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I think you misunderstand how to set up the GSO CC.
To review:
1) First, find the focal plane of your scope. Put an X of translucent scotch tape across the open 2" focuser and point the scope to the Moon and focus the moon on the tape. Measure the distance the focuser drawtube is out from its inmost position and write it down so you can return to that point if you need to at any time in the future.
2) Insert the eyepiece you have that needs the most in-focus from that point. You can try out all your 2" eyepieces if you are not sure. It'll be the one that has the least drawtube sticking out of the focuser. Measure the distance the focuser drawtube is above the focuser body. It will be a smaller number than the focal plane distance. The difference between that point and the focal plane point is the distance the focal plane in the eyepiece is above the "shoulder" of the eyepiece (what it sits on when you insert it in the focuser).
3) The GSO's "working distance" from the CC lens to the focal plane of the eyepiece is 75mm. The top of the GSO CC is only 46mm out from the lens. This is because cameras typically require more in-focus than eyepieces and need to be closer to the lens. It would be unlikely for any eyepiece to be optimized with the 46mm top alone.
4) So the difference is 29mm, and you would need a spacer of 29mm to insert between the top of the CC and the lens of the CC to get an eyepiece whose focal plane was at the shoulder to the working distance from the lens. Your in-most focusing eyepiece, however, has its focal plane above the shoulder (why it needs in focus). To get it to the 75mm working distance, you add the distance you discovered the focal plane in the eyepiece was outward from the shoulder to the 46mm. If, as an example, the eyepiece's focal plane was 10mm up from the shoulder, then that 10mm would be added to the 46mm of the top of the CC to equal 56mm.
So you still need to move the eyepiece out 75-56=19mm. So the spacer you will insert between the CC lens and the CC body would be 19mm to bring the eyepiece to the correct distance from the CC lens.
I don't know what length of spacer you'll need, but it has a 48mm thread on each end. I've seen them in the market from 5mm to 50.8mm long, so you should be able to get one in the length you need +/-1mm.
5) At this point, with that eyepiece, the CC has become a "drop-in" accessory, just like a Barlow. You insert the eyepiece in the CC, put both in the scope and focus the scope on a star. You have, by doing that, optimized the position of the CC in the light cone of the scope.
6) Every other one of your eyepieces will have to be pulled slightly out of the CC to get it to focus. You do NOT focus using the focuser from this point on. It helps to add parfocalizing rings to your eyepieces so they can simply drop in and already be in focus, i.e. it helps to parfocalize all your eyepieces on a star (which can be done in advance of using the CC). When all your eyepieces are parfocalized with that in-most focusing eyepiece, the GSO CC becomes a simple drop-in tool for all your eyepieces.
7) This works if you use an adapter and use 1.25" eyepieces. They too will need to be pulled out slightly to come to focus in the CC. Should any of them need to have more in-focus, the choice is to start again with that eyepiece and that adapter and get another spacer for the CC, or simply get a shorter 1.25" adapter. 1.25" adapters come from a height of 16.5mm above the focuser to 12.7mm below the top of the focuser. So if you need extra in-focus, there's an adapter for that.
What you end up with is the GSO CC at the correct place in the light cone from the primary, and all your eyepieces in their optimum positions relative to the CC lens. And, they're all parfocal! Convenient.
And if you add a new eyepiece, start with one of your other eyepieces, focus the scope, then switch to the new eyepiece. It'll have to be pulled out of the CC to get to focus, and you can add a parfocalizing ring to that one, too.
Due to the 1.1x magnification of the CC, it will end up, likely, needing some in-travel of the focuser from the focal plane in order to get it to the right position.
This all sounds time consuming and complex, but remember you only need to buy one spacer of the right length for one eyepiece and all your other eyepieces follow from that. The only reason you find the focal plane of the scope is to derive the position of the focal plane of one eyepiece, and determine the size of spacer you need.
Spacer = 75mm - 46mm - x, where X is the distance the focal plane of the eyepiece is above the shoulder of the eyepiece. It's unlikely your in-most focusing eyepiece has a focal plane below the shoulder, but if it had, then x would become negative.
Let me know if that is clear, because I can go over any point.
Don
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