Backfocus problem

This is a completely newbie-ish question and I apologize if it's misplaced but it's sort of a refractor-focuser-ccd question all in one. I have an SV-105 (f6.2,651mm FL), integrated 2" Feathertouch 2 speed focuser, and an ST-2000XCM. With my 2" (WO dialectric) diagonal I must run in the focuser nearly completely. So much that I don't feel I can run it past focus on that side (though it appears to focus just as it bottoms out). On the other extreme, without the diagonal, I can't run the focuser out far enough to focus, apparently needing a good 2" extension to achieve focus.

I suppose this allows for accessories like the AO8 or filter wheel
(which I don't really need with the single shot color but I
understand is possible to use). But with the focuser racked all the
way out, adding a big extension would really have the camera hanging
out in space! With my odd assortment of eyepieces (from a 55mm TV to
a 7.5mm UWAN and 5 more in between, I can focus visually with or
without the diagonal. Does this sould right?

Don Bryden said:

This is a completely newbie-ish question and I apologize if it's misplaced but it's sort of a refractor-focuser-ccd question all in one. I have an SV-105 (f6.2,651mm FL), integrated 2" Feathertouch 2 speed focuser, and an ST-2000XCM. With my 2" (WO dialectric) diagonal I must run in the focuser nearly completely. So much that I don't feel I can run it past focus on that side (though it appears to focus just as it bottoms out). On the other extreme, without the diagonal, I can't run the focuser out far enough to focus, apparently needing a good 2" extension to achieve focus.

Almost all--if not all--refractors sold in the U.S. (unlike Japan) are setup for use with diagonals. That means they cannot be used in "straight-through" fashion without an extension. The usual one folks use would just be a T-adaptor for the camera, which will usually yield enough additional out focus.