up the tube, closer to the secondary mirror. This causes the focal point to "shift" further out of the focuser, aloowing you to bring the DSLR into focus.
What you need to do is establish how much extra travel you need in your focuser, and in which direction. Do you need the focuser to go lower or higher, but you run out of travel in the mechanism?
Most likely, if you have a GSO/BinTel Dob, replacing the focuser with a low profile focuser will easily rectify the problem - and all that takes is undoing 4 screws.
Others please chime in here, but I hope I have helped in some way...
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