Well, Jim, you're going to have to have some type of primary mirror cell - to hold the mirror and allow for collimation adjustments. How are you doing that? That will take up some space (3" is pretty normal I think). Of course you could build that as an extension TO the tube instead of sitting inside it, but most people don't. So, your mirror surface will be "depth of mirror cell" PLUS "mirror thickness" UP from the bottom of the tube. Is that any help? Keep it as low as possible, because with the measurements you gave you don't have a lot of spare tube at the top - for the spider and secondary. You'll want SOME up there to help prevent stray light entering the focuser from the side... 'Course you can always add a "dew shield" extension.. Oh, and of course, you want the mirror centered in the tube - whatever the depthof mount is.. Good luck! Clear skies...

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